This is an interesting and subtle point about doing "software on GPU compute." You are in complete control over what gets computed, and are not at the mercy of

This is an interesting and subtle point about doing "software on GPU compute." You are in complete control over what gets computed, and are not at the mercy of the hardware's fixed function pipeline for stuff like rasterization rules and sampling patterns for antialiasing. So I think portable results pixel to pixel are in fact viable. Of course CPU rendering is always more straightforward than GPU, the higher performance comes at a significant cost in complexity.