Overview

The main work was fixing the refine flow. When users erased a region with SAM2 and clicked restore, the restored area came back as a white box instead of the pre-erase RGBA. Root cause: rembg_orig was not being populated under the new pipeline. After fixing the root cause I ran a repo-wide rename from rembg to matte to clean up the vocabulary. In parallel I debugged a RunPod Docker image pull-denied failure and locked down the Google-login + SQLite audit-log architecture spec. Three sessions, five commits, 3h 54m total.

Previous post: popcon Dev Log #8

graph TD
    A["Restore clicked in Refine UI"] --> B{"Source of original RGBA?"}
    B -->|"Legacy pipeline"| C["Copy from rembg_orig (white pixels if missing)"]
    B -->|"New, post-c192347"| D["Copy from pristine BiRefNet backup"]
    D --> E["Alpha-preserving restore"]
    C --> F["White-box bug"]
    F --> G["Root cause: new path never wrote rembg_orig"]

Why SAM2 Restore Left White Boxes

The opening observation in session 0 was: “before this, only the white background got removed and all effects/elements were preserved, now the effects disappear.” Clicking restore in Refine after SAM2 erase produced a visible white box even when the mask was clearly correct.

Following the backend code, backend/main.py:211 had this restore path:

# Restore logic (before fix)
for path in mask_paths:
    src = rembg_orig_dir / path.name   # original RGBA should live here
    if not src.exists():
        # fallback: copy pixels from the white-background image
        src = rembg_dir / path.name
    shutil.copy(src, restore_dst)

The problem was that the rembg_orig/ directory was empty for a lot of jobs. Two paths led there:

• Legacy jobs. The code that populated rembg_orig was still uncommitted (git status showed M backend/main.py). Every job currently sitting in /tmp/popcon/jobs/ had been processed without it.
• New-pipeline jobs. The GPU worker swapped rembg for BiRefNet in commit 5af85f2, but the backend kept writing into a directory still named rembg/, and the restore logic kept reading rembg_orig/.

So the contract “rembg_orig is where we back up the original RGBA” was being honored only on some paths. The restore depended on that contract, and when the contract was broken, the fallback grabbed the white-background image and copied its pixels — the symptom.

The Fix — Pristine BiRefNet Backup

c192347 fix(refine): hybrid SAM restore + pristine BiRefNet backup made two changes:

1. Change the restore source from rembg_orig to the pristine BiRefNet output. The RGBA that BiRefNet produces already has the correct alpha, so it’s always a valid “pre-erase” source for a frame.
2. Move the save step to right after the BiRefNet call. Instead of copying the original in front of SAM2’s erase, the BiRefNet stage writes both the working file and the backup. The restore path reads the backup.

Frontend frontend/app/refine/page.tsx, frontend/components/RembgRefineCanvas.tsx, the GPU worker’s birefnet_service.py and sam_service.py moved together. The Refine canvas now restores to the saved pristine state, and SAM2 no longer needs to stash the pre-erase frame in a separate directory.

rembg → matte Rename

After the restore fix, the user made the right catch: “wait, didn’t we already swap from rembg to BiRefNet for background removal?” Yes — commit 5af85f2 did that inside the GPU worker, but the backend kept rembg as the name everywhere:

• Directory: frames/{emoji}/rembg/ — contained BiRefNet output but was named rembg.
• Endpoint: POST /api/emoji/{id}/rembg-apply — function and route both said rembg.
• Frontend component: RembgRefineCanvas.
• API client: rembgApply(), rembgFrames.
• Types: RembgRefineCanvasProps.

A vocabulary mismatch costs two things. First, a new contributor reads “rembg” and thinks of the rembg Python package, building a wrong mental model. Second, when the backend wants to swap models again (to ToonOut for anime, say), the name has to move again.

9e8d27c refactor: rename rembg to matte across the background-removal pipeline renamed 10 files at once — backend, GPU worker, frontend. matte is a model-independent term — the VFX-industry standard word for the alpha mask produced by background removal. Swap BiRefNet for ToonOut or u2net later, and this name doesn’t change. The frontend gained MatteRefineCanvas and deleted RembgRefineCanvas in the same commit. backend/scripts/migrate_rembg_to_matte.py handles the one-time on-disk layout migration for existing jobs.

RunPod Docker Image Pull Denied

Session 1 was a separate chase. A RunPod worker was stuck on “image pull: wildboar7693/popcon-gpu-worker” forever. The real error in the logs was:

error pulling image: Error response from daemon: pull access denied for
wildboar7693/popcon-gpu-worker, repository does not exist or may require
'docker login': denied: requested access to the resource is denied

RunPod had no Docker Hub credentials registered, and the image was private. The Docker daemon kept retrying the auth-denied pull — from the RunPod UI this looked like “pending” but internally it was a failing loop. Immediate fix: flip the image to public and kill the stuck workers. Longer term: wrote a short markdown guide on how to keep the image private using RunPod’s Docker Registry Credential feature.

A supply issue was tangled in — the banner “Supply of your primary GPU choice is currently low” was also up, with 12 jobs queued. The two are unrelated. Added extra regions to fix supply; flipping the image to public fixed the auth issue.

Action-Specific Start Frame Prompts + 24-Emoji Cap

The third theme was a lighter feature. 41aea71 feat: action-specific start frame prompts + cap emoji sets at 24 did:

• Split the start-frame prompt per action. Until now start-frame generation used one generalized prompt for every action. Per-action presets in backend/presets.py now steer an “angry” frame into angry-specific face/pose directives.
• Cap emoji sets at 24. The action-selector UI previously accepted any count, and large sets would time out deep in the pipeline. Hard cap applied on both frontend and backend.

frontend/components/ActionSelector.tsx and CharacterUpload.tsx visualize the cap; backend/pipeline/start_frame_gen.py consumes the preset dictionary.

Google Login + User Logs Design Spec

The last theme is documentation, not code. 0aaae34 docs(spec): Google login + user logs design landed docs/superpowers/specs/2026-04-17-google-login-user-logs-design.md, nailing down the architecture. Four decisions:

1. Auth = Firebase Auth. At this stage we only need Google sign-in, and standing up a dedicated auth server is over-scoped. Firebase has the Google provider built-in and covers Korea KYC.
2. User + job DB = SQLite for stage 1. Small user base; SQLite handles it. Schema starts with three tables: users, jobs, events.
3. Full audit trail. No billing yet, but the events table records user actions; billing fields can be added later. users and jobs are the referenced entities; events is append-only.
4. Anonymous jobs written with user_id = NULL. Logged-out jobs persist, but don’t get claimed on login. Deferring the job-claim logic (stage 2+).

The spec is not yet implemented — it’s scaffolding for the next interval.

Commit Log

Insights

The central lesson this interval was that decoupling “name swap” from “behavior swap” compounds its own cost. When 5af85f2 swapped rembg for BiRefNet inside the GPU worker but left backend directories, frontend components, and type names as rembg, the restore path ended up depending on a contract (rembg_orig backup) that no longer existed. The symptom — a white box in a refine UI — was ambiguous enough to waste a session debugging. When refactoring, remember that names are the shell of a contract. Either change the name when you change the internals, or if you keep the name, preserve the externally-observable contract too. This interval we chose to change the name (to matte), and now swapping to ToonOut or u2net tomorrow is a weights-only change. The SQLite-first decision follows the same pattern — setting up Firebase + Postgres now looks like “investing in the future,” but until the user count makes that real, it only costs and doesn’t pay. Start with a small contract at a small stage; upgrade only when the contract starts breaking.