3.1.9.1.1 Splitting and Combining Color Planes

Planar separation is a bitmap representation in which each color plane in a bitmap is sent as separate 8-bpp bitmaps. The color schemes used in RDP 6.0 Bitmap Compression are ARGB, RGB, AYCoCg, and YCoCg. These schemes have the following color planes.

Color scheme	Color planes
ARGB	Alpha, Red, Green, Blue
RGB	Red, Green, Blue
AYCoCg	Alpha, Luma, Orange Chroma, Green Chroma
YCoCg	Luma, Orange Chroma, Green Chroma

Splitting a bitmap into constituent color planes is a straightforward operation that simply involves the transfer of data into the correct color plane. For example, the 3-pixel ARGB bitmap (A1R1G1B1; A2R2G2B2; A3R3G3B3) can be split into the four color planes (A1A2A3; R1R2R3; G1G2G3; B1B2B3). In the case of 24-bpp bitmaps (or fully opaque 32-bpp bitmaps), the alpha mask for each pixel is assumed to be 0xFF, and hence, the alpha plane MAY be excluded as it is trivial to reconstitute (see the DRAW_ALLOW_SKIP_ALPHA flag in [MS-RDPBCGR] section 2.2.7.1.3).

The combination of separate color planes into a single 24-bpp or 32-bpp bitmap is also a trivial operation and merely involves reconstructing the individual ARGB or AYCoCg pixels from the constituent color planes so that the component bytes are ordered correctly. For example, the four AYCoCg planes (A1A2A3; Y1Y2Y3; Co1Co2Co3; Cg1Cg2Cg3) can be reconstituted to produce the 3-pixel (A1Y1Co1Cg1; A2Y2Co2Cg2; A3Y3Co3Cg3) bitmap.

Note that Microsoft RDP servers incorrectly convert 24 bpp RGB bitmaps to the YCoCg color space when color loss reduction (see the DRAW_ALLOW_DYNAMIC_COLOR_FIDELITY (0x02) flag in [MS-RDPBCGR] section 2.2.7.1.2) is in effect. This results in an incorrect value for the Co component. To compensate for this issue, client decoders MUST swap the B and R components after applying the inverse transformation, which converts from the YCoCg color space to the RGB color space.