Combining Multiple Images with Enblend 4.0-753b534c819d

Enblend

This manual is for Enblend (version 4.0-753b534c819d, 4 December 2009), a tool for compositing images in such a way that the seam between the images is invisible, or at least very difficult to see.

1 Overview

Enblend overlays multiple TIFF images using the Burt-Adelson multiresolution spline algorithm.¹ This technique tries to make the seams between the input images invisible. The basic idea is that image features should be blended across a transition zone proportional in size to the spatial frequency of the features. For example, objects like trees and windowpanes have rapid changes in color. By blending these features in a narrow zone, you will not be able to see the seam because the eye already expects to see color changes at the edge of these features. Clouds and sky are the opposite. These features have to be blended across a wide transition zone because any sudden change in color will be immediately noticeable.

Enblend expects each input file to have an alpha channel. The alpha channel should indicate the region of the file that has valid image data. Enblend compares the alpha regions in the input files to find the areas where images overlap. Alpha channels can be used to indicate to Enblend that certain portions of an input image should not contribute to the final image.

Enblend does not align images. Use a tool such as hugin or PanoTools to do this. The TIFF files produced by these programs are exactly what Enblend is designed to work with. Sometimes these GUIs allow you to select feathering for the edges of your images. This treatment is detrimental to Enblend. Turn off feathering by deselecting it or setting the feather width to zero.

Enblend blends the images in the order they are specified on the command line. You should order your images according to the way that they overlap, for example from left-to-right across the panorama. If you are making a multi-row panorama, we recommend blending each horizontal row individually, and then running Enblend a last time to blend all of the rows together vertically.

Enblend reads all layers of multi-layer images, like, for example, multi-directory TIFF images². The input images are processed in the order they appear on the command line. Multi-layer images are processed from the first layer to the last before Enblend considers the next image on the command line.

Find out more about Enblend on its SourceForge web page.

2 Workflow

Enblend and Enfuse are parts of a chain of tools to assemble images.

• Enblend combines a series of pictures taken at the same location but in different directions.
• Enfuse merges photos of the same subject at the same location and same direction, but taken with varying exposure parameters.

Figure:photographic-workflow shows where Enblend and Enfuse sit in this tool chain.

Figure 2.1: Photographic workflow with Enblend and Enfuse.

Take Images

Take multiple images to form a panorama, an exposure series, a focus stack, etc.

          There is one exception with Enfuse when a single raw image is
          converted multiple times to get several – typically differently
          “exposed” – images.

Exemplary Benefits

• Many pictures taken from the same vantage point but showing different viewing directions. – Panorama
• Pictures of the same subject exposed with different shutter speeds. – Exposure series
• Images of the same subject focussed at differing distances. – Focus stack

Remaining Problem: The “overlayed” images may not fit together, that is the overlay regions may not match exactly.

Convert Images

Convert the raw data exploiting the full dynamic range of the camera and capitalize on a high-quality conversion.

Align Images

Align the images so as to make them match as well as possible.

          Again there is one exception and this is when images naturally align.
          For example, a series of images taken from a rock solid tripod with a
          cable release without touching the camera, or images taken with a
          shift lens, can align without further user intervention.

This step submits the images to affine transformations. If necessary, it rectifies the lens' distortions (e.g. barrel or pincushion), too. Sometimes even luminance or color differences between pairs of overlaying images are corrected (“photometric alignment”).

Benefit: The overlay areas of images match as closely as possible given the quality if the input images and the lens model used in the transformation.

Remaining Problem: The images may still not align perfectly, for example, because of parallax errors, or blur produced by camera shake.

Combine Images

Enblend and Enfuse combine the aligned images into one.

Benefit: The overlay areas become imperceptible for all but the most mal-aligned images.

Remaining Problem: Enblend and Enfuse write images with an alpha channel. (For more information on alpha channels see Understanding Masks.) Furthermore, the final image rarely is rectangular.

Postprocess

Postprocess the combined image with your favorite tool. Often the user will want to crop the image and simultaneously throw away the alpha channel.

View

Print

Enjoy

3 Invocation

enblend [OPTIONS] [--output=IMAGE] INPUT...

Assemble the sequence of images INPUT... into a single IMAGE.

Input images are either specified literally or via so-called response files (see below). The latter are an alternative to specifying image filenames on the command line.

3.1 Response Files

A response file contains names of images or other response filenames. Introduce response file names with an at-character (‘@’).

Enblend and Enfuse process the list INPUT strictly from left to right, expanding response files in depth-first order. (Multi-layer files are processed from first layer to the last.) The following examples only show Enblend, but Enfuse works exactly the same.

Solely image filenames.

Example:

          enblend image-1.tif image-2.tif image-3.tif

The ultimate order in which the images are processed is: image-1.tif, image-2.tif, image-3.tif.

Single response file.

Example:

          enblend @list

where file list contains

          img1.exr
          img2.exr
          img3.exr
          img4.exr

Ultimate order: img1.exr, img2.exr, img3.exr, img4.exr.

Mixed literal names and response files.

Example:

          enblend @master.list image-09.png image-10.png

where file master.list comprises of

          image-01.png
          @first.list
          image-04.png
          @second.list
          image-08.png

first.list is

          image-02.png
          image-03.png

and second.list contains

          image-05.png
          image-06.png
          image-07.png

Ultimate order: image-01.png, image-02.png, image-03.png, image-04.png, image-05.png, image-06.png, image-07.png, image-08.png, image-09.png, image-10.png,

3.1.1 Response File Format

Response files contain one filename per line. Blank lines or lines beginning with a sharp sign (‘#’) are ignored; the latter can serve as comments. Filenames that begin with an at-character (‘@’) denote other response files. Table:response-file-format states a formal grammar of response files in EBNF.

In a response file relative filenames are used relative the response file itself, not relative to the current-working directory of the application.

The above grammar might unpleasantly surprise the user in the some ways.

Whitespace trimmed at both line ends

For convenience, whitespace at the beginning and at the end of each line is ignored. However, this implies that response files cannot represent filenames that start or end with whitespace, as there is no quoting syntax. Filenames with embedded whitespace cause no problems, though.

Only whole-line comments

Comments in response files always occupy a complete line. There are no “line-ending comments”. Thus, in

          # exposure series
          img-0.33ev.tif # "middle" EV
          img-1.33ev.tif
          img+0.67ev.tif

only the first line contains a comment, whereas the second line includes none. Rather, it refers to a file called ‘img-0.33ev.tif # "middle" EV’.

Image filenames cannot start with ‘@’

An at-sign invariably introduces a response file, even if the filename's extension hints towards an image.

If Enblend or Enfuse do not recognize a response file, they will skip the file and issue a warning. To force a file being recognized as a response file add one of the following syntactic comments to the first line of the file.

     response-file: true
     enblend-response-file: true
     enfuse-response-file: true

Finally, here is an example of a valid response file.

     # 4\pi panorama!
     
     # These pictures were taken with the panorama head.
     @round-shots.list
     
     # Freehand sky shot.
     zenith.tif
     
     # "Legs, will you go away?" images.
     nadir-2.tif
     nadir-5.tif
     nadir.tif

3.1.2 Syntactic Comments

Comments that follow the format described in Table:response-file-syntactic-comment are treated as instructions how to interpret the rest of the response file. A syntactic comment is effective immediately and its effect persists to the end of the response file, unless another syntactic comment undoes it.

Unknown syntactic comments are silently ignored.

3.1.3 Globbing Algorithms

The three equivalent syntactic keys

• glob,
• globbing, or
• filename-globbing

control the algorithm that Enblend or Enfuse use to glob filenames in response files.

All versions of Enblend and Enfuse support at least two algorithms: literal, which is the default, and wildcard. See Table:globbing-algorithms for a list of all possible globbing algorithms. To find out about the algorithms in your version of Enblend or Enfuse team up the options --version and --verbose.

Example:

     # Horizontal panorama
     # 15 images
     
     # filename-globbing: wildcard
     
     image_000[0-9].tif
     image_001[0-4].tif

3.2 Common Options

Common options control some overall features of Enblend.

-a

Pre-assemble non-overlapping images before each blending iteration.

This overrides the default behavior which is to blend the images sequentially in the order given on the command line. Enblend will use fewer blending iterations, but it will do more work in each iteration.

--compression=COMPRESSION

Write a compressed output file.

Depending on the output file format Enblend accepts different values for COMPRESSION.

JPEG

COMPRESSION is a JPEG quality level ranging from 0–100.

TIFF

COMPRESSION is one of the keywords:

‘NONE’

Do not compress. This is the default.

‘DEFLATE’

Use the Deflate compression scheme also called ZIP-in-TIFF. Deflate is a lossless data compression algorithm that uses a combination of the LZ77 algorithm and Huffman coding.

‘LZW’

Use Lempel-Ziv-Welch (LZW) adaptive compression scheme. LZW compression is lossless.

‘PACKBITS’

Use PackBits compression scheme. PackBits is particular variant of run-length compression. It is lossless.

Any other format

Other formats do not accept a COMPRESSION setting.

However, VIGRA automatically compresses png-files with the Deflate method.

-h

--help

Print information on the available options and exit.

-l LEVELS

--levels=LEVELS

Use at most this many LEVELS for pyramid ³ blending if LEVELS is positive, or reduce the maximum number of levels used by −LEVELS if LEVELS is negative.

The number of levels used in a pyramid controls the balance between local and global image features (contrast, saturation, ...) in the blended region. Fewer levels emphasize local features and suppress global ones. The more levels a pyramid has, the more global features will be taken into account.

As a guideline, remember that each new level works on a linear scale twice as large as the previous one. So, the zeroth layer, the original image, obviously defines the image at single-pixel scale, the first level works at two-pixel scale, and generally, the n-th level contains image data at 2^n-pixel scale. This is the reason why an image of width×height pixels cannot be deconstructed into a pyramid of more than

$\⌊{log}_2\⁡\left(min\⁡\left(\mathit{width},\mathit{height}\right)\right)\⌋$ levels.

If too few levels are used, “halos” around regions of strong local feature variation can show up. On the other hand, if too many levels are used, the image might contain too much global features. Usually, the latter is not a problem, but is highly desired. This is the reason, why the default is to use as many levels as is possible given the size of the overlap regions. Enblend may still use a smaller number of levels if the geometry of the overlap region demands.

Positive values of LEVELS limit the maximum number of pyramid levels. Depending on the size and geometry of the overlap regions this may or may not influence any pyramid. Negative values of LEVELS reduce the number of pyramid levels below the maximum no matter what the actual maximum is and thus always influence all pyramids.

The valid range of the absolute value of LEVELS is 1 to 29.

-o

--output=FILE

Place output in FILE.

If --output is not specified, the default is to put the resulting image in a.tif.

-v

--verbose[=LEVEL]

Without an argument, increase the verbosity of progress reporting. Giving more --verbose options will make Enblend more verbose. Directly set a verbosity level with a non-negative integral LEVEL.

Each level includes all messages of the lower levels.

Level

Messages

0

only warnings and errors

1

reading and writing of images

2

mask generation, pyramid, and blending

3

reading of response files, color conversions

4

image sizes, bounding boxes and intersection sizes

5

detailed information on the optimizer runs (Enblend only)

6

estimations of required memory in selected processing steps

The default verbosity level of Enblend is 1.

-V

--version

Output information on the Enblend version.

Team this option with --verbose to inquire about configuration details, like the extra features that have been compiled in.

-w

--wrap=MODE

Blend around the boundaries of the panorama.

With this option Enblend treats the panorama of width w and height h as an infinite data structure, where each pixel P(x, y) of the input images represents the set of pixels S_P(x, y)⁴.

MODE takes the following values:

‘none’

‘open’

This is a “no-op”; it has the same effect as not giving --wrap at all. The set of input images is considered open at its boundaries.

‘horizontal’

Wrap around horizontally: $S_P\left(x,y\right)=\{P\left(x+m\⁢w,y\right):m\text{\  in\  }Z\}\text{.}$ This is useful for 360° horizontal panoramas as it eliminates the left and right borders.

‘vertical’

Wrap around vertically: $S_P\left(x,y\right)=\{P\left(x,y+n\⁢h\right):n\text{\  in\  }Z\}\text{.}$ This is useful for 360° vertical panoramas as it eliminates the top and bottom borders.

‘both’

‘horizontal+vertical’

‘vertical+horizontal’

Wrap around both horizontally and vertically: $S_P\left(x,y\right)=\{P\left(x+m\⁢w,y+n\⁢h\right):m,n\text{\  in\  }Z\}\text{.}$ In this mode, both left and right borders, as well as top and bottom borders, are eliminated.

Specifying --wrap without MODE selects horizontal wrapping.

-x

Checkpoint partial results to the output file after each blending step.

3.3 Extended Options

Extended options control the image cache, the color model, and the cropping of the output image.

-b BLOCKSIZE

Set the BLOCKSIZE in kilobytes (KB) of Enblend's image cache.

This is the amount of data that Enblend will move to and from the disk at one time. The default is 2048KB, which should be ok for most systems. See Tuning Memory Usage for details.

Note that Enblend must have been compiled with the image-cache feature for this option to be effective. Find out about extra features with enblend --version --verbose.

-c

Use the CIECAM02 color appearance model for blending colors.

The input files should have embedded ICC profiles if this option is specified. If no ICC profile is present, Enblend will assume that the image uses the sRGB color space. The difference between this option and Enblend's default color blending algorithm is very slight and will only be noticeable when areas of different primary colors are blended together.

-d

--depth=DEPTH

Force the number of bits per channel and the numeric format of the output image.

Enblend always uses a smart way to change the channel depth to assure highest image quality (at the expense of memory), whether requantization is implicit because of the output format or explicit with option --depth.

• If the output-channel width is larger than the input-channel width of the input images, the input images' channels are widened to the output channel width immediately after loading, that is, as soon as possible. Enblend then performs all blending operations at the output-channel width, thereby preserving minute color details which can appear in the blending areas.
• If the output-channel width is smaller than the input-channel width of the input images, the output image's channels are narrowed only right before it is written to disk, that is, as late as possible. Thus the data benefits from the wider input channels for the longest time.

All DEPTH specifications are valid in lowercase as well as uppercase letters. For integer format, use

8, uint8

Unsigned 8bit; range: 0..255

int16

Signed 16bit; range: −32768..32767

16, uint16

Unsigned 16bit; range: 0..65535

int32

Signed 32bit; range: −2147483648..2147483647

32, uint32

Unsigned 32bit; range: 0..4294967295

For floating-point format, use

r32, real32, float

IEEE754 single precision floating-point, 32bit wide, 24bit significant

• Minimum normalized value: $1.2\times {10}^{-38}$
• Epsilon: $1.2\times {10}^{-7}$
• Maximum finite value: $3.4\times {10}^{38}$

r64, real64, double

IEEE754 double precision floating-point, 64bit wide, 53bit significant

• Minimum normalized value: $2.2\times {10}^{-308}$
• Epsilon: $2.2\times {10}^{-16}$
• Maximum finite value: $1.8\times {10}^{308}$

If the requested DEPTH is not supported by the output file format, Enblend warns and chooses the DEPTH that matches best.

The OpenEXR data format is treated as IEEE754 float internally. Externally, on disk, OpenEXR data is represented by “half” precision floating-point numbers.

OpenEXR half precision floating-point, 16bit wide, 10bit significant

• Minimum normalized value: $9.3\times {10}^{-10}$
• Epsilon: $2.0\times {10}^{-3}$
• Maximum finite value: $4.3\times {10}^9$

-f WIDTHxHEIGHT

-f WIDTHxHEIGHT+xXOFFSET+yYOFFSET

Set the size of the output image manually to WIDTH×HEIGHT. Optionally specify the X-OFFSET and Y-OFFSET, too.

This option is useful when the input images are cropped TIFF files, such as those produced by nona. The stitcher nona is part of Hugin. See Helpful Programs.

-g

Save alpha channel as “associated”. See the TIFF documentation for an explanation.

Gimp (before version 2.0) and Cinepaint (see Helpful Programs) exhibit unusual behavior when loading images with unassociated alpha channels. Use option -g to work around this problem. With this flag Enblend creates the output image with the associated alpha tag set, even though the image is really unassociated alpha.

--gpu

Use the graphics card – in fact the graphics processing unit (GPU) – to accelerate some computations.

This is an experimental feature that may not work on all systems. In this version of Enblend, 4.0-753b534c819d, only mask optimization strategy 1 benefits from this option.

Note that GPU-support must have been compiled into Enblend for this option to be available. Find out about this feature with enblend --version --verbose.

-m CACHESIZE

Set the CACHESIZE in megabytes (MB) of Enblend's image cache.

This is the amount of memory Enblend will use for storing image data before swapping to disk. The default is 1024MB which is good for systems with 3–4gigabytes (GB) of RAM. See Tuning Memory Usage for details.

Note that Enblend must have been compiled with the image-cache feature for this option to be effective. Find out about extra features with enblend --version --verbose.

3.4 Mask Generation Options

These options control the generation and the usage of masks.

--anneal=TAU[:DELTA-E-MAX[:DELTA-E-MIN[:K-MAX]]]

Set the parameters of the Simulated Annealing optimizer used in Optimizer Strategy 1 (see Table:optimizer-strategies).

TAU

TAU is the temperature reduction factor in the Simulated Annealing; it also can be thought of as “cooling factor”. The closer TAU is to one, the more accurate the annealing run will be, and the longer it will take.

Append a percent sign (‘%’) to specify TAU as a percentage.

Valid range:

$0<\mathit{TAU}<1\text{.}$

The default is 0.75; values around 0.95 are reasonable. Usually, slower cooling results in more converged points.

DELTA-E-MAX

DELTA-E-MIN

DELTA-E-MAX and DELTA-E-MIN are the maximum and minimum cost change possible by any single annealing move.

Valid range:

$0<\mathit{DELTA-E-MIN}<\mathit{DELTA-E-MAX}\text{.}$

In particular they determine the initial and final annealing temperatures according to:

$\begin{array}{c}{\mathit{T}}_{{initial}}=\frac{\mathit{DELTA-E-MAX}}{log\&ApplyFunction;\left(\mathit{K-MAX}/\left(\mathit{K-MAX}-2\right)\right)}\\ {\mathit{T}}_{{final}}=\frac{\mathit{DELTA-E-MIN}}{log\&ApplyFunction;\left({\mathit{K-MAX}}^2-\mathit{K-MAX}-1\right)}\end{array}$

The defaults are: DELTA-E-MAX: 7000.0 and DELTA-E-MIN: 5.0.

K-MAX

K-MAX is the maximum number of “moves” the optimizer will make for each line segment. Higher values more accurately sample the state space, at the expense of a higher computation cost.

Valid range:

$\mathit{K-MAX}\ge \mathrm{}\text{3.}$

The default is 32. Values around 100 seem reasonable.

--coarse-mask[=FACTOR]

Use a scaled-down version of the input images to create the seam line. This option reduces the number of computations necessary to compute the seam line and the amount of memory necessary to do so. It is the default.

If omitted FACTOR defaults to 8, this means, option --coarse-mask shrinks the overlapping areas by a factor of 8 ×

8. With FACTOR = 8 the total memory allocated during a run of Enblend shrinks approximately by 80% and the maximum amount of memory in use at a time is decreased to 60% (Enblend compiled with image cache) or 40% (Enblend compiled without image cache).

Valid range: FACTOR = 1, 2, 3,....

Also see Table:mask-generation.

	--no-optimize	--optimize
--fine-mask	Use NFT mask.	Vectorize NFT mask, optimize vertices with simulated annealing and Dijkstra's shortest path algorithm, fill vector contours.
--coarse-mask	Scale down overlap region, compute NFT mask and vectorize it, fill vector contours.	Scale down overlap region, vectorize NFT mask, optimize vertices with simulated annealing and Dijkstra's shortest path algorithm, fill vector contours.

Table 3.4: Various options that control the generation of masks. All mask computations are based on the Nearest-Feature Transformation (NFT) of the overlap region.

--dijkstra=RADIUS

Set the search RADIUS of the Dijkstra Shortest Path algorithm used in Optimizer Strategy 2 (see Table:optimizer-strategies).

A small value prefers straight line segments and thus shorter seam lines. Larger values instruct the optimizer to let the seam line take more detours when searching for the best seam line.

Valid range:

$\mathit{RADIUS}\ge 1\text{.}$

Default: 25pixels.

--fine-mask

Instruct Enblend to employ the full-size images to create the seam line, which can be slow. Use this option, for example, if you have very narrow overlap regions.

Also see Table 3.4.

--load-masks[=IMAGE-TEMPLATE]

Instead of generating masks, use those in IMAGE-TEMPLATE. The default is mask-%n.tif.

See --save-masks below for details.

--mask-vectorize=DISTANCE

Set the mask vectorization DISTANCE Enblend uses to partition each seam. Thus, break down the seam to segments of length DISTANCE each.

If Enblend uses a coarse mask (--coarse-mask) or Enblend optimizes (--optimize) a mask it vectorizes the initial seam line before performing further operations. See Table 3.4 for the precise conditions. DISTANCE tells Enblend how long to make each of the line segments called vectors here.

The unit of DISTANCE is pixels unless it is a percentage as explained in the next paragraph. In fine masks one mask pixel corresponds to one pixel in the input image, whereas in coarse masks one pixel represents for example 8pixels in the input image.

Append a percentage sign (‘%’) to DISTANCE to specify the segment length as a fraction of the diagonal of the rectangle including the overlap region. Relative measures do not depend on coarse or fine masks, they are recomputed for each mask. Values around 5%–10% are a good starting point.

This option massively influences the mask generation process! Large DISTANCE values lead to shorter, straighter, less wiggly, less baroque seams that are on the other hand less optimal, because they run through regions of larger image mismatch instead of avoiding them. Small DISTANCE values give the optimizers more possibilities to run the seam around high mismatch areas.

What should never happen though, are loops in the seam line. Counter loops with higher weights of DISTANCE-WEIGHT (in option --optimizer-weights), larger vectorization DISTANCEs, TAUs (in option --anneal) that are closer to one, and blurring of the difference image with option --smooth-difference. Use option --visualize to check the results.

Valid range:

$\mathit{DISTANCE}\ge 4\text{.}$ Enblend limits DISTANCE so that it never gets below 4 even if it has been given as a percentage. The user will be warned in such cases.

Default: 4pixels for coarse masks and 20pixels for fine masks.

--no-optimize

Turn off seam line optimization. Combined with option --fine-mask this will produce the same type of mask as Enblend version 2.5, namely the result of a Nearest-Feature Transform (NFT).⁵

Also see Table 3.4.

--optimize

Use a two-strategy approach to route the seam line around mismatches in the overlap region. This is the default. Table:optimizer-strategies explains these strategies; also see Table 3.4.

Stragegy 1: Simulated Annealing

Tune with option --anneal = TAU : DELTA-E-MAX : DELTA-E-MIN : K-MAX.

Simulated-Annealing

Stragegy 2: Dijkstra Shortest Path

Tune with option --dijkstra = RADIUS.

Dijkstra algorithm

Table 3.5: Enblend's two strategies to optimize the seam lines between images.

--optimizer-weights=DISTANCE-WEIGHT[:MISMATCH-WEIGHT]

Set the weights of the seam-line optimizer. If omitted, MISMATCH-WEIGHT defaults to 1.

The seam-line optimizer considers two qualities of the seam line:

• The distance of the seam line from its initial position, which has been determined by NFT (see option --no-optimize).
• The total “mismatch” accumulated along it.

The optimizer weights DISTANCE-WEIGHT and MISMATCH-WEIGHT define how to weight these two criteria. Enblend up to version 3.2 used 1:1. This version of Enblend (4.0-753b534c819d) uses 8.0:1.0.

A large DISTANCE-WEIGHT pulls the optimized seam line closer to the initial postion. A large MISMATCH-WEIGHT makes the seam line go on detours to find a path along which the mismatch between the images is small. If the optimized seam line shows cusps or loops (see option --visualize), reduce MISMATCH-WEIGHT or increase DISTANCE-WEIGHT.

Both weights must be non-negative. They cannot be both zero at the same time. Otherwise, their absolute values are not important as Enblend normalizes their sum.

--save-masks[=IMAGE-TEMPLATE]

Save the generated masks to IMAGE-TEMPLATE. The default is mask-%n.tif. Enblend saves masks as 8bit grayscale (single channel) images. For accuracy we recommend to choose a lossless format.

Use this option if you wish to edit the location of the seam line by hand. This will give you images of the right sizes that you can edit to make your changes. Later, use --load-masks to blend the project with your custom seam lines.

IMAGE-TEMPLATE defines a template that is expanded for each input file. In a template a percent sign (‘%’) introduces a variable part. All other characters are copied literally. Lowercase letters refer to the name of the respective input file, whereas uppercase ones refer to the name of the output file (see Common Options). Table:mask-template-characters lists all variables.

A fancy mask filename template could look like this:

          %D/mask-%02n-%f.tif

It puts the mask files into the same directory as the output file (‘%D’), generates a two-digit index (‘%02n’) to keep the mask files nicely sorted, and decorates the mask filename with the name of the associated input file (‘%f’) for easy recognition.

--smooth-difference=RADIUS

Smooth the difference image prior to seam-line optimization to get a shorter and – on the length scale of RADIUS – also a straighter seam-line. The default is not to smooth.

If RADIUS is larger than zero Enblend blurs the difference images of the overlap regions with a Gaussian filter having a radius of RADIUSpixels. Values of 0.5 to 1.5pixels for RADIUS are good starting points; use option --visualize to directly judge the effect.

When using this option in conjunction with  --coarse-mask=FACTOR, keep in mind that the smoothing occurs after the overlap regions have been shrunken. Thus, blurring affects a FACTOR×FACTOR times larger area in the original images.

Valid range:

$0.0\le \mathit{RADIUS}\text{.}$

--visualize[=VISUALIZE-TEMPLATE]

Create an image according to VISUALIZE-TEMPLATE that visualizes the mask optimization process. The default is vis-%n.tif.

The image shows Enblend's view of the overlap region and how it decided to route the seam line. If you are experiencing artifacts or unexpected output, it may be useful to include this visualization image in your bug report. See Bug Reports.

VISUALIZE-TEMPLATE defines a template that is expanded for each input file. In a template, a percent sign (‘%’) introduces a variable part; all other characters are copied literally. Lowercase letters refer to the name of the respective input file, whereas uppercase ones refer to the name of the output file (see Common Options). Table:mask-template-characters lists all variables.

Visualization Image The visualization image shows the symmetric difference of the pixels in the rectangular region where two images overlap. The larger the difference the lighter shade of gray it appears in the visualization image. Enblend paints the non-overlapping parts of the image pair – these are the regions where no blending occurs – in dark red. Table:visualization-colors shows the meanings of all the colors that are used in seam-line visualization images.

dark red

Non-overlapping parts of image pair.

various shades of gray

Difference of the pixel values in the overlap region.

dark blue

Location of an optimizer sample.

medium green

First sample of a line segment.

light green

Any other but first sample of a line segment.

bright cyan

State space sample inside the Dijkstra radius.

bright magenta

Non-converged point.

dark yellow

Initial seam line as generated by the NFT.

Enblend marks a non-movable (“frozen”) endpoint of a seam-line segment with a bright white cross, whereas it uses a light orange diamond to denote an endpoint that the optimizer may move around.

bright yellow

Final seam line.

Table 3.6: Colors used in seam-line visualization images.

4 Understanding Masks

A binary mask indicates for every pixel of an image if this pixel must be considered in further processing, or ignored. For a weight mask, the value of the mask determines how much the pixel contributes, zero again meaning “no contribution”.

Masks arise in two places: as part of the input files and as separate files, showing the actual pixel weights prior to image blendung or fusion. We shall explore both occurrences in the next sections.

4.1 Masks in Input Files

Each of the input files for Enfuse and Enblend can contain its own mask. Both applications interpret them as binary masks no matter how many bits per image pixel they contain.

Use ImageMagick's identify or, for TIFF files, tiffinfo to inquire quickly whether a file contains a mask. Helpful Programs shows where to find these programs on the web.

     $ identify -format "%f %m %wx%h %r %q-bit" remapped-0000.tif
     remapped-0000.tif TIFF 800x533 DirectClassRGBMatte 8-bit
                                                  ^^^^^ mask

     $ tiffinfo remapped-0000.tif
     TIFF Directory at offset 0x1a398a (1718666)
       Subfile Type: (0 = 0x0)
       Image Width: 800 Image Length: 533
       Resolution: 150, 150 pixels/inch
       Position: 0, 0
       Bits/Sample: 8
       Sample Format: unsigned integer
       Compression Scheme: PackBits
       Photometric Interpretation: RGB color
       Extra Samples: 1<unassoc-alpha>            <<<<< mask
       Orientation: row 0 top, col 0 lhs
       Samples/Pixel: 4                           <<<<< R, G, B, and mask
       Rows/Strip: 327
       Planar Configuration: single image plane

The “Matte” part of the image class and the “Extra Samples” line tell us that the file features a mask. Also, many interactive image manipulation programs show the mask as a separate channel, sometimes called “Alpha”. There, the white (high mask value) parts of the mask enable pixels and black (low mask value) parts suppress them.

The multitude of terms all describing the concept of a mask is confusing.

Mask

A mask defines a selection of pixels. A value of zero represents an unselected pixel. The maximum value (“white”) represents a selected pixel and the values between zero and the maximum are partially selected pixels. See Gimp-Savy.

Alpha Channel

The alpha channel stores the transpacency value for each pixel, typically in the range from zero to one. A value of zero means the pixel is completely transparent, thus does not contribute to the image. A value of one on the other hand means the pixel is completely opaque.

Matte

The notion “matte” as used by ImageMagick refers to an inverted alpha channel, more precisely: 1 - alpha. See ImageMagick for further explanations.

Enblend and Enfuse only consider pixels that have an associated mask value other than zero. If an input image does not have an alpha channel, Enblend warns and assumes a mask of all non-zero values, that is, it will use every pixel of the input image for fusion.

Stitchers like nona add a mask to their output images.

Sometimes it is helpful to manually modify a mask before fusion. For example to suppress unwanted objects (insects and cars come into mind) that moved across the scene during the exposures. If the masks of all input images are black at a certain position, the output image will have a hole in that position.

4.2 Weight Mask Files

...

5 Tuning Memory Usage

The default configuration of Enblend and Enfuse assumes a system with 3–4GB of RAM.

If Enblend and Enfuse have been compiled with the “image-cache” feature, they do not rely on the operating system's memory management, but use their own image cache in the file system. To find out whether your version uses the image cache say

     enblend --verbose --version

or

     enfuse --verbose --version

Enblend and Enfuse put the file that holds the image cache either in the directory pointed to by the environment variable TMPDIR, or, if the variable is not set, in directory /tmp. It is prudent to ensure write permissions and enough of free space on the volume with the cache file.

The size of the image cache is user configurable with the option ‘-m CACHE-SIZE’ (see Extended Options). Furthermore, option ‘-b BUFFER-SIZE’ (see Extended Options) allows for fine-tuning the size of a single buffer inside the image cache. Note that CACHE-SIZE is given in megabytes, whereas the unit of BUFFER-SIZE is kilobytes.

Usually the user lets the operating system take care of the memory management of all processes. However, users of Enblend or Enfuse might want to control the balance between the operating systems' Virtual Memory system and the image cache for several reasons.

• Paging in or out parts of a process' image runs at kernel level and thus can make user processes appear unresponsive or “jumpy”. The caching mechanism of Enblend and Enfuse of course runs as a user process, which is why it has less detrimental effects on the system's overall responsiveness.
• The image cache has been optimized for accesses to image data. All algorithms in Enblend and Enfuse have been carefully arranged to play nice with the image cache. An operating system's cache has no knowledge of these particular memory access patterns.
• The disk access of the operating system to the swap device has been highly optimized. Enblend and Enfuse on the other hand use the standard IO-layer, which is a much slower interface.
• Limiting the amount of image cache prevents Enblend and Enfuse from eating up most or all RAM, thereby forcing all user applications into the swap.

The CACHE-SIZE should be set in such a way as to reconcile all of the above aspects even for the biggest data sets, that is, projects with many large images.

Table:cache-size-settings suggests some cache- and buffer-sizes for different amounts of available RAM.

On systems with considerably more than 4GB of RAM it is recommended to run Enblend or Enfuse versions without image cache.

6 Helpful Additional Programs

Several programs and libraries have proven helpful when working with Enfuse and Enblend.

Raw Image Conversion

• DCRaw is a universal raw-converter written by David Coffin.
• UFRaw, a raw converter written by Udi Fuchs and based on DCRaw, adds a GUI (ufraw), versatile batch processing (ufraw-batch), and some additional features such as cropping, noise reduction with wavelets, and automatic lens error correction.

Image Alignment and Rendering

• ALE, David Hilvert's anti-lamenessing engine for the real die-hard command-line users aligns, filters, and renders images.
• Hugin is a GUI that aligns and stitches images.

  It comes with several command line tools, like nona to stitch panorama images, align_image_stack to align overlapping images for HDR or create focus stacks, and fulla to correct lens errors.

• PanoTools the successor of Helmut Dersch's original PanoTools offers a set of command-line driven applications to create panoramas. Most notable are PTOptimizer and PTmender.

Image Manipulation

• CinePaint is a branch of an early Gimp forked off at version 1.0.4. It sports much less features than the current Gimp, but offers 8bit, 16bit and 32bit color channels, HDR (for example floating-point TIFF, and OpenEXR), and a tightly integrated color management system.
• The Gimp is a general purpose image manipulation program. At the time of this writing it is still limited to images with only 8bits per channel.
• ImageMagick and its clone GraphicsMagick are general purpose command-line driven image manipulation programs, for example, convert, display, identify, and montage.

High Dynamic Range

• OpenEXR offers libraries and some programs to work with the EXR HDR format.
• PFSTools create, modify, and tonemap high-dynamic range images.

Libraries

• LibJPEG is a library for handling the JPEG (JFIF) image format.
• LibPNG is a library that handles the Portable Network Graphics (PNG) image format.
• LibTIFF offers a library and utility programs to manipulate the ubiquitous Tagged Image File Format, TIFF.

  The nifty tiffinfo command quickly inquires the properties of TIFF files.

Meta-Data Handling

• EXIFTool reads and writes EXIF meta data. In particular it copies meta data from one image to another.
• LittleCMS is the color management library used by Hugin, DCRaw, UFRaw, Enblend, and Enfuse. It supplies some binaries, too. tifficc, an ICC color profile applier, is of particular interest.

Appendix A Bug Reports

     Most of this appendix was taken from the
     Octave documentation.

Bug reports play an important role in making Enblend and Enfuse reliable and enjoyable.

When you encounter a problem, the first thing to do is to see if it is already known. On the package's SourceForge homepage click “Develop” and on the development page click “Tracker”. Search the trackers for your particular problem. If it is not known, then you should report the problem.

In order for a bug report to serve its purpose, you must include the information that makes it possible to fix the bug.

A.1 Have You Really Found a Bug?

If you are not sure whether you have found a bug, here are some guidelines:

• If Enblend or Enfuse get a fatal signal, for any options or input images, that is a bug.
• If Enblend or Enfuse produce incorrect results, for any input whatever, that is a bug.
• If Enblend or Enfuse produce an error message for valid input, that is a bug.
• If Enblend or Enfuse do not produce an error message for invalid input, that is a bug.

A.2 How to Report Bugs

The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it. Often people omit facts because they think they know what causes the problem and they conclude that some details do not matter. Play it safe and give a specific, complete example.

Keep in mind that the purpose of a bug report is to enable someone to fix the bug if it is not known. Always write your bug reports on the assumption that the bug is not known.

Try to make your bug report self-contained. If we have to ask you for more information, it is best if you include all the previous information in your response, as well as the information that was missing.

To enable someone to investigate the bug, you should include all these things:

• The exact version and configuration of Enblend or Enfuse. You can get this by running it with the options --version and --verbose.
• A complete set of input images that will reproduce the bug. Strive for a minimal set of small⁶ images.
• The type of machine you are using, and the operating system name and its version number.
• A complete list of any modifications you have made to the source. Be precise about these changes. Show a diff for them.
• Details of any other deviations from the standard procedure for installing Enblend and Enfuse.
• The exact command line you use to call Enblend or Enfuse, which then triggers the bug.

  Examples:

                ~/local/bin/enblend -v \
                    --fine-mask \
                    --optimizer-weights=3:2 --mask-vectorize=12.5% \
                    image-1.png image-2.png
  

  or:

                /local/bin/enfuse \
                    --verbose \
                    --exposure-weight=0 --saturation-weight=0 --entropy-weight=1 \
                    --gray-projector=l-star \
                    --entropy-cutoff=1.667% \
                    layer-01.ppm layer-02.ppm layer-03.ppm
  

  If you call Enblend or Enfuse from within a GUI like, for example, Hugin or KImageFuser by Harry van der Wolf, copy&paste or write down the command line that launches Enblend or Enfuse.

• A description of what behavior you observe that you believe is incorrect. For example, “The application gets a fatal signal,” or, “The output image contains black holes.”

  Of course, if the bug is that the application gets a fatal signal, then one cannot miss it. But if the bug is incorrect output, we might not notice unless it is glaringly wrong.

A.3 Sending Patches for Enblend or Enfuse

If you would like to write bug fixes or improvements for Enblend or Enfuse, that is very helpful. When you send your changes, please follow these guidelines to avoid causing extra work for us in studying the patches. If you do not follow these guidelines, your information might still be useful, but using it will take extra work.

• Send an explanation with your changes of what problem they fix or what improvement they bring about. For a bug fix, just include a copy of the bug report, and explain why the change fixes the bug.
• Always include a proper bug report for the problem you think you have fixed. We need to convince ourselves that the change is right before installing it. Even if it is right, we might have trouble judging it if we do not have a way to reproduce the problem.
• Include all the comments that are appropriate to help people reading the source in the future understand why this change was needed.
• Do not mix together changes made for different reasons. Send them individually.

  If you make two changes for separate reasons, then we might not want to install them both. We might want to install just one.

• Use the version control system to make your diffs. Prefer the unified diff format: hg diff --unified 4.
• You can increase the probability that your patch gets applied by basing it on a recent revision of the sources.

Appendix B Authors

Andrew Mihal (acmihal@users.sourceforge.net) has written Enblend and Enfuse.

Contributors

• Pablo d'Angelo (dangelo@users.sourceforge.net) added the contrast criteria.
• Joe Beda: Win32 porting up to version 3.2.
• Kornel Benko, kornelbenko@users.sourceforge.net: CMake support for version 4.0.
• Roger Goodman: Proofreading of the manuals.
• Max Lyons.
• Mark aka mjz: Win32 porting up to version 3.2.
• Thomas Modes, tmodes@users.sourceforge.net: Win32 porting of version 4.0.
• Ryan Sleevi, ryansleevi@users.sourceforge.net: Win32 porting of version 4.0.
• Christoph Spiel (cspiel@users.sourceforge.net) added the gray projectors, the LoG-based edge detection, an O(n)-algorithm for the calculation of local contrast, entropy weighting, and various other features.
• Brent Townshend, btownshend@users.sourceforge.net: HDR support.

Thanks to Simon Andriot and Pablo Joubert for suggesting the Mertens-Kautz-Van Reeth technique and the name “Enfuse”.

Appendix C GNU Free Documentation License

Version 1.2, November 2002

     Copyright © 2000, 2001, 2002 Free Software Foundation, Inc.
     51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA
     
     Everyone is permitted to copy and distribute verbatim copies
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1. PREAMBLE

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   This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

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Program Index

• ale: Helpful Programs
• align_image_stack (Hugin): Helpful Programs
• cinepaint: Helpful Programs
• cinepaint: Extended Options
• convert (ImageMagick): Helpful Programs
• dcraw: Helpful Programs
• dcraw: Workflow
• display (ImageMagick): Helpful Programs
• exiftool: Helpful Programs
• exrdisplay (OpenEXR): Helpful Programs
• fulla (Hugin): Helpful Programs
• gimp: Extended Options
• gimp: Helpful Programs
• gimp: Workflow
• gm (GraphicsMagick): Helpful Programs
• hugin: Overview
• hugin: Helpful Programs
• hugin: Extended Options
• hugin: Workflow
• identify (ImageMagick): Understanding Masks
• identify (ImageMagick): Helpful Programs
• montage (ImageMagick): Helpful Programs
• nona (Hugin): Extended Options
• nona (Hugin): Helpful Programs
• PanoTools: Overview
• PanoTools: Workflow
• pfshdrcalibrate (PFScalibration): Helpful Programs
• pfsin (PFSTools): Helpful Programs
• pfsout (PFSTools): Helpful Programs
• pfstmo_* (PFStmo): Helpful Programs
• pfsview (PFSTools): Helpful Programs
• PTmender (PanoTools): Helpful Programs
• PTOptimizer (PanoTools): Helpful Programs
• tifficc (LittleCMS): Helpful Programs
• tiffinfo (libtiff): Understanding Masks
• tiffinfo (libtiff): Helpful Programs
• ufraw: Workflow
• ufraw: Helpful Programs
• ufraw-batch: Helpful Programs

Syntactic-Comment Index

• enblend-response-file: Response Files
• enfuse-response-file: Response Files
• filename-globbing: Response Files
• glob: Response Files
• globbing: Response Files
• response-file: Response Files

Option Index

• --anneal: Mask Generation Options
• --coarse-mask: Mask Generation Options
• --compression: Common Options
• --depth: Extended Options
• --dijkstra: Mask Generation Options
• --fine-mask: Mask Generation Options
• --gpu: Extended Options
• --help: Common Options
• --levels: Common Options
• --load-masks: Mask Generation Options
• --mask-vectorize: Mask Generation Options
• --no-optimize: Mask Generation Options
• --optimize: Mask Generation Options
• --optimizer-weights: Mask Generation Options
• --output: Common Options
• --save-masks: Mask Generation Options
• --smooth-difference: Mask Generation Options
• --verbose: Common Options
• --version: Common Options
• --visualize: Mask Generation Options
• --wrap: Common Options
• -a: Common Options
• -b: Tuning Memory Usage
• -b: Extended Options
• -c: Extended Options
• -d: Extended Options
• -f: Extended Options
• -g: Extended Options
• -h: Common Options
• -l: Common Options
• -m: Tuning Memory Usage
• -m: Extended Options
• -o: Common Options
• -v: Common Options
• -V: Common Options
• -w: Common Options
• -x: Common Options

General Index

• ‘#’ (response file comment): Response Files
• 360° panoramas: Common Options
• ‘@’ (response file prefix): Response Files
• a.tif: Common Options
• affine transformation: Workflow
• algorithms, globbing: Response Files
• alpha channel: Overview
• alpha channel: Workflow
• alpha channel, associated: Extended Options
• anneal parameters: Mask Generation Options
• authors, list of: Authors
• binary mask: Understanding Masks
• bits per channel: Extended Options
• blur difference image: Mask Generation Options
• bug reports: Bug Reports
• Burt-Adelson multiresolution spline: Overview
• channel width: Extended Options
• channel, alpha: Overview
• CIECAM02: Extended Options
• coarse mask: Mask Generation Options
• color appearance model: Extended Options
• color space, sRGB: Extended Options
• colors, visualization image: Mask Generation Options
• compression: Common Options
• compression, deflate: Common Options
• compression, JPEG: Common Options
• compression, LZW: Common Options
• compression, packbits: Common Options
• conversion, raw: Workflow
• default output filename: Common Options
• deflate compression: Common Options
• Dijkstra radius: Mask Generation Options
• double precision float, IEEE754: Extended Options
• feathering, detrimental effect of: Overview
• filename, literal: Invocation
• fine mask: Mask Generation Options
• format of response file: Response Files
• free documentation license (FDL): FDL
• frozen seam-line endpoint: Mask Generation Options
• general index: General Index
• glob(7): Response Files
• globbing algorithm ‘literal’: Response Files
• globbing algorithm ‘none’: Response Files
• globbing algorithm ‘sh’: Response Files
• globbing algorithm ‘shell’: Response Files
• globbing algorithm ‘wildcard’: Response Files
• globbing algorithms: Response Files
• GNU free documentation license: FDL
• GPU (Graphics Processing Unit): Extended Options
• grammar, response file: Response Files
• grammar, syntactic comment: Response Files
• graphics processing unit: Extended Options
• half precision float, OpenEXR: Extended Options
• helpful programs: Helpful Programs
• Hugin: Bug Reports
• ICC profile: Extended Options
• IEEE754 double precision float: Extended Options
• IEEE754 single precision float: Extended Options
• image cache: Tuning Memory Usage
• image cache, block size: Extended Options
• image cache, cache size: Extended Options
• image cache, location: Tuning Memory Usage
• image colors, visualization: Mask Generation Options
• image, multi-layer: Overview
• image, visualization: Mask Generation Options
• index, general: General Index
• index, option: Option Index
• index, program: Program Index
• index, syntactic-comment: Syntactic-Comment Index
• input mask: Understanding Masks
• invocation: Invocation
• JPEG compression: Common Options
• KImageFuser: Bug Reports
• lens distortion, correction of: Workflow
• levels, pyramid: Common Options
• LibJPEG: Helpful Programs
• LibPNG: Helpful Programs
• LibTiff: Helpful Programs
• literal filename: Invocation
• load mask: Mask Generation Options
• loops in seam line: Mask Generation Options
• LZW compression: Common Options
• mask template character, ‘%’: Mask Generation Options
• mask template character, ‘b’: Mask Generation Options
• mask template character, ‘B’: Mask Generation Options
• mask template character, ‘d’: Mask Generation Options
• mask template character, ‘D’: Mask Generation Options
• mask template character, ‘e’: Mask Generation Options
• mask template character, ‘E’: Mask Generation Options
• mask template character, ‘f’: Mask Generation Options
• mask template character, ‘F’: Mask Generation Options
• mask template character, ‘i’: Mask Generation Options
• mask template character, ‘n’: Mask Generation Options
• mask template character, ‘P’: Mask Generation Options
• mask template character, ‘p’: Mask Generation Options
• mask template characters, table of: Mask Generation Options
• mask, binary: Understanding Masks
• mask, coarse: Mask Generation Options
• mask, fine: Mask Generation Options
• mask, generation: Mask Generation Options
• mask, input files: Understanding Masks
• mask, load: Mask Generation Options
• mask, optimization visualization: Mask Generation Options
• mask, save: Mask Generation Options
• mask, vectorization distance: Mask Generation Options
• mask, weight: Understanding Masks
• masks, understanding: Understanding Masks
• memory, tuning usage of: Tuning Memory Usage
• multi-directory TIFF: Overview
• multi-layer image: Overview
• nearest-feature transform (NFT): Mask Generation Options
• Octave: Bug Reports
• OpenEXR, data format: Extended Options
• OpenEXR, half precision float: Extended Options
• optimize seam: Mask Generation Options
• optimize strategy: Mask Generation Options
• optimize, anneal parameters: Mask Generation Options
• optimizer weights: Mask Generation Options
• optimizer, simulated annealing: Mask Generation Options
• option index: Option Index
• options, common: Common Options
• options, extended: Extended Options
• options, mask generation: Mask Generation Options
• order, of processing: Response Files
• output file compression: Common Options
• output filename, default: Common Options
• output image, set size of: Extended Options
• overview: Overview
• packbits compression: Common Options
• parallax error: Workflow
• photometric alignment: Workflow
• problem reports: Bug Reports
• processing order: Response Files
• profile, ICC: Extended Options
• program index: Program Index
• programs, helpful additional: Helpful Programs
• pyramid levels: Common Options
• radius, Dijkstra: Mask Generation Options
• raw conversion: Workflow
• response file: Response Files
• response file: Invocation
• response file, comment (‘#’): Response Files
• response file, force recognition of: Response Files
• response file, format: Response Files
• response file, grammar: Response Files
• response file, syntactic comment: Response Files
• save mask: Mask Generation Options
• seam line, loops: Mask Generation Options
• seam optimization: Mask Generation Options
• seam-line endpoint, frozen: Mask Generation Options
• simulated annealing optimizer: Mask Generation Options
• single precision float, IEEE754: Extended Options
• smooth difference image: Mask Generation Options
• SourceForge: Overview
• SourceForge: Bug Reports
• SourceForge, tracker: Bug Reports
• sRGB color space: Extended Options
• syntactic comment, grammar: Response Files
• syntactic comment, response file: Response Files
• syntactic-comment index: Syntactic-Comment Index
• TIFF, multi-directory: Overview
• tiffcopy: Overview
• tiffsplit: Overview
• TMPDIR: Tuning Memory Usage
• tracker, SourceForge: Bug Reports
• transformation, affine: Workflow
• understanding masks: Understanding Masks
• visualization image: Mask Generation Options
• visualization image colors: Mask Generation Options
• visualization of mask optimization: Mask Generation Options
• weight mask: Understanding Masks
• weights, optimizer: Mask Generation Options
• workflow: Workflow
• workflow with Enblend: Workflow
• workflow with Enfuse: Workflow
• wrap around: Common Options