Intel Iris Gallium3D Picks Up Conservative Rasterization Support

On top of Intel's new open-source OpenGL driver seeing some hefty performance optimizations, the Iris Gallium3D driver has picked up another OpenGL extension ahead of the Mesa 19.1 branching.

Iris Gallium3D now supports INTEL_conservative_rasterization alongside the existing support in the i965 driver. INTEL_conservative_rasterization is the several year old Intel extension for seeing if all fragments are at least partially covered by a polygon rather than the default rasterization mode of including fragments with at least one sample covered by a polygon.

Regular rasterization includes fragments with at least one sample covered by a polygon. Conservative rasterization includes all fragments that are at least partially covered by the polygon.

In some use cases, it is also important to know if a fragment is fully covered by a polygon, i.e. if all parts of the fragment are within the polygon. An application may, for example, want to process fully covered fragments different from the "edge" pixels. This extension adds an option for the fragment shader to receive this information via gl_SampleMaskIn[].

This extension affects only polygons in FILL mode and specifically does not imply any changes in processing of lines or points.

Conservative rasterization automatically disables polygon antialiasing rasterization if enabled by POLYGON_SMOOTH.

The support was merged on Tuesday for this driver although simple in nature thanks to previous Intel conservative rasterization work having done all the heavy-lifting that was re-used by this new Gallium3D driver.

This Intel Gallium3D driver will be making its maiden voyage with Mesa 19.1 due for release at the end of May. However, the classic "i965" OpenGL driver will remain the default until further notice.