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diff --git a/doc/CrushedDisplyBlacks.html b/doc/CrushedDisplyBlacks.html new file mode 100644 index 0000000..3038384 --- /dev/null +++ b/doc/CrushedDisplyBlacks.html @@ -0,0 +1,85 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html> +<head> + <title>Crushed Display Blacks</title> + <meta http-equiv="content-type" + content="text/html; charset=ISO-8859-1"> + <meta content="Graeme Gill" name="author"> +</head> +<body> +<h2 style="text-decoration: underline; font-weight: bold;">Crushed +Display Blacks<br> +</h2> +Often people create a display profile, and then notice that when they +try and display some images using the profile, that the darkest blacks +in the image all get crushed into the black of the display. Why does +this happen ?<br> +<br> +There are many reasons this may happen, but here is a common one:<br> +<br> +The image has blacks that are darker than the black of the display, and +the color management intent being used clips out of gamut colors. So +all the blacks that are darker than the display black get mapped to the +display black. To avoid this, some sort of <span + style="font-weight: bold;">Gamut Mapping</span> that maps that black +of the source image to the black of the display, while preserving the +distinction between all the rest of the colors needs to be used.<br> +<br> +Some popular synthetic colorspaces have a perfect (and unrealistic) +zero black, for instance <span style="font-weight: bold;">sRGB</span> +and <span style="font-weight: bold;">AdobeRGB</span>. Real world +display profiles have non-zero blacks, so transforming between these +two using a colorimetric intent will clip the blacks, and loose the +shadow details.<br> +<br> +<h4 style="text-decoration: underline;">What performs gamut mapping ?</h4> +Typically there are only two mechanisms available to perform gamut +mapping. The main one is a pre-cooked (static) gamut mapping built into +cLUT type ICC profiles. The second is an on-the-fly (dynamic) gamut +mapping performed by the CMM (Color Management Module). A limited form +of the latter is Adobe BPC (Black point compensation), which is also +available sometimes with applications or systems that use lcms. (Little +cms).<br> +<br> +<h4 style="text-decoration: underline;">How do I fix it ?</h4> +There are two ways of avoiding the black crush. One is to turn on BPC +if it is available in the system you are using. Sometimes it may only +be available for certain intents.<br> +<br> +The second way of fixing it is to create your display profile with +appropriate gamut mapping, and make sure that it gets used.<br> +<br> +Shaper/Matrix type ICC profiles do not support gamut mapping, since +there is only one transformation in them and it does not have the +necessary flexibility to incorporate gamut mapping. Shaper/Matrix +profiles are always colorimetric intent. So it is necessary to create a +cLUT based Display profile if gamut mapping is to be incorporated into +the profile. (Note that not all systems accept cLUT based Display +profiles). Creating cLUT profiles that incorporate appropriate gamut +mapping depends on the profile creation tools, and not all tools give +adequate control over gamut mapping to reliably fix this problem.<br> +<br> +<h4 style="text-decoration: underline;">OK, so how do I fix it using +Argyll ?</h4> +You can usually fix this problem using Argyll by simply creating a cLUT +based profile (the default), and telling colprof what the source +colorspace is going to be.<br> +<br> +i.e. say your source images are in sRGB space, then:<br> +<br> + colprof -v -S sRGB.icm -D "My +Display" MyDisplayProfile +<br> +<br> +[It's usually safer to use the sRGB profile provided by Argyll than use +an sRGB profile of unknown origin. Find it in the ref directory.]<br> +<br> +This will create 3 separate B2A cLUT tables, one for colorimetric +intent, one for Perceptual intent, and one for Saturation intent. Both +Perceptual and Saturation tables will have appropriate gamut mapping +for a source colorspace of sRGB. So it is just a matter of making sure +that either Perceptual or Saturation intent is used when making use of +the display profile.<br> +<br> +</body> +</html> |