1. A method of performing coverage merging in a shading stage of a graphics system, the method comprising: performing a draw call on primitives and rasterizing the primitives into blocks of pixels; selecting the draw call for merge testing of individual blocks; performing a depth test on the individual blocks; in response to the depth test being satisfied, merging partially covered fragments of the same draw call of one of the blocks of pixels to form a merged block of pixels; and performing shading of the merged block of pixels on a draw call basis.