glossary: add gamut and tone mapping

These definitions are from my own head and BT.2408.

Fixes: #38



Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

doc/glossary.md

@@ -119,6 +119,19 @@ color volume needs to be specified separately.
 See also: [target color volume](#target-color-volume),
 [diagrams][plots:color-volumes]
 
+#### gamut mapping
+
+Gamut mapping refers to a color transformation that fits an input
+[color volume](#color-volume) into or onto a destination color volume by e.g.
+expansion, contraction, shifting the white point, or clipping.
+
+Gamut mapping tends to modify the [colorimetry](#colorimetry) of an image.
+One possible goal is to keep the image details visible and appearance similar
+to those of the image in its mastering environment. Other goals might include
+producing a gray-scale image, or arbitrary white-balance adjustment.
+
+Gamut mapping may be a part of [color re-rendering](#color-re-rendering).
+
 #### image
 
 Conceptually a two-dimensional array of [pixel](#pixel)s. The pixels may be
@@ -126,6 +139,11 @@ stored in one or more [memory buffer](#memory-buffer)s. Has width and height in
 pixels, a [pixel format](#pixel-format) and a [modifier](#modifier) (implicit
 or explicit).
 
+#### inverse tone mapping ([BT.2408][bt2408])
+
+[Color re-rendering](#color-re-rendering) by expanding the dynamic range,
+for converting SDR content to HDR content for example.
+
 #### linear values, light-linear
 
 See [tristimulus](#tristimulus-values).
@@ -149,6 +167,16 @@ An absolute unit of luminance: cd/m², candelas per square meter. Candela is a
 measure of luminous intensity, accounting for the human eye sensitivity to
 light.
 
+#### picture formation
+
+Picture formation is the act of processing data to become a picture.
+
+A physics-based rendering of a scene is not a usable picture as is. Likewise
+RAW images from digital cameras are not usable pictures. These data sets relate
+to the measured or simulated physical radiometric power. They need to be
+developed to become displayable pictures that depict what and how they are
+intended to.
+
 #### pitch
 
 See [stride](#stride).
@@ -226,6 +254,24 @@ some code points will never be used, which is wasteful for storage.
 See also: [container color volume](#container-color-volume),
 [diagrams][plots:color-volumes]
 
+#### tone mapping ([BT.2408][bt2408])
+
+[Color re-rendering](#color-re-rendering) by compressing the dynamic range,
+for converting HDR content to SDR content for example.
+
+#### tone mapping (in games)
+
+Tone mapping in the context of game rendering engines tends to refer to
+[picture formation](#picture-formation).
+
+#### tone mapping (color re-rendering)
+
+Tone mapping is similar to [gamut mapping](#gamut-mapping), except tone mapping
+tends to emphasize dynamic range adjustment particularly in a HDR context,
+while gamut mapping is a term used more in pre-HDR contexts.
+
+Tone mapping may be a part of [color re-rendering](#color-re-rendering).
+
 #### trichromatic system
 
 A trichromatic system defines the three primary colors and their relative
@@ -333,3 +379,4 @@ Apple
 [e-ILV:colour stimulus]: https://cie.co.at/eilvterm/17-23-002
 [plots:color-volumes]: ../plots/index.md#primary_container_target_color_volumespy
 [drm_fourcc.h]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/drm/drm_fourcc.h?h=v6.0#n362
+[bt2408]: https://www.itu.int/pub/R-REP-BT.2408 "Report ITU-R BT.2408-7 - Guidance for operational practices in HDR television production"