Why do I have to switch T(v) texture coordinates while importing OpenGL to Direct3D?

Question

I am importing my code from OpenGL to Direct3D. My D3DTS_PROJECTION uses D3DXMatrixPerspectiveFovRH, and my D3DTS_VIEW uses D3DXMatrixLookAtRH to set a view equal to OpenGL's view. My question is why do I have to switch all of my 1.0000 Tex(v) texture coordinates to "minus value" in D3D to get equal texture mapping as in OpenGL.

OpenGL:

//T(u)   T(v)
1.0000,  1.0000,
0.0000,  1.0000, 
1.0000,  0.0000,
1.0000,  0.0000,
0.0000,  1.0000, 
0.0000,  0.0000,     

0.0000,  1.0000,
0.0000,  0.0000,
1.0000,  1.0000,
1.0000,  1.0000,
0.0000,  0.0000,
1.0000,  0.0000,
// ... 

Direct3D:

//           T(u)     T(v)
D3DXVECTOR2( 1.0000f, -1.0000f)
D3DXVECTOR2( 0.0000f, -1.0000f)
D3DXVECTOR2( 1.0000f,  0.0000f)
D3DXVECTOR2( 1.0000f,  0.0000f)
D3DXVECTOR2( 0.0000f, -1.0000f)
D3DXVECTOR2( 0.0000f,  0.0000f)

D3DXVECTOR2( 0.0000f, -1.0000f) 
D3DXVECTOR2( 0.0000f,  0.0000f) 
D3DXVECTOR2( 1.0000f, -1.0000f) 
D3DXVECTOR2( 1.0000f, -1.0000f) 
D3DXVECTOR2( 0.0000f,  0.0000f)
D3DXVECTOR2( 1.0000f,  0.0000f)
// ... 

Is it because Direct3D's origin (0.0) of texture coordinates lies in different place than in OpenGL?

Answer 1

For texture spaces, in Direct3D (0, 0) is top-left, in OpenGL (0, 0) is bottom-left. Therefore the v-coordinate will be upside down in one of these APIs.

However, I wouldn't recommend negating the v-coordinate as this will only work if you're using a sampler with wrapping.

You can fix the v-coordinate as follows:

v = 1.0f - v;