Image Effect - Screen coordinates

Questions & Answers
1

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 !

2

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.