Image Effect - Screen coordinates

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Hey everyone, I am creating an image effect and I am wondering what is the difference between those ways to calculate coordinates.

 half2 uv = MultiplyUV(UNITY_MATRIX_TEXTURE0, v.texcoord);
 o.uv = uv;
 #if UNITY_UV_STARTS_AT_TOP
     if (_MainTex_TexelSize.y < 0)
     { uv.y = 1 - uv.y; }
 #endif
 o.screenPos = uv;

and

 o.uv = v.texcoord;
 o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
 o.screenPos = ComputeScreenPos(o.pos);

Thank you !

Answer 1

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Answer by zach-r-d · Aug 12, 2015 at 03:45 PM

The first way only works in image effect/screen-space shaders, but is more accurate. Basically, the uv coordinate of a fragment in the screen-space plane will be equal to its normalized viewport coordinate. So that value can be trivially manipulated to get the screen-space coordinate (although UV_STARTS_AT_TOP needs to be accounted for). In that code sample I think to get the screen position in pixel coordinates requires the last line to be "o.screenPos = uv*_ScreenParams.xy" though.

The second way works in any kind of shader, but will not be quite as accurate, and potentially more expensive to compute. It takes a world-space position, multiplies it by the MVP matrix to get it into viewport coordinates, then translates that into pixel coordinates.

tl;dr use the first one when making an image effect, second when making a normal (object) shader.

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MaT227 · Aug 12, 2015 at 08:58 PM 0

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Thank you very much for your answer. Could you explain your last line a bit more ? o.screenPos = uv *_ScreenParams.xy;

zach-r-d · Aug 12, 2015 at 11:17 PM 1

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Sure. UV coordinates go from (0,0) to (1,1) whereas screen coordinates go from (0,0) to (screenWidth, screenHeight). So multiplying by _ScreenParams.xy is necessary to get the pixel coordinates.

MaT227 · Aug 13, 2015 at 06:40 AM 1

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Thank again !

Image Effect - Screen coordinates

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