Re: [Algorithms] negative UV coords

[Jon W. <jw...@gm...>]

Jason Hughes wrote:
> In this one specific case, where the U or V has been negated (but not 
> both), and the texture addressing mode is set to wrap, yes the texture 
> is mirrored.

That is only true when you have MIRROR texture addressing mode turned 
on. This is easy to verify in your modeling tool, which assumes WRAP: 
the texture is tiled continuously in all directions, including the 
negative directions.

However, the texture MAP may be mirrored on the SCREEN if your texture 
coordinate gradients don't match your texture memory lay-out and 
triangle mapping. Most people think that in Direct3D, V goes down-to-up, 
and in OpenGL, V goes up-to-down. However, that's just an artifact of 
D3D typically loading BMP textures, and OpenGL typically loading JPG or 
PNG textures (in the samples that are around). The actual definition is 
simple: V grows in the direction of the data you pass to the texture. If 
you pass data with the bottom-most scanline first, V grows upwards. If 
you pass data with the topmost scanline first, V grows downwards.

Sincerely,

jw