Overdraw occurs in graphics applications where scenes are built using multiple layers of objects that overlap, and are rendered in a back-to-front order. High levels of overdraw can cause poor performance on some devices, where pixels are unnecessarily shaded multiple times.
The Mali Overdraw chart shows the number of fragments shaded per output pixel. Ideally this number should be less than 3.
This video explains the problem and provides some tips to avoid it.
Refer to our optimization advice for reducing overdraw. If the rendering order of objects and blending levels are acceptable, the next step is to check whether you have complex meshes that may result in very small triangles. With bigger triangles, Mali GPUs can use hidden surface removal (FPK) to stop rendering of objects that are behind other opaque objects. However, if triangles are too small to hit any sample points during rasterization, they can not be used as hidden surface occluders during FPK, and this can result in more overdraw. Check the FPK kill rate in the Early ZS chart to see if this number is low.
Explore overdraw further with Graphics Analyzer
Use Graphics Analyzer to explore your object geometry in more detail. You can capture the level of overdraw in a scene, and explore it to see which objects are causing the problem.