diff --git a/internal/shaderir/program.go b/internal/shaderir/program.go
index d53414332..b5deea34d 100644
--- a/internal/shaderir/program.go
+++ b/internal/shaderir/program.go
@@ -45,8 +45,9 @@ type VertexFunc struct {
 	Block Block
 }
 
-// FragmentFunc takes pseudo in-params, and the number is len(varyings) + 1.
-// The last param represents the coordinate of the fragment (gl_FragCoord in GLSL)
+// FragmentFunc takes pseudo in-params, and the number is len(varyings) + 2.
+// If index == len(varyings), the param represents the coordinate of the fragment (gl_FragCoord in GLSL).
+// If index == len(varyings)+1, the param is an out-param representing the color of the pixel (gl_FragColor in GLSL).
 type FragmentFunc struct {
 	Block Block
 }