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• Alpha interpolation in a pixel shader

  - by c4sh

  How does the interpolation in a fragment shader work when it comes to the alpha parameter? I'm programming a shader with SharpDX, DirectX11. My idea is to interpolate 2 3d points of a segment, so that I'll have the position interpolated in between in the pixel shader. But I want to know what happens with the alpha parameter when that position is blocked by another polygon. For instance, if alpha is 1.0 at the left end of my segment and 0.0 at the other one. What is the value of alpha in the middle, 0.5? Or does it depend on the visibility at that point (meaning it could be, for instance, 1.0 OR 0.0 depending on if that part of the segment is hidden by a poolygon?

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• How can I find a position between 4 vertices in a fragment shader?

  - by c4sh

  I'm creating a shader with SharpDX (DirectX11 in C#) that takes a segment (2 points) from the output of a Vertex Shader and then passes them to a Geometry Shader, which converts this line into a rectangle (4 points) and assigns the four corners a texture coordinate. After that I want a Fragment Shader (which recieves the interpolated position and the interpolated texture coordinates) that checks the depth at the "spine of the rectangle" (that is, in the line that passes through the middle of the rectangle. The problem is I don't know how to extract the position of the corresponding fragment at the spine of the rectangle. This happens because I have the texture coordinates interpolated, but I don't know how to use them to get the fragment I want, because the coordinate system of a) the texture and b) the position of my fragment in screen space are not the same. Thanks a lot for any help.

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