transparent-bg

Transparent background

The problem (see research 13 + 04)

Almost every modern T2I VAE is RGB-only. Asking Imagen 3/4 or Gemini 2.5/3 Flash Image for a "transparent logo" triggers one of two failures:

1. Flat background — DALL·E 3, Stable Diffusion families render a white/gray background that must be matted.
2. Checkerboard as RGB pixels — Gemini/Imagen literally render the gray-and-white 8×8 tile pattern (because Photoshop screenshots in their training data show transparency that way). This is the "weird boxes" users report. It cannot be fixed by prompting.

Fix hierarchy (apply first that fits)

1. Route to a native-RGBA provider

Provider	Mechanism
gpt-image-1 / gpt-image-1.5	API param background: "transparent" + output_format: "png" or "webp"
Ideogram 3 Turbo	Dedicated /ideogram-v3/generate-transparent endpoint; set rendering_speed: "TURBO" for Turbo tier
Recraft V3	native SVG output (alpha is trivial); rasterize if raster needed
LayerDiffuse on SDXL / Flux	In-diffusion-loop transparency adapter; better edges than post-matte

2. Post-process matte (for everything else)

Matting model	License	Strength
BiRefNet	MIT	Default choice 2026; best soft-edge handling
BRIA RMBG-2.0	CC-BY-NC-4.0 (hosted API for commercial)	Best overall quality
U²-Net	Apache-2.0	Legacy fallback
rembg	wrapper around U²-Net / BiRefNet	Easy CLI integration
SAM 2	Apache-2.0	For complex/multi-object scenes; 2-stage (segment → matte)

3. Difference matting (for semi-transparency: glass, hair, smoke)

1. Generate the same subject on pure-white background.
2. Generate again with same seed on pure-black background.
3. alpha = (white_version - black_version + 255) / 2 in luminance.
4. Works for true semi-transparent objects where rembg/BiRefNet produce hard-cutout artifacts.

4. Vectorize-and-drop (for flat marks)

1. Generate raster at 1024² on white background.
2. BiRefNet matte → K-means quantize to N colors → vtracer or potrace.
3. SVG has alpha trivially. Rasterize at target sizes with @resvg/resvg-js for RGBA PNGs.

Validation (never ship without)

Tier-0 alpha validator (see packages/mcp-server/src/pipeline/validate.ts):

1. Check PNG/WebP header has alpha channel type (RGBA, not RGB).
2. Reject if no pixels have alpha < 255 (it's opaque).
3. Reject if >5% of pixels have alpha ∈ [0.05, 0.95] AND FFT signature shows gray-tile band frequency (Gemini fake checkerboard).
4. Alpha coverage: subject pixels (alpha > 0.5) should occupy 30–85% of frame (sanity check).
5. Premultiplied-alpha check: no RGB values where alpha==0 (cleanup artifacts).

Prompt scaffold for fallback matting flow

Do not ask for transparency in the prompt. Ask for pure white:

[SUBJECT, concrete].
Centered, isolated, no surrounding context or props.
Solid pure white #FFFFFF background.
Clean silhouette with distinct outline.
No drop shadow, no ground plane, no reflection.
1:1 square, 1024x1024.

Then post-process with BiRefNet / BRIA.

Never do this

• "transparent background" as a prompt to Imagen or Gemini (checkerboard result).
• Trust output_format: "png" alone to produce alpha (PNG supports alpha, but the model has to emit it).
• Algorithmic #FFFFFF → alpha=0 thresholding (loses anti-aliasing, creates jagged edges).
• Post-resize RGBA with non-premultiplied alpha (dark fringes).

Output

transparent/
├── mark.png         # RGBA, validated
├── mark@2x.png      # optional high-DPI
├── mark.svg         # if vector path chosen
└── meta.json        # alpha_coverage, matting_method, validation_hash