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Processing order of vertices and fragments in arm gpu

Shawn Chang
When the ARM GPU processes vertices and fragments, in what order are they distributed and processed? Vertices are in index order? What about fragment? Can we try to control the order of the index on the cpu side to improve the efficiency of the GPU?
For example: Does the optimization of the link below improve the gpu of arm? If there is an improvement, what features of the GPUs they use? github.com/.../meshoptimizer
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  • 0 in reply to Peter Harris
    Peter Harris said:
    Older Mali GPUs (Utgard, Midgard) render vertices in incrementing index order, between min and max referenced index, ignoring the actual order in the index buffer. 

    In relatively new GPUs, calculations are done in the order of indexbuff. The vertices shared by multiple triangles may be calculated multiple times. What about these old GPUs you mentioned? Will there be multiple calculations?
    For example, drawing two triangles with 0,1,2 and 2,1,3 in indexbuff. When the GPU is calculating, will the vertices of the two indexes of 1 and 2 be executed twice?

    In other words, older Mali GPUs (Utgard, Midgard) simply render all vertices once in incrementing index order, between min and max referenced index?

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  • 0 in reply to Peter Harris
    Peter Harris said:
    Older Mali GPUs (Utgard, Midgard) render vertices in incrementing index order, between min and max referenced index, ignoring the actual order in the index buffer. 

    In relatively new GPUs, calculations are done in the order of indexbuff. The vertices shared by multiple triangles may be calculated multiple times. What about these old GPUs you mentioned? Will there be multiple calculations?
    For example, drawing two triangles with 0,1,2 and 2,1,3 in indexbuff. When the GPU is calculating, will the vertices of the two indexes of 1 and 2 be executed twice?

    In other words, older Mali GPUs (Utgard, Midgard) simply render all vertices once in incrementing index order, between min and max referenced index?

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  • +1 Arm Employee Badge in reply to Shawn Chang

    In relatively new GPUs, calculations are done in the order of indexbuff. The vertices shared by multiple triangles may be calculated multiple times.

    Yes, may be shaded multiple times but there is post-transform caching, so it will only happen if you have bad locality in your index buffers. In general we don't see much reshading for sensibly ordered meshes.

    What about these old GPUs you mentioned? Will there be multiple calculations? In other words, o lder Mali GPUs (Utgard, Midgard) simply render all vertices once in incrementing index order, between min and max referenced index?

    It's guaranteed not to reshade, but may shade non-referenced vertices between min and max (various optimizations exists to minimize this, but YMMV). It's therefore recommended to tightly pack used indices - no unreferenced vertex between min and max.

    HTH, 
    Pete