A graphics processing pipeline includes a fragment shader 8 and can carry out blending either by means of a blend shading software routine in a blend shader 10, or by using fixed function, dedicated processing hardware blending units 12. When the programmable blend shading path using the blend shader 10 is selected, the blend shader 10 is configured to provide the alpha value 110 generated by the fragment shading operation as an output to the multisample coverage operations unit 105 and the alpha test unit 106 of the fixed function hardware unit pipeline. This allows the alpha test on the alpha values generated by the fragment shader 8 still to be performed using the multisample coverage operations hardware unit 105 and the alpha test hardware unit 106, instead of having to emulate those operations in the blend shading software routine in the blend shader 10.