- This topic has 3 replies, 2 voices, and was last updated 7 years, 6 months ago by
.
-
Posts
-
In what scenarios should you enable CIECAM02 gamut mapping and/or “exotic” rendering intents like “Luminance preserving perceptual”?
If your source material viewing conditions don’t match your actual viewing conditions, CIECAM02 appearance modeling can be used to approximate the appearance of the material under the original viewing conditions under your actual viewing conditions.
E.g. consider typical video material (or stills grabbed from video) that has usually been mastered in a “dim” environment, and you want to view it under brighter conditions, preserving the appearance. You would then (e.g.) pick “Monitor in darkened environment” as source viewing condition and “Monitor in bright environment” as target viewing condition. The overall result would be somewhat similar to lowering the viewing gamma.
Note that CIECAM02 is a pretty sophisticated model that allows tweaking quite a few parameters, most of which are not exposed in the GUI but can be altered via additional command line parameters to Argyll’s colprof and collink – you’ll have to familiarize yourself with the command line usage of these tools for more advanced usage.The rendering intents are described in the documentation under the 3D LUT section, and the description should help in picking an intent that is appropriate for your needs.
In the gear dialog, if the selected viewing conditions for target and source are the same, will the result be the same?
Second question, you mentioned some of the intents behave differently regarding the tone curve but I wasn’t sure if this effect is subtle or if some intents actually override the source tone curve.
Florian, can I ask you then, what tone curve will the color managed application use…
It solely depends on the profiles used and the rendering intent. A relative colorimetric rendering will usually reproduce the source profile tone response in the destination (i.e. the display). A perceptual rendering may do something else, e.g. CIECAM02 gamut and appearance mapping (if a profile incorporates such a mapping).
I created a synthetic profile based on the sRGB preset modified with my tone curve settings. I couldn’t find my black and white levels as cd/m2 in the log so I copied them from “report on calibrated device”. I wasn’t sure if “profile class:input device profile” was correct.
If I want to use my custom tone curve while editing, do I provide a synthetic source profile in the gear icon dialog, or do I select it as the source profile in my photo editor?
For reference I’ve compiled all descriptions of the various intents I could find.
“Absolute colorimetric with white point scaling”
behaves almost exactly like “Absolute colorimetric”, but will scale the source colorspace down to make sure the source whitepoint isn’t clipped.
…The aw intent, Absolute Colorimetric with scaling to fit white point, is very similar to the a intent, except that it will scale the source colorspace down in order to make sure that the source white point isn’t clipped by the gamut of the destination. This might be used in some print proofing situations where the source white is lightly lighter than the destination white (as an alternative to using the -w flag), or it may be useful in some soft proofing situations where the differences in white point of the display destination would cause clipping of the source white point. When the -v flag is on, the scaling factor used will be displayed during execution.
…Map absolute Jab to Jab and clip out of gamut.“Absolute colorimetric” / “White Point Matched Appearance”
is intended to reproduce colors exactly. Out of gamut colors will be clipped to the closest possible match. The destination whitepoint will be altered to match the source whitepoint if possible, which may get clipped if it is out of gamut.
…The a intent, Absolute Colorimetric, is intended to reproduce colors exactly, irrespective of the white point of the each medium. This is done using CIECAM02 Jab appearance colorspace by forcing the source and destination to have a common white point (but other aspects of the individual viewing conditions are active), and colors are mapped directly from source to destination, clipping any out of gamut colors to the closest match. This is equivalent to the ICC Absolute Colorimetric intent, and is often used for proofing purposes.
…scale source to avoid clipping the white point, and
map absolute Jab to Jab and clip out of gamut.“Absolute appearance”
maps colors from source to destination, trying to match the appearance of colors as closely as possible, but may not exactly map the whitepoint. Out of gamut colors will be clipped to the closest possible match.
…The aa intent, Absolute Appearance, simply maps the Jab colors directly from source to destination, clipping any out of gamut colors to the closest match. This attempts to match the exact appearance of colors as closely as possible, but may not exactly map the white point of the source to the destination, depending on how different the viewing conditions are.
…Map Jab to Jab and clip out of gamut.___________________________________
“Relative colorimetric”
is intended to reproduce colors exactly, but relative to the destination whitepoint which will not be altered to match the source whitepoint. Out of gamut colors will be clipped to the closest possible match. This intent is useful if you have calibrated a display to a custom whitepoint that you want to keep.
…The r intent is like Absolute Appearance mode, but maps the white point from source to destination precisely, and otherwise maps the Jab colors directly from source to destination, clipping any out of gamut colors to the closest match. This is equivalent to the ICC Relative Colorimetric intent.
…Align neutral axes and linearly map white point, then
map Jab to Jab and clip out of gamut.“Luminance matched appearance” / “Luminance axis matched Appearance”
linearly compresses or expands the luminance axis from white to black to match the source to the destination space, while not otherwise altering the gamut, clipping any out of gamut colors to the closest match. The destination whitepoint is not altered to match the source whitepoint.
…The la intent, Luminance matched appearance, linearly compresses or expands the the luminance axis from white to black to match the source to the destination space, while not otherwise altering the gamut, clipping any out of gamut colors to the closest match. This is often useful for appearance based soft proofing.
…Align neutral axes and linearly map white and black points, then
map Jab to Jab and clip out of gamut.
___________________________________
“Perceptual”
uses three-dimensional compression to make the source gamut fit within the destination gamut. As much as possible, clipping is avoided, hues and the overall appearance is maintained. The destination whitepoint is not altered to match the source whitepoint. This intent is useful if the destination gamut is smaller than the source gamut.
…The p intent, Perceptual, uses “knee” type 3 Dimensional compression to make the source gamut fit within the destination gamut. As much as possible, clipping is avoided, hues and the overall appearance is maintained. The white point is mapped precisely from source to destination.This is equivalent to the ICC Perceptual intent.
…Align neutral axes and perceptually map white and black points,
perceptually compress out of gamut and map Jab to Jab.“Perceptual appearance”
uses three-dimensional compression to make the source gamut fit within the destination gamut. As much as possible, clipping is avoided, hues and the overall appearance is maintained. The destination whitepoint is altered to match the source whitepoint. This intent is useful if the destination gamut is smaller than the source gamut.
…The pa intent, Perceptual Appearance uses “knee” type 3 Dimensional compression to make the source gamut fit within the destination gamut. As much as possible, clipping is avoided, hues and the overall appearance is maintained. The white point is not mapped from source to destination, allowing the apperance parameters to alter the chromatic mapping.“Luminance preserving perceptual” (Argyll CMS 1.8.3+)
uses compression to make the source gamut fit within the destination gamut, but very heavily weights the preservation of the luminance value of the source, which will compromise the preservation of saturation. No contrast enhancement is used if the dynamic range is reduced. This intent may be of use where preserving the tonal distinctions in images is more important than maintaining overall colorfulness or contrast.
“Preserve saturation”
uses three-dimensional compression and expansion to try and make the source gamut match the destination gamut, and also favours higher saturation over hue or lightness preservation. The destination whitepoint is not altered to match the source whitepoint.
…The lp intent, Luminance Preserving Perceptual Appearance uses compression to make the source gamut fit within the destination gamut, but very heavily weights the preservation of the Luminance value of the source, which will compromise the preservation of saturation. No contrast enhancement is used if the dynamic range is reduced. This intent may be of use where preserving the tonal distinctions in images is more important than maintaining overall colorfulness or contrast.“Preserve saturation”
uses three-dimensional compression and expansion to try and make the source gamut match the destination gamut, and also favours higher saturation over hue or lightness preservation. The destination whitepoint is not altered to match the source whitepoint.
…The ms intent, Saturation, uses 3 Dimensional compression and expansion to try and make the source gamut exactly match the destination gamut, and also favours higher saturation over hue or lightness preservation. The white point is mapped precisely from source to destination.
…Align neutral axes and perceptually map white and black points,
perceptually compress and expand to match gamuts and map
Jab to Jab.
___________________________________
“Saturation”
uses the same basic gamut mapping as “Preserve saturation”, but increases saturation slightly in highly saturated areas of the gamut.
…The s intent, Enhanced Saturation, uses the same basic gamut mapping as ms, Saturation, but increases saturation slightly in highly saturated areas of the gamut. This is equivalent to the ICC Saturation intent. The white point is mapped precisely from source to destination.
…Same as “ms” but enhance saturation.In the gear dialog, if the selected viewing conditions for target and source are the same, will the result be the same?
The same as what?
Second question, you mentioned some of the intents behave differently regarding the tone curve but I wasn’t sure if this effect is subtle or if some intents actually override the source tone curve.
There’s no overriding. First of all, only table-based (LUT) profiles support different rendering intents via different tables. It’s enough that only one profile in the chain (usually the destination profile) is table-based and has one or two tables that do gamut mapping (i.e. are not the same as the colorimetric table). Profiling software creators are free to implement these rendering intents in whichever way they like. Usually, some assumptions are made about the source, and the gamut mapping tweaked accordingly to give “pleasing” results. Argyll CMS makes no assumptions by default and thus only creates a colorimetric table, unless you define CIECAM02 parameters for the other intent(s).
If I want to use my custom tone curve while editing, do I provide a synthetic source profile in the gear icon dialog, or do I select it as the source profile in my photo editor?
In most situations that makes little sense – e.g. in a photo editing workflow, images may be in ProPhoto, AdobeRGB or sRGB already, so converting to another intermediate space before going to display would be just another unnecessary step that could impact the overall quality of the conversion, and if you want to have a CIECAM02 appearance mapping as part of e.g. the perceptual rendering, then you can use the respective image profile unaltered as source profile parameter when creating the display profile (on the command line you have even more options, i.e. you could specify an image gamut in addition – although for a general-use display profile, this may not be desirable in my opinion, because it makes the profile less versatile). Even if you develop photos yourself using a RAW converter that allows outputting the final images in arbitrary RGB spaces, it usually makes no sense to not use one of the widely used common profiles like ProPhoto, AdobeRGB, sRGB etc.
