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>> In
>> practise every modeller and renderer will convert it to two triangles, so
>> I
>> guess that can be thought of the "standard" way.
>
> It might be the "standard" way, but it produces an incorrect texturing
> (with non-rectangular quads).
More specifically, when trying to map a rectangular section of a texture
onto a non-rectangular quad in model-space. In practise this rarely happens
because you usually match your texture coordinates to the mesh (so that the
texture does not get distorted) - ie your 2 and 3 in texture space would be
moved to match how they look in model space.
In screen-space, when quads pretty much always become distorted and
non-rectangular due to perspective, the GPU accounts for this by
interpolating the texture coordinates divided by w (the homogenous
coordinate) to ensure the problem you illustrated in the other reply is
avoided.
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