>> 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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