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LockBox() with volume textures of non-power-2 dimensions

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I'm working with a GeForce 6800 Ultra and I've run into difficulties generating volume textures properly. According to the texture caps for the card, the maximum volume extent is 511 and it allows non-power-of-two sized textures. However, it seems that internally it is actually using a 512x511xDepth block of memory when I use a 511x511xDepth size texture. First, I create a 511x511xDepth texture object. Next, I use LockBox(), specifying a box and array ranks with the same dimensions. I fill this array and call UnlockBox(). When the texture is rendered, it is skewed as if each row contained 512 pixels and I had written contiguous rows of 511 (each subsequent row is offset by one less than the previous). I tried using the LockedBox out parameter in LockBox(). In my case (2 bytes per pixel), the box it returns has row pitch of 1024 (512*2) and slice pitch of 523264 (row pitch * 511). I could use that information to adjust what I lock. So the question is, how can I know what is stored before the call to LockBox()? If there is not a better way, I guess I can just call LockBox() just to get the pitch information before the texture is actually generated. Any notes or suggestions are appreciated.

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