Question by Lizfin · Jul 11, 2019 at 01:38 PM · shader graphics shader writing directx 11

Shader: Access current pixel screen position from within #ifdef SHADER_API_D3D11

Hello,

basically, what I want to do is access the current fragments screen coordinate. I know, I can do it like so uint xcoord = _ScreenParams.x * screenPos.x / screenPos.w; and similar for the y coordinate. But this does not work if I call it from within a #ifdef ... #endif block. I need the coordinate to get the right index for a StructuredBuffer. This happens inside a function call from inside the surf function.

For debugging reasons, I used screenPos.xy/screenPos.w as the red component of the object's color as I traced it down to this problem.
This works

 float bub = screenPos.x/screenPos.w;
 return bub;

but this does not

 float bub = screenPos.x/screenPos.w;
 #ifdef SHADER_API_D3D11
     bub = screenPos.x/screenPos.w;
 #endif
 return bub;

The first one produces a nice red to black gradient (as expected) but the second one produces only black.

Oddly enough, this works too (gradient)

 float bub = screenPos.x/screenPos.w;
 #ifdef SHADER_API_D3D11
     bub = screenPos.x/screenPos.w;
 #else
     bub = screenPos.x/screenPos.w;
 #endif
 return bub;

but this doesn't (black)

 float bub = screenPos.x/screenPos.w;
 #ifdef SHADER_API_D3D11
     bub = screenPos.x/screenPos.w;
 #else
     bub = 1; //this does not get executed
 #endif
 return bub;

I also tried to render the x component only and the w component only. From within the block, it gives me a nearly completely red object and black (or nearly black) object respectively. From outside it is a gradient or at least reddish color for both.
I can't wrap my head around this behaviour. It appears strange to me.

Calculating the index outside also does not work. Using a hardcoded index does, though.

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