BD Mask Flatten

BD Mask Flatten takes an image with transparency and lays it flat onto a background, so downstream nodes get a plain RGB image and never have to think about alpha. It sounds trivial - every image editor has "flatten" - but it's one of the workhorse nodes of this pack's game-asset prep, because it handles the three things that always complicate a real flatten: the background, the composite math, and the dreaded edge bleed.

How it works

The composite math is standard alpha blending: image × alpha + background × (1 − alpha). Semi-transparent pixels become background-blended rather than cut, which is exactly what you want when the result might later be chroma-keyed - percentage-based recovery only works if the semi-transparency was preserved, not snapped to 0 or 1. The output is always RGB (no alpha channel), which is the point: downstream nodes like BD Pack Channels or save nodes won't trip over unexpected transparency.

The mask input is optional but overrides everything: if you pass one, it becomes the alpha. If you don't, the node uses the image's own alpha channel (or treats RGB input as fully opaque). That's a flexible design - you can flatten a mask you got from SAM3 onto any background even if the source image had no alpha at all.

The inputs that matter

• background_color - white/black by default, but the interesting ones are the chroma keys (red/green/blue/magenta) and checker for debug, plus custom with a hex color. Game pipelines flatten onto a key color when they want to re-extract later.
• flatten_mode - the power move. Beyond alpha_composite there's grayscale (luminance, then composite) and image_to_red / image_to_green / image_to_blue, which pack the image's grayscale into a single RGB channel with the background filling the others. That's how you build engine-packed maps (e.g. AO in red, roughness in green, metallic in blue) without leaving ComfyUI.
• edge_pad_pixels - the game-dev trick worth the most. It runs a Voronoi edge pad: foreground RGB extends outward into transparent areas by N pixels using the nearest foreground color. This is the standard "alpha bleed" that prevents the UV-edge halos you get when a game engine samples a texture at a silhouette boundary and hits transparent black. 16–32 is typical for game textures; the pad happens before flattening, so the background only shows beyond the padded zone.
• mask_threshold - >0 binarizes the alpha (hard edges) instead of the default soft continuous alpha. Use it when you want crisp edges, not partial transparency.

You also get mask_strength (multiply alpha before compositing) and invert_mask (flatten the outside instead of the inside). Outputs are the flattened image and alpha_used - the actual alpha that went in, for debugging or reuse.

Where it fits

The README's Mask Tools template chains it with BD Luminance Mask, crop-to-mask, and fill-holes; the game-packing template uses it before channel packing. It's the "make this presentable" node: whatever weird masked thing you're holding, this turns it into a clean flattened texture that exports without surprises.

Installing

Same as every node here - ComfyUI Manager, search "BrainDead", install; or clone into custom_nodes and pip install -r requirements.txt. Restart, and it's under 🧠BrainDead/Segmentation. Keep ComfyUI current (V3 API pack).

The habit worth forming: if your flattened textures show dark fringes in-engine, it's almost certainly missing edge_pad_pixels, not a wrong background. Set it before you export, not after you see the halo.