FL Paste By Mask

The generic "put image B onto image A, only where the mask is white" job - a piece that shows up constantly in inpaint-and-composite workflows, region-swap tools, and anything that generates a piece separately then needs it stitched back in. FL_PasteByMask handles the resizing, blending, and feathering that job actually needs, instead of you gluing together a resize node, a mask-multiply, and an alpha composite by hand every time.

How it works

You give it a base image, an image to paste, and a mask locating where the paste should land. The interesting part is resize_behavior, because "how should the pasted image fit the masked area" has more than one reasonable answer depending on what you're doing: resize stretches the paste to exactly fill the mask's bounding box (can distort aspect ratio), keep_ratio_fill scales to cover the area while preserving aspect ratio (may crop overflow), keep_ratio_fit scales to fit inside while preserving aspect ratio (may leave gaps), source_size pastes at the paste-image's native size regardless of mask dimensions, and source_size_unmasked does the same but ignores the mask shape for sizing purposes. Picking the wrong one is the single most common way this node produces a result that looks "almost right but stretched." Once sized, blend_mode (normal through difference - nine total) controls how the paste combines with what's underneath, and feather_amount softens the mask edge so the seam isn't a hard cutout line.

The inputs and outputs that matter

• image_base, image_to_paste, mask - the three required pieces. Note the mask here is typed IMAGE, not ComfyUI's native MASK type - if you're feeding it from a standard mask output, you may need a conversion step first.
• resize_behavior - the decision that most affects how correct the final composite looks; pick based on whether preserving the paste's aspect ratio matters more than exactly filling the masked region.
• blend_mode and feather_amount (0–100) - how the edges and colors blend. Start with normal and a small feather (a handful of pixels) unless you have a specific compositing reason for something else.
• mask_mapping_optional (MASK_MAPPING) - for batch workflows where your base images and paste images aren't a clean 1:1 pairing, this lets you specify which mask/paste goes with which base index instead of assuming positional order.
• Output: IMAGE - the composited result.

How to install it

Via ComfyUI Manager: search "ComfyUI_Fill-Nodes" and install, or manually:

cd ComfyUI/custom_nodes
git clone https://github.com/filliptm/ComfyUI_Fill-Nodes

then restart ComfyUI. Standard image-compositing math, no extra dependency for this node. The pack overall is a large, general-purpose collection from filliptm, one prolific solo author, also covering PDF tools, GPT/Gemini/Fal API wrappers, and a full KSampler family; this node's needs are modest by comparison.

Common issues & troubleshooting

Pasted content looks stretched or squashed. Almost always resize_behavior set to resize on a paste image whose aspect ratio doesn't match the mask's bounding box. Switch to keep_ratio_fill or keep_ratio_fit depending on whether you'd rather crop overflow or leave a gap.

Hard, visible seam around the pasted content. Bump feather_amount up - even a small amount of edge softening does most of the work of hiding a composite seam, and it's a common thing to forget to set since it defaults to 0.

Wrong image lands on the wrong base in a batch. If you're running batches of unequal size or non-1:1 pairings, check mask_mapping_optional - without it the node assumes positional (index-matched) pairing, which silently produces wrong results if your batches aren't actually aligned that way.

Mask input rejected. Remember this node wants IMAGE-typed mask data, not ComfyUI's native MASK type - a direct wire from a mask-generating node may need a mask-to-image conversion step first.