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1px Bordered Quad

Started by dr4cula Aug 1, 2014 at 10:09 AM 1 reply 1.2k views

Original Post

Aug 1, 2014 at 10:09 AM

Hello,

I'm having a bit of trouble rendering a quad with exactly 1px border but can't figure out for the life of me how to do it. I'm using basic vertex and pixel shaders where the position I pass in is in clip space. I thought this would do the trick:


float dx = 1.0f / static_cast<float>(width_);
float dy = 1.0f / static_cast<float>(height_);
 
D3DXVECTOR2 coords[6] = {D3DXVECTOR2(-1.0f + dx, -1.0f + dy), D3DXVECTOR2(-1.0f + dx, 1.0f - dy), D3DXVECTOR2(1.0f - dx, 1.0f - dy),
D3DXVECTOR2(-1.0f + dx, -1.0f + dy), D3DXVECTOR2(1.0f - dx, 1.0f - dy), D3DXVECTOR2(1.0f - dx, -1.0f + dy)};

When I use 2.0f * dx or 2.0f * dy I get a 2px border everywhere. Which I guess means my method should be correct? I'm really confused...

Thanks in advance!

Erik Rufelt

189

Erik Rufelt

Aug 1, 2014 at 11:38 AM

Probably the rasterization rules that only draw pixels where the center is inside it (or perhaps the edges, check the rasterization rules for your DX version). Try offsetting all coords a half pixel (dx and dy are half pixel offsets). As of now you begin your line on a pixel edge, and include half a pixel in each direction, so you include two halves of two different adjacent pixels, instead of including one full pixel.

When you do 2.0f / width then you include one full pixel in each direction, which includes the pixel centers of the adjacent pixels in both directions.

dr4cula

Author

dr4cula

Aug 1, 2014 at 1:46 PM

Probably the rasterization rules that only draw pixels where the center is inside it (or perhaps the edges, check the rasterization rules for your DX version). Try offsetting all coords a half pixel (dx and dy are half pixel offsets). As of now you begin your line on a pixel edge, and include half a pixel in each direction, so you include two halves of two different adjacent pixels, instead of including one full pixel.

When you do 2.0f / width then you include one full pixel in each direction, which includes the pixel centers of the adjacent pixels in both directions.

Yeah, I figured that it had something to do with the triangle rasterization rules but didn't really know how to fix the issue. Shifting all coordinates by an extra 1.0f / (dimLength * 2.0f) solved the issue. Thanks!

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