MESA_tile_raster_order Added To The OpenGL Registry

The new OpenGL extension MESA_tile_raster_order proposed by Eric Anholt at Broadcom has now been merged to the Khronos registry.

The MESA_tile_raster_order extension was developed by Anholt while working on the VC4 driver stack for the Raspberry Pi. The purpose of MESA_tile_raster_order is for boosting the performance of uncomposited X11 when using GLAMOR.

The new specification explains, "This extension extends the sampling-from-the-framebuffer behavior provided by GL_ARB_texture_barrier to allow setting the rasterization order of the scene, so that overlapping blits can be implemented. This can be used for scrolling or window movement within in 2D scenes, without first copying to a temporary."

Eric does have patches pending for the VC4 Gallium3D driver. He explained with that work, "This extension came from trying to get vc4's window movement in X11 to be as fast as it used to be with software rendering. Without the extension, overlapping copies need to first get read to a temporary, while with the extension the vc4 driver can be forced to rasterize the tiles in a specific order so that overlapping blits are safe."