How You Fail To Implement Skinning

Chances are, as a graphics programmer, you’ve spent some time implementing some engine and shader code that does vertex skinning. I don’t mean anything clever, none of your dual or more quaternion methods, just a bunch of weighted matrix * vertex transforms indexing some kind of palette. Like many “simple” graphics tasks, writing skinning from scratch is one of those things that despite its simplicity, is a complete bug factory – see also, writing a GL application from scratch, rendering offscreen the right way up, etc.

In the spirit of OpenGL: Why Is Your Code Producing a Black Window?, I thought it would be good to make a list of possible mistakes and how to recognize them. Even though you may have implemented it before, it’s possible with all of this spare compute power, that you may be re-implementing it again soon.

Personally I don’t think these kinds of mistakes are a sign of a bad graphics programmer – I’m sure it happens to the best. However, I think how you approach investigating the bug and how quickly you are able to fix the mistake is more indicative of experience with the GPU tools available and problem solving. I’d even say that it could make a good interview test (if you’re inclined to do those things) – something like “@InternDept is stuck implementing their own skinning – can you help them?” and before breaking out the tools, you could even show pictures of what the result currently looks like and have the interviewee guess.

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