Absolute luminance or bust!

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Basic Usage

Syntax: colorist convert  [input]        [output]       [OPTIONS]
        colorist identify [input]                       [OPTIONS]
        colorist generate                [output.icc]   [OPTIONS]
        colorist generate [image string] [output image] [OPTIONS]
        colorist modify   [input.icc]    [output.icc]   [OPTIONS]
        colorist calc     [image string]                [OPTIONS]

Basic Options:
    -h,--help                : Display this help
    -j,--jobs JOBS           : Number of jobs to use when working. 0 for as many as possible (default)
    -v,--verbose             : Verbose mode.
    --cmm WHICH,--cms WHICH  : Choose Color Management Module/System: auto (default), lcms, colorist (built-in, uses when possible)
    --deflum LUMINANCE       : Choose the default/fallback luminance value in nits when unspecified (default: 80)
    --hlglum LUMINANCE       : Alternative to --deflum, hlglum chooses an appropriate diffuse white for --deflum based on peak HLG lum.
                               (--hlglum and --deflum are mutually exclusive as they are two ways to set the same value.)

Input Options:
    -i,--iccin file.icc      : Override source ICC profile. default is to use embedded profile (if any), or sRGB@deflum
    --frameindex INDEX       : Choose the source frame from an image sequence (AVIF only, defaults to frame 0)

Output Profile Options:
    -o,--iccout file.icc     : Override destination ICC profile. Disables all other output profile options
    -a,--autograde           : Enable automatic color grading of max luminance and gamma (disabled by default)
    -c,--copyright COPYRIGHT : ICC profile copyright string.
    -d,--description DESC    : ICC profile description.
    -g,--gamma GAMMA         : Output gamma (transfer func). 0 for auto (default), "pq" for PQ, "hlg" for HLG, or "source" to force source gamma
    -l,--luminance LUMINANCE : ICC profile max luminance, in nits. "source" to match source lum (default), or "unspecified" not specify
    -p,--primaries PRIMARIES : Color primaries. Use builtin (bt709, bt2020, p3) or in the form: rx,ry,gx,gy,bx,by,wx,wy
    -n,--noprofile           : Do not write the converted image's profile to the output file. (all profile options still impact image conversion)

Output Format Options:
    -b,--bpc BPC             : Output bits-per-channel. 8 - 16, or 0 for auto (default)
    -f,--format FORMAT       : Output format. auto (default), avif, bmp, jpg, jp2, j2k, png, tiff, webp
    -q,--quality QUALITY     : Output quality for supported output formats. (default: 90)
    -r,--rate RATE           : Output rate for for supported output formats. If 0, codec uses -q value above instead. (default: 0)
    -t,--tonemap TM          : Set tonemapping. auto (default), on, or off. Tune with optional comma separated vals: contrast=1.0,clip=1.0,speed=1.0,power=1.0
    --yuv YUVFORMAT          : Choose yuv output format for supported formats. 444 (default), 422, 420, yv12
    --quantizer MIN,MAX      : Choose min and max quantizer values directly instead of using -q (AVIF only, 0-63 range, 0,0 is lossless)
    --tiling ROWS,COLS       : Enable tiling when encoding (AVIF only, 0-6 range, log2 based. Enables 2^ROWS rows and/or 2^COLS cols)
    --codec READ,WRITE       : Specify which internal codec to be used when decoding (AVIF only, auto,auto is default, see libavif version below for choices)
    --speed SPEED            : Specify the quality/speed tradeoff when encoding (AVIF only, [0-10] range. auto = default (let the codec decide), 0=best quality, 10=fastest)

Convert Options:
    --resize w,h,filter      : Resize dst image to WxH. Use optional filter (auto (default), box, triangle, cubic, catmullrom, mitchell, nearest)
    --rotate cwTurns         : Rotate image cwTurns clockwise
    -z,--rect,--crop x,y,w,h : Crop source image to rect (before conversion). x,y,w,h
    --composite FILENAME     : Composite FILENAME on top of input. Must be identical dimensions to input.
    --composite-gamma GAMMA  : When compositing, perform sourceover blend using this gamma (default: 2.2)
    --composite-premultiplied: When compositing, assume composite image's alpha is premultiplied (default: false)
    --composite-tonemap TM   : When compositing, determines if composite image is tonemapped before blend. auto (default), on, or off
    --composite-offset x,y   : When compositing, offsets source image onto destination image
    --hald FILENAME          : Image containing valid Hald CLUT to be used after color conversion
    --stats                  : Enable post-conversion stats (MSE, PSNR, etc)

Identify / Calc Options:
    -z,--rect x,y,w,h        : Pixels to dump. x,y,w,h
    --json                   : Output valid JSON description instead of standard log output

Modify Options:
    -s,--striptags TAG,...   : Strips ICC tags from profile


–cmm, –cms

Choose which color management module to use when performing color math (colorist/ccmm or littlecms/lcms). By default, colorist will try to use its own internal CMM whenever possible, but will fall back to LittleCMS’ conversion code if the profile contains unsupported tone curves or A2B tags, etc.

–deflum, –hlglum

There is no requirement for an ICC profile to contain a lumi tag, and in the case of its absence, colorist must internally supply a default/fallback luminance in order for any calculations to make sense. Setting --deflum allows the user to specify how bright any profiles without a lumi tag should be considered to be.

When converting to or from HLG with unspecified luminance (recommended), the current default luminance value (set with --deflum) is used for diffuse white, which maps to a 75% signal in HLG, and the max luminance is calculated with it. If you want to choose a max luminance in which to scale absolute luminance images into HLG, use --hlglum and provide the luminance you’d like a 100% HLG signal to represent (during conversion).

Example: If a display renders diffuse white at 203 nits, it will render a 75% HLG signal at 203 nits, and a 100% HLG signal at 1000 nits. Therefore, --hlglum 1000 is synonymous with --deflum 203.

-a, –autograde

Enable automatic color grading. “Grading” is currently a bit overstated, but the tool’s name is “colorist” and it is close enough to one potential goal of color grading, and leaves room for further enhancements. The general idea is to choose an “optimal” (read: better) tone curve and max luminance during the conversion process.

Turning this on and then specifying a luminance (-l) AND gamma (-g) will make this a useless switch.

-b, –bpc

Choose an output bit depth (8 - 16). By default, convert will try to use the bit depth of the source image, and generate will choose an 8-bit image. In either case, choosing an output file format incapable of 16-bit will automatically force it to 8-bit. Currently, only J2K/JP2 supports bpc (9-15).

Write a copyright into the copyright tag (cprt) of the ICC profile of any output file generated. If not used, the copyright tag will default to whatever LittleCMS uses for its default (“No copyright, use freely”).

-d, –description

Write a description into the description tag (desc) of the ICC profile of any output file generated. If not used, Colorist will make one up based on the contents of the ICC profile.

-f, –format

Force a specific output file format. Most of the time this is not required as Colorist will infer the format from the output file extension, but if you wanted to choose a nonstandard output filename, this is the switch for you.

-g, –gamma

Choose a specific gamma curve for the tone curves in the ICC profile (for all channels). Similar to -b, convert will try to use the source image’s gamma by default, and generate will use a gamma of 2.4 as it is sRGB’s gamma (very common).

-h, –help

Show the help/syntax text shown in Basic Usage, and quit.

-j, –jobs

Choose the number of threads to spawn when performing any operation that has been multithreaded, such as pixel transformations or automatic grading. By default, Colorist chooses the number of cores available in the system. Running colorist -h will show how many cores Colorist detects (and will use by default) after displaying the syntax.


When using identify or calc, this will disable all log output and instead emit a single JSON object output that contains the requested information. If an error occurs, the JSON will only contain a single key named “error”.

-l, –luminance

Set a max luminance in the lumi tag of the ICC profile, and use this max in any luminance scaling that needs to be performed. For example, if the source image’s max luminance claims to be 10,000 nits and you specify -l 300 for the output luminance, all pixels in the scene will have their luminance scaled up and either clipped or tonemapped to 300 nits (see -t).

Leveraging this option along with --deflum should allow for smooth conversions to and from relative luminance images.

-p, –primaries

Sets the color primaries for the output ICC profile, in the form rx,ry,gx,gy,bx,by,wx,wy. These values are readily available for any RGB color space (ex. Rec 709’s color space parameters). Specifying this in convert will almost certainly cause a color space conversion to occur.

There are a handful of builtin primaries for convenience:

-n, –noprofile

By default, colorist will try to write an ICC profile to the destination file, even if it is sRGB. This disables all ICC profile chunk writing.

-q, –quality

Choose a lossy quality (0-100) for any output file format that supports it (JPG, JP2 if not using -2, WebP). The lower the value, the lower the file size and quality. For WebP and JP2 (without -2), 100 is lossless.

See --quantizer for an explanation of how quality is mapped to AVIF encoding.

-r, –rate

Choose a “rate” for JP2 output compression. This effectively puts a hard ceiling on the size of the output JP2 file, and can be treated as a divisor on the input source data. For example, a 3840x2160, 16-bit image is (3840 * 2160 * 8 == 66,355,200) bytes in size (~63 MB). Specifying -2 200 will make the output file size roughly (66,355,200 / 200 = 331776) bytes, or 324 KB.

The JP2 compressor will do everything it can to blur/ruin your image if you don’t give it enough breathing room, but it will hit your requested rate. Also, be sure to experiment to make sure my math isn’t really wrong.


Resizes the destination image during conversion. -r expects 1-2 numbers, either comma separated or in the form WxH, representing the destination dimensions. If either of the dimensions is absent, colorist will use the aspect ratio of the source image to choose an appropriate value for the other dimension.

A third, optional argument (separated from the dimensions with a comma) can specify the filter to be used when resizing. This is the list of valid filter values (explanations here taken directly from STB library’s image_resize):

If unspecified or auto (default), colorist will use catmullrom for scaling up, and mitchell for scaling down.

-t, –tonemap

Forces tonemapping to be on or off. When scaling from a large luminance range down to a smaller range, any values that are too bright will not “fit” in the new range. With tonemapping disabled, those pixels will simply be clipped to the max value, but any pixels that do fit in the smaller range will be untouched. With tonemapping enabled, the entire image is subtly darkened to make room for a a bit of extra granularity in the top of the smaller range, which is used to distinguish super bright pixels. This spares having large white blobs of no definition from being in the converted image at the cost of a bit of darkening.

Automatic grading will automatically turn this off if you allow it to choose a max luminance, as it will never choose a luminance that will cause a pixel to clip. Use this switch (-t off) to achieve this with a manually specified max luminance.


Choose yuv output format for supported formats. auto (default), 444, 422, 420, yv12

–quantizer MIN,MAX

Quantizer Ranges: 0-63

The quality emitted by AVIF encoder is controlled by two quantizer values (a minimum and maximum). The higher these numbers are, the worse the image quality will be. For example, choosing [0,0] will create a lossless AVIF, and choosing [63,63] will encode something that barely looks like the original image.

If this option isn’t used, colorist will map the single -q quality value to these, by slowly ramping up the maximum quantizer first as you turn down the quality from 100, until it caps out at 63 (e.g. Q=99 will have a max quantizer of 1). The min quantizer will begin to ramp up from 0 in the 60s until it hits 63 right at Q=1, making Q=1 have quantizer settings of [63,63].

Due to how the encoder uses these values, there can be a bit of a plateau in the Q30-Q50 range, but dual ramp provides a reasonable single dial for quality. Q100->Q60 has a reasonable descent in quality, and Q30->Q1 really trashes the image.

Use this option if you want to specify your own min/max quantizers instead.


AVIF only. Enables tiling when encoding, and is log2 based (as these values are simply passed through to the encoder and it requests it as such).

Example: --tiling 2,3 will create 4 rows and 8 columns during encoding.

-v, –verbose

Verbose mode. Ironically, that’s it for this one.

-z, –rect, –crop

When using identify, it will dump the basic information about the image such as the dimensions, bit depth, and embedded ICC profile. By default, it also dumps the colors of a handful of pixels from the upper left corner. If you want to choose an alternate rectangle for that pixel information, use this. Choosing a width and height of 0 will disable pixel dumping during identify (-z 0,0,0,0).

When using convert, it will crop the source image (prior to conversion) to the requested rect.

–composite, –composite-gamma, –composite-premultiplied, –composite-tonemap

After converting the source image to the destination profile, but before writing it out to disk, this will read in a second image file and composite it on top of the image. The composite image is first converted into the same destination profile, then the two images are blended in a blend-friendly gamma space (chosen by --composite-gamma). By default, the composite image is assumed to not be premultiplied alpha, but it can be enabled with --composite-premultiplied. If the composite image is a larger luminance range than the destination profile’s max luminance, how tonemapping should behave can be adjusted with --composite-tonemap.

Currently, this command requires that the composite image is the already same dimensions as the destination (post-conversion/resize) image.


A Hald CLUT is a 3D color lookup table; effectively a 3D texture (a cube of colors) laid out correctly in a regular image. Nobody seems to know where the name “Hald” came from.

When using this option, just before writing to disk, colorist will “look up” every pixel’s final raw value in the Hald and replace it with the interpolated value sampled from it.

Image Strings

The generate command offers a means to create basic test images, using an “image string” as its input. The general idea of an image string is to describe a big list of colors along with (optional) image dimensions, and to spread those colors across the image as evenly as possible. The goal is to be able to make simple test images as tersely as possible, but if you’re stubborn enough, you can probably make almost anything.

As a basic reminder, this is the form of the command:

colorist generate "#ff0000" image.png

In the above commandline "#ff0000" is the image string. Whether or not the quotes are necessary is up to your shell, but it good practice to use them. It specifies a single color in the typical web color format used in CSS, one of many ways to specify a color. Since this is the only information in the image string, Colorist will count a total of 1 color in the color list, and will select a 1x1 16-bit image to house this single color. Any of the other options specified above will inform these decisions, such as making an 8-bit image with -b 8.

Let’s add some more information:

colorist generate "1024x1024,#ff0000" image.png

Colorist will count a total of 1 color in the color list, and instead of choosing a 1x1 image to spread the colors across, it’ll honor the 1024x1024 dimensions request, so it’ll just fill the whole image with red.

Quick tips:

How about a second color?

colorist generate "1024x1024,#ff0000,#00ff00" image.png

This creates the same 1024x1024 image, but it shares the pixels evenly across the two colors (red for the first half, then green). Reordering or adding colors will continue the pattern.

What if I wanted 75% red and 25% green?

colorist generate "1024x1024,#ff0000,x3,#00ff00" image.png

The x3 in there says to pretend that the previous statement was listed three times in a row. Colorist will then count a total of four colors, and will spread those four colors across the requested image size.

How about a gradient?

colorist generate "1024x1024,#ff0000..#000000" image.png

The .. syntax creates an interpolation (gradient) between the two colors. Colorist will count 256 images in the color list, and since there are more than 256 pixels in the requested image, it will spread those evenly across the image, creating the gradient.

How about two gradients?

colorist generate "1024x1024,#ff0000..#000000,#000000..#00ff00" image.png

Colorist counts 512 colors here, and spreads them accordingly. What if I wanted my red gradient to take up 75%, and the green one only 25%? Let’s try it.

colorist generate "1024x1024,#ff0000..#000000,x3,#000000..#00ff00" image.png

Oops! x3 is making 3 red-to-black gradients, followed by 1 red-to-black gradient. I didn’t want to repeat my gradient, I wanted to stretch my gradient. We can achieve this specifying the color range, such as:

colorist generate "1024x1024,#ff0000.768.#000000,#000000..#00ff00" image.png

How about a vertical gradient? This is where basic rotation comes into play.

colorist generate "1024x1024,#ff0000..#000000,cw" image.png

This rotates the image clockwise once after generating it. You can use multiple cw or ccw statements if you want, but ultimately their rotations will all be summed up and potentially cancelled out, and a single rotation will be performed at the end.

How about some color squares for testing?

colorist generate "400x100,#ff0000,#00ff00,#0000ff,#ffffff" image.png

A lot of the trick with using image strings is describing a color count that corresponds evenly with the pixel count of the requested image dimension. Try to make the color list’s count to be a factor of the pixel count for best results.

There are many ways to define a color. For example, simply parenthesizing some numbers works!

colorist generate "1024x1024,(255,0,0),(0,255,0)" image.png

This should recreate our red/green image from earlier. How about a gradient?

colorist generate "1024x1024,(255,0,0)..(0,0,0)" image.png

Another red/green version:

colorist generate "1024x1024,rgb(255,0,0)..#000000" image.png


colorist generate "1024x1024,rgb16(65535,0,0)..#000000" image.png

All possible color formats:

#ffffff                               // 8-bit
(255,0,0)                             // 8-bit
rgb(255,0,0)                          // 8-bit
rgba(255,0,0,255)                     // 8-bit
rgb16(65535,0,0)                      // 16-bit
rgba16(65535,0,0,65535)               // 8-bit
f(1.0,0,0)                            // 32-bit, can't be used in a gradient
float(1.0,0,0)                        // 32-bit, can't be used in a gradient
XYZ(0.385127, 0.716909, 0.0970615)    // 32-bit, can't be used in a gradient
xyY(0.321181, 0.597874, 0.716909)     // 32-bit, can't be used in a gradient

Still want more? Read the Cookbook!