3.1.9.1.3 Chroma Subsampling and Super-Sampling

Chroma subsampling is a compression technique that employs a lower resolution for representing the chroma planes (Co and Cg) while keeping the luminosity plane (Y) at full resolution. This method relies on the human eye being less sensitive to color than differences in luminosity. Support for chroma subsampling is advertised in the Bitmap Capability Set (see [MS-RDPBCGR] section 2.2.7.1.3).

The subsampling algorithm used employs a simple averaging of four neighboring pixels in a 2-by-2 grid. This reduces the information needed to be sent by half in each dimension, resulting in a four-fold decrease for a two-dimensional bitmap per chroma channel. If an odd number of rows or columns is present, the subsampling is performed on a 2-by-1 or 1-by-2 grid, or not at all for a corner when the bitmap has both an odd number of rows and columns.

Super-sampling of a compressed chroma plane simply results in the expansion of a subsampled pixel into a 2-by-2 grid of pixels. Because this expansion results in an area with the same dimensions as the original bitmap, odd row and column counts imply that edge pixels will be expanded into either a 2-by-1, 1-by-2 or 1-by-1 grid of pixels.

The following figure shows averaging (first stage) and final subsampling (second stage) being applied to a chroma plane of a 3-by-3 bitmap as well as the super-sampling required to reconstruct the chroma plane.

Figure 14: Chroma subsampling and super-sampling