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Quickman · Jul 12, 2006

Ex-Cyber said:

As for the curved quad hack, can you summarize what the actual deal is with regard to how/why it works? I've heard several descriptions of it and none of them square with my understanding of VDP1. Frequently I see it described as "non-coplanar" quads, but this makes no sense to me (isn't VDP1 purely 2D, or is this referring to modeling technique?). I had assumed it was a matter of it not being able to generate correct scanlines for concave quads and basically skewing that portion of the quad; I never wrote a software renderer, but my limited understanding is that it's simple to scale from width A to width B along one dimension of the polygon and to reflect that scaling along a line of symmetry if necessary, but that anything more complex than that quickly makes the algorithm hairy. However, my reading on this was quite triangle-centric, which leaves out concavity entirely. Any chance you can enlighten me?
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I'm not Christina, but I'll have a go.

The VDP1 is, as you say, primarily a sprite engine (hence the reliance on quads - a three-sided sprite doesn't happen) and is a logical extension of the previous 2D graphics chips like the VDP in the Megadrive. It has a concept of "deformed sprites" which is effectively a quad (the SDK we all know and love to hate translates the coordinates into settings for the VDP1 to handle in drawing the quad). However, when the sprite itself has to cover a bend (a concave quad) it breaks down - it's not clever enough to resolve it as two triangles or any other workaround because it can't do texture mapping, so it tries its best (at this point my knowledge runs thin) and the result is this curved effect which is used.

I'm probably extremely wrong in that. =P

SANiK · Jul 14, 2006

Quickman's extremely right in that. =P

Quick "tutorial" that I drew up that covers it:

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