ComfyUI_UltimateSGUpscale/docs/plans/2026-02-25-differential-diffusion-seam-fix-design.md

Differential Diffusion Seam Fix

Problem

The current seam fix pass uses binary masks (1.0/0.0) with SetLatentNoiseMask. This creates hard transitions at band edges that can themselves become visible artifacts. Differential diffusion allows gradient masks where the value controls per-pixel denoise intensity, producing smoother seam repairs.

Design

GenerateSeamMask Node Changes

Add a mode combo input:

• binary (default): Current behavior. Output is 1.0 inside seam bands, 0.0 outside.
• gradient: Linear falloff from 1.0 at seam center to 0.0 at band edge. Value at distance d from center: max(0, 1.0 - d / half_w). Where horizontal and vertical bands overlap (grid intersections), take max of both values.

The seam_width parameter keeps the same meaning in both modes.

Workflow Changes

Add one DifferentialDiffusion node (node 24) inside the Seam Fix group. It wraps the model before it reaches the seam fix KSampler:

• Checkpoint → DifferentialDiffusion → Seam Fix KSampler (replaces direct Checkpoint → KSampler link)
• All other wiring unchanged. SetLatentNoiseMask still passes the mask to the latent.

Tests

• Existing binary tests pass with explicit mode="binary"
• Gradient tests: center=1.0, edge=0.0, midpoint~0.5, intersection uses max