GdkRGB is a small module of GDK that can render RGB images to any of the possible X visual formats. For example, it will automatically do color reduction and dithering when rendering to an 8-bit pseudocolor visual, or it will use the original data when rendering to a 24-bit truecolor visual.
GdkRGB achieves very high-quality output by using a color cube for color reduction, and a large error diffusion matrix. It can also do dithering for 16-bit truecolor modes, which makes them look almost as good as full 24-bit truecolor visuals. Also, GdkRGB can perform operations on several pixels in parallel, thus increasing performance.
The GNOME Canvas uses GdkRGB to render the final RGB buffers to the screen, thus achieving high quality and high performance.