Why does FRAGCOORD produce visible seams?

FRAGCOORD samples at pixel center (offset 0.5). Multiplying by VIEWPORT_SIZE division produces UVs aligned to pixel grid; without it you get half-pixel mismatches.

What's SCREEN_PIXEL_SIZE?

vec2(1.0/screen_w, 1.0/screen_h). Use to convert pixel offsets to UV space when sampling neighbors.

For pixel-perfect retro look, snap with floor(FRAGCOORD.xy) + 0.5. Eliminates sub-pixel filtering artifacts.

Fix: Godot Shader FRAGCOORD Half-Pixel Seam

Quick answer: Convert FRAGCOORD to UV via division by VIEWPORT_SIZE. Use SCREEN_PIXEL_SIZE for pixel-grain offsets.

Custom blur shader sampling FRAGCOORD shows seams between adjacent pixels. Half-pixel offset misaligns sampling.

The Fix

// Convert to UV correctly
vec2 uv = FRAGCOORD.xy / VIEWPORT_SIZE;

// Pixel-precise neighbor sample
vec2 px = SCREEN_PIXEL_SIZE;
vec3 n = (
    texture(SCREEN_TEXTURE, uv + vec2( px.x, 0.0)).rgb +
    texture(SCREEN_TEXTURE, uv + vec2(-px.x, 0.0)).rgb +
    texture(SCREEN_TEXTURE, uv + vec2(0.0,  px.y)).rgb +
    texture(SCREEN_TEXTURE, uv + vec2(0.0, -px.y)).rgb
) * 0.25;

VIEWPORT_SIZE division produces 0..1 UV. SCREEN_PIXEL_SIZE provides pixel-step offsets. No half-pixel surprises.

Verifying

Seams gone. 4-tap blur produces smooth output. Sample at FRAGCOORD without conversion: visible grid pattern.

“Divide by VIEWPORT. Pixel size for steps.”

Related Issues

For SCREEN_TEXTURE banding, see SCREEN_TEXTURE. For shader UV flip, see UV flip.

Convert. Pixel size. Smooth.