avutil/pixfmt: improve definition of AVColorRange

As it was brought up that the current documentation leaves things
as specific to YCbCr only, ICtCp and RGB are now mentioned.
Additionally, the specifications on which these definitions of
narrow and full range are defined are mentioned.

This way, the documentation of AVColorRange should now match how
most people seem to read interpret it at this point, and thus
flagging RGB AVFrames as full range is valid not only according to
common sense, but also the enum definition.

libavutil/pixfmt.h

@@ -530,12 +530,60 @@ enum AVColorSpace {
 };
 
 /**
- * MPEG vs JPEG YUV range.
+ * Visual content value range.
+ *
+ * These values are based on definitions that can be found in multiple
+ * specifications, such as ITU-T BT.709 (3.4 - Quantization of RGB, luminance
+ * and colour-difference signals), ITU-T BT.2020 (Table 5 - Digital
+ * Representation) as well as ITU-T BT.2100 (Table 9 - Digital 10- and 12-bit
+ * integer representation). At the time of writing, the BT.2100 one is
+ * recommended, as it also defines the full range representation.
+ *
+ * Common definitions:
+ *   - For RGB and luminance planes such as Y in YCbCr and I in ICtCp,
+ *     'E' is the original value in range of 0.0 to 1.0.
+ *   - For chrominance planes such as Cb,Cr and Ct,Cp, 'E' is the original
+ *     value in range of -0.5 to 0.5.
+ *   - 'n' is the output bit depth.
+ *   - For additional definitions such as rounding and clipping to valid n
+ *     bit unsigned integer range, please refer to BT.2100 (Table 9).
  */
 enum AVColorRange {
     AVCOL_RANGE_UNSPECIFIED = 0,
-    AVCOL_RANGE_MPEG        = 1, ///< the normal 219*2^(n-8) "MPEG" YUV ranges
-    AVCOL_RANGE_JPEG        = 2, ///< the normal     2^n-1   "JPEG" YUV ranges
+
+    /**
+     * Narrow or limited range content.
+     *
+     * - For luminance planes:
+     *
+     *       (219 * E + 16) * 2^(n-8)
+     *
+     *   F.ex. the range of 16-235 for 8 bits
+     *
+     * - For chrominance planes:
+     *
+     *       (224 * E + 128) * 2^(n-8)
+     *
+     *   F.ex. the range of 16-240 for 8 bits
+     */
+    AVCOL_RANGE_MPEG        = 1,
+
+    /**
+     * Full range content.
+     *
+     * - For RGB and luminance planes:
+     *
+     *       (2^n - 1) * E
+     *
+     *   F.ex. the range of 0-255 for 8 bits
+     *
+     * - For chrominance planes:
+     *
+     *       (2^n - 1) * E + 2^(n - 1)
+     *
+     *   F.ex. the range of 1-255 for 8 bits
+     */
+    AVCOL_RANGE_JPEG        = 2,
     AVCOL_RANGE_NB               ///< Not part of ABI
 };