% Use a uniform to prevent the compiler from optimizing. [pixel shader] uniform float i; float4 main() : SV_TARGET { float4 color = float4(0.5, 0.4, 0.3, 0.2); color.g = color[i]; color.b = 0.8; return color; } [test] uniform 0 float 2 todo draw quad probe all rgba (0.5, 0.3, 0.8, 0.2)