Graphics processing systems

The present disclosure relates to a graphics processors and graphics processing systems. In the graphics processor, the rasterizer may operate to identify pairs of fragments for a primitive being rendered for which not all the sampling positions in the fragments are covered by the primitive. When the fragments reach the fragment shader, corresponding execution threads may be spawned for execution by the fragment shader to process the fragments. A first part of the fragment shader program that uses the helper threads of the thread groups may then be executed. There may then be a merge instruction in the fragment shader program which operates to cause the active threads of the thread groups to be merged into a single, combined thread group. Following this thread group merger, the remaining program steps of the fragment shader program may be executed for the merged thread group.