Deferred rendering inaccurate UV coordinate

[ottoville]

When I was writing a program inspired by deferred rendering sample, I noticed that current UV mapping algorithm have minimum resolution of 2x2 pixels.

Here is how to see prove that issue:
When adding following lines at the end of "fragmentDeferredRendering.wgsl" before return statement:

    let fragCoordI=vec2<i32>(round(coord.xy));
    if(fragCoordI.x > 500) {
      result=vec3<f32>(1.0,0.0,0.0);  //red
    }
    if(fragCoordI.x > 501) {
      result=vec3<f32>(0.0,1.0,0.0);  //green
    }
    if(fragCoordI.x > 502) {
      result=vec3<f32>(0.0,0.0,1.0); //blue
    }
    if(fragCoordI.x > 503) {
      result=vec3<f32>(1.0,1.0,0.0); //yellow
    }
    if(fragCoordI.x > 504) {
      result=vec3<f32>(0.0,1.0,1.0); //cyan
    }
    if(fragCoordI.x > 505) {
      result=vec3<f32>(1.0,0.0,1.0); //pink
    }

Expected result is 1px thin red, green, blue, yellow and cyan line to be drawn and right side of image to be pink.
Here is a screenshot from actual results:

When zooming to very close, it is visible that only green and yellow line is visible, both being 2px wide:

The 2x2 pixel resolution is also clearly visible when looking aliasing of edges of the squares in the sample page

[kainino0x]

@shrekshao

[shrekshao]

Seems to be round is behaving incorrectly in chrome canary win
Here's 4x4 texture attachment with image-rendering: pixelated; for canvas css style
in fragmentDeferredRendering.wgsl I added

var cc:vec2<i32> = vec2<i32>(floor(coord.xy));
//var cc:vec2<i32> = vec2<i32>(round(coord.xy));
var colors = array<vec3<f32>, 8>(
      vec3<f32>(1.0, 0.0, 0.0),
      vec3<f32>(1.0, 0.5, 0.0),
      vec3<f32>(1.0, 1.0, 0.0),
      vec3<f32>(0.0, 1.0, 0.0),
      vec3<f32>(0.0, 1.0, 1.0),
      vec3<f32>(0.0, 0.5, 1.0),
      vec3<f32>(0.0, 0.0, 1.0),
      vec3<f32>(0.5, 0.0, 1.0)
);
result = colors[(cc.x + cc.y) % 8];

var cc:vec2<i32> = vec2<i32>(floor(coord.xy));

var cc:vec2<i32> = vec2<i32>(round(coord.xy));

I will try testing on other platform, come up with a minimal test and report to tint (likely a tint bug)

Thanks for the report!

[shrekshao]

Filed at https://bugs.chromium.org/p/tint/issues/detail?id=1089