glossary: add color volumes

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

doc/glossary.md

 ---
-SPDX-FileCopyrightText: 2020 Collabora, Ltd.
+SPDX-FileCopyrightText: 2020-2021 Collabora, Ltd.
 SPDX-License-Identifier: MIT
 ---
 
@@ -29,6 +29,29 @@ color channel as well.
 A tuple of numbers, representing a color in some color system. Each
 element in the tuple is a [color channel](#color-channel) value.
 
+#### color volume
+
+A three dimensional space of [color value](#color-value)s. This
+includes both "color" (e.g. hue and saturation) and brightness
+(luminance).
+
+#### container color volume
+
+This is the color volume addressable by a given signal container format
+([pixel format](#pixel-format), encoding, defining color space). An
+example is BT.2020/PQ as is standard for driving many consumer HDR
+monitors.
+
+As a special case, the container color volume with floating-point
+[pixel format](#pixel-format)s is practically infinite, because
+negative and greater than 1.0 values can be represented. The defining
+color space and dynamic range are likely anchored to channel values 0.0
+and 1.0, but due to extrapolation the representable color volume is not
+limited by these. The target color volume needs to be specified
+separately.
+
+See also: [target color volume](#target-color-volume)
+
 #### image
 
 Conceptually a two-dimensional array of [pixel](#pixel)s. The pixels
@@ -85,6 +108,22 @@ In [DRM](#DRM) [KMS](#KMS):
 * A hardware composition element capable of holding one framebuffer
 ([image](#image)) and having position and stacking order on screen.
 
+#### target color volume
+
+The color volume intended to be usable by a given video or image
+signal. That is, the signal has been produced targeting this color
+volume in the mastering process. An example is using HDR static
+metadata (SMPTE ST 2086) to describe the image content when the
+container is BT.2020/PQ.
+
+Traditional container formats and ICC v2-v4 profiles assume that
+container color volume equals target color volume. This makes the most
+use of the pixel storage. Using target color volume smaller than the
+container color volume means that some code points will never be used,
+which is wasteful for storage.
+
+See also: [container color volume](#container-color-volume)
+
 #### Wayland
 
 A window system protocol.