DirectX 11 Direct Texel Mapping

Question

I am trying to render to a texture in DirectX 11 (C++ API) using an orthographic projection matrix. In the horizontal dimension, I am taking advantage of the "projection" that occurs, but in the vertical dimension, I am wanting my vertices to map directly to a specific texel row.

I have created a texture that is 128x128. I set it as my render target and set up a viewport as follows:

D3D11_VIEWPORT textureViewport;
textureViewport.TopLeftX = 0.0f;
textureViewport.TopLeftY = 0.0f;
textureViewport.Height = 128.0f;
textureViewport.Width = 128.0f;
textureViewport.MinDepth = 0.0f;
textureViewport.MaxDepth = 1.0f;

I provide a projection matrix created like this:

XMMATRIX projection = XMMatrixOrthographicOffCenterLH(50.0f, 250.0f, 128.0f, 0.0f, 256.0f, 0.0f);

Notice my left and right values do not match my texture width, but the bottom to top values do match the height. With this I would expect that if my vertex shader transforms a coordinate with a Y-value of 0.0f using the above projection matrix for SV_Position, it would be drawn to the top row of the texture. A coordinate with a Y-value of 127.0f would be drawn to the bottom row of the texture.

However, after giving it data that I would expect to fill the entire texture (I'm drawing many vertices as a point list), the bottom row of my texture is blank. And I notice the top row of my texture doesn't appear to contain the vertices I expect it to. It seems like everything is shifted up a row (or a partial row).

I have read the page on the DirectX 10 Coordinate System, but I must not be fully understanding this. I thought this was telling me that I don't have to offset anything by 0.5 like you would have to in DirectX 9. But maybe I have that backwards?

What's the proper way to control what texel row my vertices get drawn to?

I don't know if this is any consequence, but I will note that I am not ever rendering this texture to the screen. I am saving the texture to an image file.

Tim Coolman Tim Coolman · Accepted Answer · 2014-05-27T17:15:11

I have solved this problem. After rereading the article I linked in my question, I realized I had previously misunderstood the coordinate system. If I modify my orthographic matrix as follows, then my vertices are transformed to land right on the center of texel rows rather than on the border between texel rows. All data is now being drawn to the texture as desired, and I no longer have the issue of the blank bottom row.

XMMATRIX projection = XMMatrixOrthographicOffCenterLH(50.0f, 250.0f, 127.5f, -0.5f, 256.0f, 0.0f);

DirectX 11 Direct Texel Mapping

1 Answers