Add documentation on recommend channel swizzles
* the following config settings:
*
* max(config.v_rgba_radius, config.a_scale_radius)
+ *
+ * Common compressor usage
+ * =======================
+ *
+ * One of the most important things for coding image quality is to align the
+ * input data channel count with the ASTC color endpoint mode. This avoids
+ * wasting bits encoding channels you don't need in the endpoint colors.
+ *
+ * | Input data | Encoding swizzle | Sampling swizzle |
+ * | ---------- | ---------------- | ---------------- |
+ * | 1 channel | RRR1 | .g |
+ * | 2 channels | RRRG | .ga |
+ * | 3 channels | RGB1 | .rgb |
+ * | 4 channels | RGBA | .rgba |
+ *
+ * The 1 and 2 channel modes recommend sampling from "g" to recover the
+ * luminance value as this provide best compatibility with ETC1. For ETC the
+ * luminance data will be stored as RGB565 where the green channel has the
+ * best quality. For ASTC any of the rgb channels can be used - the same data
+ * will be returned for all three.
+ *
+ * When using the normal map compression mode ASTC will store normals as a two
+ * channel X+Y map. Input images must contain unit-length normalized and should
+ * be passed in using the RRRG swizzle. The Z component can be programatically
+ * recovered in shader code, using knowledge that the vector is unit length.
*/
#ifndef ASTCENC_INCLUDED
