pixel formats: Documents the internal representation

This adds a comment to cogl-types.h where we define all the
CoglPixelFormat enums to give lots of information about the internal
representation of the format and to explain how new formats should be
allocated.

This information came from the discussion in bug #660188

Reviewed-by: Neil Roberts <neil@linux.intel.com>

cogl/cogl-types.h

@@ -168,6 +168,72 @@ typedef struct _CoglTextureVertex       CoglTextureVertex;
 #define COGL_AFIRST_BIT         (1 << 6)
 #define COGL_PREMULT_BIT        (1 << 7)
 
+/* XXX: Notes to those adding new formats here...
+ *
+ * First this diagram outlines how we allocate the 32bits of a
+ * CoglPixelFormat currently...
+ *
+ *                             4 bits for flags
+ *                             |--|
+ *  enum        unused             4 bits for the bytes-per-pixel
+ *                                 and component alignment info
+ *  |------| |---------------|     |--|
+ *  00000000 xxxxxxxx xxxxxxxx PFBA0000
+ *                             ^ premult
+ *                              ^ alpha first
+ *                               ^ bgr order
+ *                                ^ has alpha
+ *
+ * The most awkward part about the formats is how we use the last 4
+ * bits to encode the bytes per pixel and component alignment
+ * information. Ideally we should have had 3 bits for the bpp and a
+ * flag for alignment but we didn't plan for that in advance so we
+ * instead use a small lookup table to query the bpp and whether the
+ * components are byte aligned or not.
+ *
+ * The mapping is the following (see discussion on bug #660188):
+ *
+ * 0     = undefined
+ * 1, 8  = 1 bpp (e.g. A_8, G_8)
+ * 2     = 3 bpp, aligned (e.g. 888)
+ * 3     = 4 bpp, aligned (e.g. 8888)
+ * 4-6   = 2 bpp, not aligned (e.g. 565, 4444, 5551)
+ * 7     = YUV: undefined bpp, undefined alignment
+ * 9     = 2 bpp, aligned
+ * 10    = undefined
+ * 11    = undefined
+ * 12    = 3 bpp, not aligned
+ * 13    = 4 bpp, not aligned (e.g. 2101010)
+ * 14-15 = undefined
+ *
+ * Note: the gap at 10-11 is just because we wanted to maintain that
+ * all non-aligned formats have the third bit set in case that's
+ * useful later.
+ *
+ * Since we don't want to waste bits adding more and more flags, we'd
+ * like to see most new pixel formats that can't be represented
+ * uniquely with the existing flags in the least significant byte
+ * simply be enumerated with sequential values in the most significant
+ * enum byte.
+ *
+ * Note: Cogl avoids exposing any padded XRGB or RGBX formats and
+ * instead we leave it up to applications to decided whether they
+ * consider the A component as padding or valid data. We shouldn't
+ * change this policy without good reasoning.
+ *
+ * So to add a new format:
+ * 1) Use the mapping table above to figure out what to but in
+ *    the lowest nibble.
+ * 2) OR in the COGL_PREMULT_BIT, COGL_AFIRST_BIT, COGL_A_BIT and
+ *    COGL_BGR_BIT flags as appropriate.
+ * 3) If the result is not yet unique then also combine with an
+ *    increment of the last sequence number in the most significant
+ *    byte.
+ *
+ * The last sequence number used was 0 (i.e. no formats currently need
+ *                                      a sequence number)
+ * Update this note whenever a new sequence number is used.
+ */
 /**
  * CoglPixelFormat:
  * @COGL_PIXEL_FORMAT_ANY: Any format
@@ -198,19 +264,20 @@ typedef struct _CoglTextureVertex       CoglTextureVertex;
  * @COGL_PIXEL_FORMAT_ARGB_2101010_PRE: Premultiplied ARGB, 32 bits, 10 bpc
  * @COGL_PIXEL_FORMAT_ABGR_2101010_PRE: Premultiplied ABGR, 32 bits, 10 bpc
  *
- * Pixel formats used by COGL. For the formats with a byte per
+ * Pixel formats used by Cogl. For the formats with a byte per
  * component, the order of the components specify the order in
  * increasing memory addresses. So for example
  * %COGL_PIXEL_FORMAT_RGB_888 would have the red component in the
  * lowest address, green in the next address and blue after that
- * regardless of the endinanness of the system.
+ * regardless of the endianness of the system.
  *
- * For the 16-bit formats the component order specifies the order
+ * For the formats with non byte aligned components the component
- * within a 16-bit number from most significant bit to least
+ * order specifies the order within a 16-bit or 32-bit number from
- * significant. So for %COGL_PIXEL_FORMAT_RGB_565, the red component
+ * most significant bit to least significant. So for
- * would be in bits 11-15, the green component would be in 6-11 and
+ * %COGL_PIXEL_FORMAT_RGB_565, the red component would be in bits
- * the blue component would be in 1-5. Therefore the order in memory
+ * 11-15, the green component would be in 6-11 and the blue component
- * depends on the endianness of the system.
+ * would be in 1-5. Therefore the order in memory depends on the
+ * endianness of the system.
  *
  * When uploading a texture %COGL_PIXEL_FORMAT_ANY can be used as the
  * internal format. Cogl will try to pick the best format to use