Why do dFdx/dFdy give wrong results inside an if?

GPU derivatives need data from neighboring pixels. Inside a divergent branch (some pixels take, some don't), neighbors may not have computed the same value. Hoist the derivative call outside the branch.

What's the right pattern for AA edges?

Compute the value v unconditionally, take fwidth(v), then smoothstep(0 - fw, 0 + fw, v) for an anti-aliased threshold. No branches, derivatives valid everywhere.

Are texture samples affected?

Yes — texture sampling uses derivatives for mipmap selection. A texture call inside an if can produce wrong mip. Use textureGrad or textureLod to specify mip explicitly when branching.

Fix: Godot 4 Shader Conditional dFdx dFdy Banding

Quick answer: Compute derivative inputs unconditionally; branch only on the final value. Use fwidth(v) + smoothstep for AA edges. For texture sampling inside conditionals, use textureLod with explicit mip.

A signed-distance icon shader uses dFdx inside an if to anti-alias an edge. The AA bands or breaks because the derivative was wrong on the divergent side.

The Symptom

Visible banding at edges, especially near pixel-boundary thresholds. Smooth gradient shows discontinuities.

The Fix

// Bad — dFdx inside divergent branch
if (some_condition) {
    float w = fwidth(uv.x);
    color = smoothstep(0.5 - w, 0.5 + w, uv.x);
}

// Good — derivative computed unconditionally
float w = fwidth(uv.x);
float aa = smoothstep(0.5 - w, 0.5 + w, uv.x);
if (some_condition) {
    color = vec4(aa);
}

Compute fwidth at the top of fragment, branch on the use later. Derivatives valid for every pixel.

Texture Mip in Branches

vec4 c = textureLod(tex, uv, 0.0);   // explicit mip 0
if (cond) ALBEDO = c.rgb;

textureLod skips automatic mip selection (which depends on derivatives), making sampling safe inside conditionals.

Verifying

Render the edge close to the camera. AA should be smooth. With derivatives in branches: bands. Hoist; bands disappear.

“Derivative outside branches. Smoothstep on the result. Edges anti-alias.”

Related Issues

For shader uniform array, see uniform array. For shader uniform runtime, see runtime uniform.

Hoist derivatives. Smooth edges.