remove-ai-watermarks/docs/module-internals.md

Module internals

This page documents the current implementation contract. It intentionally avoids experiment logs, corpus counts, and calibration history. Those records live in the verification plan and the research archive listed in the documentation index.

Read the relevant section before changing a subsystem. When this page and the code disagree, the code and its tests are authoritative and this page must be updated in the same change.

Architecture

The package has four main paths:

flowchart LR
    Input[Input file] --> Identify[Identify provenance]
    Input --> Visible[Visible mark removal]
    Input --> Invisible[Diffusion regeneration]
    Input --> Metadata[Metadata stripping]

    Identify --> Report[ProvenanceReport]
    Visible --> VisibleOutput[Localized and filled image]
    Invisible --> InvisibleOutput[Regenerated image]
    Metadata --> MetadataOutput[Container with AI metadata removed]

The all command runs visible removal, optional invisible regeneration, and metadata stripping in that order.

Command line interface

cli.py owns command parsing and user-facing exit behavior.

Important contracts:

• Single-image arguments reject directories.
• visible writes no output when no registered mark is selected and exits with EXIT_NO_VISIBLE_MARK.
• invisible writes no output when no supported local signal is found, unless --force is supplied.
• The two no-signal conditions currently share exit code 2.
• Hard processing and write failures exit with code 1.
• all can still write the completed visible and metadata stages when the diffusion dependencies are unavailable, but exits with code 1 so the partial result is not reported as complete.
• batch counts per-file failures and exits nonzero if any file failed or an applicable invisible stage was skipped because its dependencies were absent.

The decorators for diffusion options are shared by invisible, all, and batch. The runtime help generated by Click is the source of truth for option names and defaults.

The deprecated --auto option does not select a pipeline or change adaptive polishing. _resolve_auto_polish emits a warning and returns the explicit polish value unchanged.

Regression coverage:

• test_cli.py
• test_cli_robustness.py
• test_optional_deps.py

High-level Python API

api.py provides:

• remove_visible
• visible_provenance

The package root exposes both lazily through __getattr__, keeping a plain package import free of the heavier image and model imports.

For path inputs, remove_visible reads provenance metadata, preserves alpha, and optionally writes and strips metadata. Array inputs are treated as BGR arrays and have no file provenance or separate alpha plane.

When no visible mark is removed, a same-format path copy preserves the original bytes. write_noop=False leaves the requested output path untouched instead.

Regression coverage:

• test_api.py
• test_image_io.py

video.py provides the high-level video entry point:

• identify_video
• inspect_video_metadata
• remove_video_all
• remove_video_batch
• remove_video_invisible
• remove_video_metadata
• remove_video_visible

The video API validates both the supported extension and container signature, then delegates all metadata detection and stripping to metadata.py. It requires a separate same-container output, defaulting to <source>_clean, so the product path does not overwrite an original. The package root exposes all functions lazily.

identify_video runs the same stable-mark selection helper as remove_video_visible, so a provenance report cannot authorize a mark that the removal path would reject. It reports an empty local result as unknown rather than clean. Identification skips the separate per-frame timestamp probe because it never encodes frames. remove_video_all is the predictable-output composition: visible removal plus verified metadata stripping by default, with a same-container passthrough when neither signal exists. The lossy invisible removal stage is an explicit opt-in through the oracle-certified profile. remove_video_batch applies those contracts sequentially across a top-level directory, returns every per-file failure, and byte-copies visible no-ops so a successful output set has no silent holes. An invisible batch loads one VAE runtime and reuses it across every compatible file; a failed model load is reported per file without retrying the same multi-GB initialization.

Native MP4/MOV TC260 labels follow TC260-PG-20257A: moov.udta.meta.keys maps an AIGC key to a raw JSON value in ilst. _internal/isobmff.py walks those nested boxes by seeking, so detection reaches a tail moov without reading the preceding mdat. The MP4/MOV/M4V/M4A removal path first validates the top-level box walk, then copies the source to a sibling temporary file in bounded chunks. Supported C2PA/JUMBF/AI-label boxes become same-size free boxes with blank payloads; TC260 removal changes the four-byte key to free and blanks only the validated JSON value with same-length spaces. This preserves every box size, stco/co64 offset, encoded stream byte, and source-sized memory bound. Publication is atomic, and a malformed top-level walk is copied unchanged. A generic AIGC key whose value has no TC260 field is ignored.

_internal/ebml.py provides the corresponding bounded Matroska/WebM reader. It seeks over clusters and accepts only a Segment.Tags.Tag.SimpleTag pairing TagName=AIGC with a JSON TagString carrying a TC260 field. The existing ffmpeg stream-copy path removes those container tags without transcoding the encoded streams.

_internal/riff.py and _internal/flv.py implement the remaining normative TC260 video placements. The RIFF walker reads only AVI LIST/INFO/AIGC children. The FLV walker skips media tags and parses the AMF0 script.onMetaData.AIGC string. Both require a recognized TC260 JSON field and use the verified ffmpeg stream-copy path for removal.

video_encoding.py owns the ffmpeg command and pipe lifecycle shared by visible removal and invisible regeneration. It centralizes container codecs, optional audio stream copying, metadata/chapter policy, encode-failure reporting, and atomic same-directory publication. Each mapped stream is allowed to reach its own end, so a copied audio tail is not shortened to the frame-input duration. Both the raw-BGR and timestamped-NUT stdin modes redirect ffmpeg stderr to a temporary file while frames are written. Waiting to read diagnostics until after stdin closed allowed stderr backpressure to stop ffmpeg's frame reads, which in turn blocked the producer before it could close stdin. The file consumes no pipe capacity or RAM while ffmpeg runs; completion reports a bounded head and tail when diagnostics are unusually large. Aborts release it even when ffmpeg has already exited. A real subprocess regression writes diagnostics beyond pipe capacity while streaming frames, checks bounded failure reporting, and the Linux full-clip CI job guards the complete path. Frame encoding and source-audio copying run as two ffmpeg processes in sequence. The streaming encoder has only the frame pipe as input, so input probing or demux queues cannot deadlock the producer against a second input. After that pipe reaches EOF, a finite stream-copy mux combines the encoded video with the source audio and applies the requested metadata/chapter policy. Both stages use sibling temporary files, and only the completed mux is published atomically. The mux also redirects diagnostics to disk and reports only a bounded head and tail. Command regressions assert the single-input encoder and final map targets; failure regressions cover bounded mux diagnostics and atomic cleanup. probe_video_encode_profile reads the first source video stream with ffprobe and preserves the supported properties that survive the 8-bit BGR boundary: yuv420p/yuv422p/yuv444p chroma sampling, recognized color tags, encoder time base, MP4/MOV track timescale, source pixel format, and component depth. Both raw-CFR and timestamped-NUT inputs use ffmpeg's passthrough FPS mode. This keeps one encoded frame per supplied frame when an older ffmpeg receives a fine-grained source encoder time base such as 1/90000; implicit synchronization can otherwise synthesize thousands of duplicate frames between CFR timestamps. HDR transfer functions and component depths above 8 bits are rejected before encoding so the OpenCV boundary cannot silently reduce them to SDR 8-bit. probe_video_timestamps reads authoritative per-frame display PTS through ffprobe. OpenCV timestamps are only a count-matched fallback when ffprobe is unavailable or fails; this avoids decoder anomalies such as one spurious negative first-frame timestamp turning a CFR clip into false VFR. A uniform sequence keeps the cheap raw-BGR pipe unless the source starts at a non-zero PTS. A variable or offset sequence is packetized by the lazy PyAV bridge as rawvideo in an in-memory NUT stream with explicit PTS. System ffmpeg reads that stream with -fps_mode passthrough; -copyts additionally retains a non-zero video start and the corresponding copied-audio offset. No temporary frame sequence or second video encoder is introduced.

video_temporal.py owns the shared optical-flow maps and temporal residual metric. Visible removal uses stabilize_filled_frame after the selected image backend: it works on a bounded crop around adjacent masks, backward-warps the prior cleaned frame, requires high warped-mask coverage, and gates blending on an unmasked source-context ring. Only covered current-mask pixels change. Scene cuts, disjoint marks, and poor motion matches therefore retain the independent current-frame fill. The same module supplies the motion-compensated metric used by the invisible-video sweep.

video_invisible.py implements the oracle-certified video SynthID removal engine. It samples frames uniformly, resizes to a VAE-aligned geometry, encodes each frame to latent space, applies one seeded spatial-noise field across the entire sequence, and decodes fresh pixels. Reusing a single noise field avoids independent frame-to-frame noise. The shipped path retains only one configured frame batch, updates PSNR and temporal residuals incrementally, and streams BGR frames directly to the video-only ffmpeg encoder. A separate stream-copy mux then adds optional source audio and drops all source metadata. The result is written through same-directory temporary files and atomically replaced only after both stages succeed.

The engine returns PSNR and a motion-compensated temporal-residual ratio as quality measurements. Neither is a watermark detector. The high-level result reports completed removal without a separate verification-status flag. The companion scripts/video_synthid_sweep.py imports the same engine helpers to build a matched control and candidate grid, preventing research and shipped regeneration paths from drifting. The full-clip oracle floor is noise_std=0.15: on the public eight-second Veo carrier, 0.10 remained detected while 0.15 did not.

video_visible.py implements the first pixel stages for Sora, Veo, Seedance, Dola, Hailuo, and Kling. The Sora detector searches a normalized frame with a fully synthetic mascot-and-text silhouette at several scales. The Veo detector uses separate synthetic silhouettes for the current four-point diamond and legacy Veo text. Seedance uses a synthetic rounded boxed-AI silhouette, while Dola uses an OpenCV-font Dola AI silhouette. Hailuo uses a synthetic waveform, MINIMAX/Hailuo text, separator, and ring. Kling combines synthetic font variants with a ring approximation of its swirl; the logo path rescues wordmarks whose version or font differs, while the edge and white-label gates reject recurring scene texture. All fixed-mark searches are bounded to the expected lower-frame area and calibrated independently. A strong relocated Veo diamond may bypass the known layout anchors, but weak free-corner matches never enter the temporal arbiter.

The default auto route decodes each frame once, shares its grayscale and normalized representations across all detectors, and caches resized synthetic template features for the fixed stream geometry. Provider confidence scales are not comparable: selection applies each provider's temporal arbiter and takes the first stable result in specificity order (sora, veo, seedance, dola, hailuo, kling). An explicit mark uses the same scan path with one candidate. Removal also collects authoritative per-frame timestamps for the encoder, while identification omits that unused ffprobe pass.

Every per-frame result is untrusted. The provider-specific stabilization wrappers share one recurrence implementation, while retaining separate visual floors and minimum-run policy. Provenance can relax a low-contrast run only after recurring visual evidence exists. Sora transition frames follow the nearest confirmed moving position only with Sora provenance. Veo, Seedance, Dola, Hailuo, and Kling additionally require candidates to remain anchored to the start of a run. This rejects slowly drifting scene details that still have high frame-to-frame overlap. Hailuo and Kling do not infer provenance from technical encoder tags; their confirmed public samples carried no provider metadata.

Removal runs in a second decode pass. Sora, legacy Veo text, Dola text, Seedance, Hailuo, and Kling use box masks. Seedance deliberately fills the complete localized box: a synthetic outline mask passed repeat detection but left part of the real translucent border visible during visual end-to-end review. Hailuo expands beyond the matched core to cover both provider icons. Kling expands around the wordmark or swirl to include the version and optional PRO suffix. The square Veo diamond uses a synthetic shape mask so transparent corners do not erase unrelated pixels. Every mask goes through the shared watermark_registry.fill backends. ffmpeg encodes the changed video stream and copies optional audio. The default OpenCV fill is the speed floor; structured backgrounds need MI-GAN or LaMa for better reconstruction. Invisible video stages must continue to reuse the image and metadata implementations rather than copying their logic.

Regression coverage:

• test_video.py, including a real ffmpeg full-clip Sora/OpenCV path that generates a synthetic marked MP4 with AAC audio and C2PA provenance, runs both remove_video_visible and the composed remove_video_all API without mocks, and verifies complete removal, frame count, frame rate, duration, untouched-region PSNR, paired temporal deltas inside the filled region, byte-identical copied audio packets, source stream properties, metadata stripping, and a large-mdat metadata case that rejects any full-source read_bytes() call. CI installs ffmpeg explicitly for this test so the integration gate cannot silently skip.

Metadata and provenance

C2PA

_internal/c2pa.py reads C2PA with the official c2pa-python reader first. Its byte-level PNG parser remains a fallback for partial and synthetic fixtures that the official reader rejects.

Vendor attribution comes from the registry in _internal/constants.py. Derived issuer and platform maps should not be maintained separately.

Metadata scanning and stripping

metadata.py contains the shared metadata scanners and remove_ai_metadata.

Key contracts:

• scan_head is the shared cached input for bounded byte scans.
• JPEG stripping walks metadata segments and preserves the entropy-coded image scan.
• ISOBMFF containers use _internal/isobmff.py.
• Native MP4/MOV TC260 AIGC entries are read from moov.udta.meta.keys/ilst and blanked without changing box sizes.
• Native MKV/WebM TC260 AIGC entries are read from Segment.Tags.Tag.SimpleTag and removed through the ffmpeg stream-copy path.
• Native AVI and FLV TC260 entries are read from LIST/INFO/AIGC and script.onMetaData.AIGC, respectively, then removed through ffmpeg stream copying.
• Supported non-ISOBMFF audio and video containers use ffmpeg stream copying.
• The low-level remover is fail-safe and can copy an undecodable file through unchanged.
• A caller that reports success must use strip_and_verify, which scans the written output for surviving markers. If the metadata-preserving decoder rejected the container but image_io can still decode its raster, strip_and_verify normalizes that raster and scans again. A truly undecodable file keeps the surviving-marker result.

Detection and removal must stay in parity. A new marker is incomplete until the scanner can find it, the remover can reach every supported placement, and a test proves that it no longer appears in the output.

Regression coverage:

• test_metadata.py
• test_metadata_internals.py
• test_security_clamp.py

Provenance report

identify.py separates file-backed metadata extraction from verdict logic:

• extract_provenance_evidence reads the supported metadata signals into ProvenanceEvidence.
• evidence_from_metadata_record normalizes an externally collected nested metadata record into the same evidence type without file access. Diagnostic values under error and kind are excluded from evidence while nested raw bytes remain available through encoded binary fields.
• identify_from_evidence evaluates that evidence without reopening the source.
• identify preserves the path-based API and adds the optional registered visible-mark and open invisible-watermark decoders after extraction.

The detect extra composes the shared pixels runtime with PyWavelets. Its in-tree dwt_dct.py decoder preserves the upstream matrix algorithm without installing Torch or non-headless OpenCV. The upstream MIT notice ships inside the wheel under licenses/.

is_ai_generated is True or None; absence of evidence is not reported as a human-made verdict. ai_source_kind distinguishes fully generated content from AI-enhanced composites when the source metadata provides that distinction.

TrustMark is reported as a watermark signal but does not by itself assert AI origin because it can also protect human-authored content.

Regression coverage:

• test_identify.py
• test_trustmark_detector.py
• test_invisible_watermark.py

Visible mark removal

Registry and decision flow

watermark_registry.py is the only visible-mark registry. mark_keys() supplies the CLI choices, so the CLI must not maintain a separate mark list.

Automatic removal has three distinct stages:

1. Perception: each registered detector produces strict and relaxed candidates.
2. Decision: the pure decide arbiter applies sensitivity and corroborating provenance.
3. Action: each selected mark is localized to a mask and passed to the shared fill function.

sensitivity="strict" never relaxes a detector. sensitivity="auto" can relax one only when metadata or a sufficiently strong same-product sibling confirms that product. The removed blanket assume_ai mode is rejected explicitly.

The Jimeng pill has an additional decision gate because its visual detector is weaker than the other registered marks. Keep that policy in the registry, not inside unrelated detector engines.

remove_auto_marks removes every selected mark, not only the strongest one. This matters for images that carry marks in more than one corner.

Regression coverage:

• test_watermark_registry.py
• test_api.py

Gemini sparkle

gemini_engine.py uses a multi-scale shape search and a false-positive gate. Its captured sparkle assets serve detection and mask geometry only. Pixel recovery is performed by the shared fill backend.

detect_sparkle_confidence uses a process-wide shared engine because its loaded assets and template ladder are immutable.

Regression coverage:

• test_gemini_engine.py

Text mark engines

_text_mark_engine.py provides common localization, detection front ends, template caching, rival comparison, and footprint construction.

Each vendor module supplies a TextMarkConfig and only the behavior that cannot be represented by the shared base:

• doubao_engine.py
• jimeng_engine.py
• qwen_engine.py
• kling_engine.py
• yuanbao_engine.py
• samsung_engine.py
• runninghub_engine.py
• baidu_engine.py
• liblib_engine.py

The detector and removal mask must use compatible geometry. A detector that fires while producing an empty or misplaced mask is a removal failure even if the detection test passes.

Yuanbao uses the polarity-independent contrast front end because its standard two-line mark can be light on dark scenes or dark on light scenes. Its detector and footprint both use the same best-match box. The separate one-line overlay variant is not covered.

The capture-less Jimeng pill lives in pill_engine.py. It uses a synthetic silhouette for detection and a fixed top-left footprint.

Each engine has a corresponding test module under tests/. Shared behavior is covered by:

• test_text_mark_engine.py
• test_text_mark_faint_mask.py
• test_text_mark_memory.py

Fill backends and region erasing

region_eraser.py implements the same backends used by visible removal and the user-directed erase command:

• cv2
• migan
• lama

watermark_registry.resolve_backend selects LaMa first, then MI-GAN, then OpenCV for auto. A memory-constrained caller should explicitly select MI-GAN or OpenCV instead of relying on auto.

MI-GAN and LaMa crop around the mask before model inference and paste back only masked pixels. Their model sessions are loaded lazily. MI-GAN uses the inverse mask polarity expected by its ONNX model.

Regression coverage:

• test_region_eraser.py
• test_inpaint_fallback.py

Invisible watermark regeneration

Profiles and strength

_internal/watermark_profiles.py is the source of truth for:

• profile aliases;
• default model identifiers;
• default steps and seeds;
• vendor-adaptive strength resolution;
• the minimum viable step calculation.

The current profiles are controlnet, sdxl, qwen, and qwen-zimage. For serverless cold starts, InvisibleEngine.preload(global_only=True) loads the mandatory Qwen stage and YuNet while leaving the optional Z-Image and SAM face stack lazy until a face is detected. The default preload() still loads every stage. default is a legacy alias for sdxl. There is no content-dependent automatic router.

invisible_engine.py handles image sizing, optional pre-upscaling, postprocessing, and the public engine interface. It delegates model execution to _internal/watermark_remover.py.

The Python engine and CLI do not have identical defaults for every optional postprocessing argument. Integrations that require reproducibility should pass the relevant values explicitly.

The standard Qwen and ControlNet prompts are calibrated model inputs, and the ControlNet edge map uses fixed Canny thresholds of 100 and 200. Treat those values as behavioral compatibility contracts: a refactor must preserve them, and any deliberate change requires image-quality evaluation rather than only a unit-test pass. Exact prompt and edge-map regression guards live in test_platform.py and test_invisible_engine.py.

Regression coverage:

• test_watermark_profiles.py
• test_invisible_engine.py
• test_img2img_runner.py
• test_platform.py

CPU offload

CPU offload is enabled only when requested on CUDA. The standard Diffusers profiles call enable_model_cpu_offload. The qwen-zimage profile uses the same flag to force both its stacks out of automatic device residency.

Residency is otherwise chosen from the card's total VRAM, once per stack: resolve_global_model_residency gates the mandatory Qwen stack at RESIDENT_GLOBAL_MODEL_MIN_VRAM_GIB and resolve_face_model_residency gates the optional Z-Image stack at RESIDENT_FACE_MODEL_MIN_VRAM_GIB.

Below the global floor, _qwen_vram_config streams the stack from disk, which is what makes a 20B model runnable on a consumer card. At or above it, streaming is pure waste and the weights stay on the GPU. The difference is not marginal: DiffSynth offloads by dropping the weights to the meta device and re-reading every parameter through its DiskMap on the next onload, and the pipeline moves between text encoder, transformer and VAE on each pass. Measured on an H100 (80 GiB) in August 2026, a warm global pass took 37.3 s at 0.8 GiB resident with the streaming config, against 2.2 s at 28.7 GiB with the stack resident; both stacks resident peaked at 48.0 GiB. Faster storage cannot close that gap, because the cost is the reload itself rather than the read.

The resident config deliberately passes no "disk" value anywhere. DiffSynth latches disk_offload once, from offload_dtype, so leaving the sentinel in place while pointing every device at CUDA would keep the meta-drop and re-read.

Regression coverage:

• test_cpu_offload.py
• test_qwen_zimage_pipeline.py

Qwen plus Z-Image

_internal/qwen_zimage_pipeline.py implements the fixed CUDA-only two-stage profile:

1. Qwen Image with Canny conditioning regenerates the frame.
2. YuNet locates faces, SAM builds masks, and Z-Image regenerates the selected face regions.

The profile rejects a custom model identifier. Its global and face model stack is fixed by the implementation. When tiling is enabled, only the global stage is tiled; the face stage runs once after the tiles are blended.

How the face stage actually composites

The mechanism is easy to misread from the parameter names, so state it plainly. _run_faces crops the expanded box from the original image, resizes it toward the 768 px face guide, runs Z-Image over the entire crop, resizes back, and only then merges through composite_face, which cross-fades on a Gaussian-blurred SAM mask with feather=10. The base it merges into is the global Qwen result.

Two consequences follow, and both matter when tuning:

• Everything inside the crop is regenerated, including the pixels the mask later discards. The generation is therefore conditioned on a fully noised neighbourhood, not on an intact one. An alternative design passes the mask into the sampler as a latent noise mask, so only masked pixels are ever denoised and the edge transition happens inside the generation rather than as a post-hoc blend. That approach has never been tried in this runtime and is an open lever, particularly since the face stage is the largest measured quality contributor: removing it costs 3.5 dB inside the face boxes on one fixture and 6.1 dB on another.
• FACE_DENOISE_SCALE = 0.5 is best understood as compensation for the above. Regenerating a whole crop and blending is a stronger operation than denoising only inside a mask, so the halved strength brings the visible result back into range. Read it as coupled to the compositing design rather than as an independently calibrated constant: changing the compositing without revisiting the scale would change output strength by roughly a factor of two.

The maintained implementation preserves the previously oracle-tested strength, conditioning, crop, and sampler parameters as compatibility contracts. Its Python orchestration, YuNet integration, SAM selection, masks, sizing helpers, and pixel compositing are implemented for this runtime. Changing a calibrated model input requires the same provider-oracle and identity evaluation as a model change.

SDXL plus Z-Image

_internal/sdxl_zimage_pipeline.py runs the same two-stage recipe on an SDXL global pass. SdxlZImagePipeline subclasses QwenZImagePipeline and overrides only _run_global and preload, so the face stage is inherited rather than copied and cannot drift between the profiles; a test asserts the shared methods are the same objects.

Four things are architecture-bound and swap with the model: the ControlNet (xinsir/controlnet-canny-sdxl-1.0), the four-step distillation LoRA (ByteDance/SDXL-Lightning at its documented strength 1.0, not the reference graph's 0.8, which belongs to a different LoRA), the sampler (Euler with trailing spacing, no AuraFlow shift), and the latent grid (8 px against Qwen's 16).

Strength is architecture-bound too, and that is the easy mistake. An SDXL global pass leaves SynthID at the strength Qwen needs: verified through the Gemini app on a native 2816x1536 original, 0.154 is FOUND while 0.20, 0.25 and 0.30 are clean. So this profile takes a vendor policy (SDXL_ZIMAGE_OPENAI_STRENGTH 0.15, SDXL_ZIMAGE_GEMINI_STRENGTH 0.25, unknown following Gemini) rather than resolution_adaptive_denoise. Flat values are what was measured; no size dependence has been established for this stage, so none is asserted.

requested_steps exists because the two runtimes truncate differently. DiffSynth sets sigma_start = denoising_strength and runs every requested step across the shortened sigma range; Diffusers img2img truncates the step count (init_timestep = int(steps * strength)), so asking it for four steps at 0.15 executes zero and returns a bare VAE round-trip.

This profile is not deployed. Before it could be, it needs the other three Gemini originals, OpenAI re-verified at 0.15, a flat-graphic content class, and a low resolution case -- every verdict so far comes from one fixture and one seed.

Measured provider boundaries for qwen-zimage

Both ends of the shipped curve now have oracle verdicts, and the shipped curve clears everything it has been tested at:

oracle	fixture size	detected at	clean from
openai.com/verify	1.57 MP	0.06	0.08
Gemini app	4.33 MP	0.08	0.10
Gemini app	0.57 MP	--	0.0896 (the curve's own value)
Gemini app	1.40 MP	--	0.1066 (the curve's own value)

Read the last two rows before concluding the curve's low end is under-driven. Against the 4.33 MP Gemini boundary the sub-1 MP rungs of 0.084-0.094 look short, but at those sizes the curve's own values verify clean, which is what a resolution-scaled requirement would predict. There is no measured size at which the shipped curve fails, so it is left alone.

Static prompt embeddings

Both stages prompt with module constants, and at CFG 1.0 DiffSynth's PipelineUnitRunner reuses the positive embedding for the negative side instead of encoding it. So exactly one embedding per stage is ever computed, from text that cannot vary at runtime, which makes it cacheable across containers rather than only within one pipeline.

_cache_static_prompt_embeddings therefore persists what the text encoder produced under _model_cache_dir()/prompt-embeddings, keyed by cache version, model id, pipeline output params, and the exact prompt string. Once that file exists, _load_qwen and _load_zimage drop the text-encoder ModelConfig from the model stack entirely and serve the stored tensors instead. Measured on an H100 volume in August 2026, that removes 15.45 GiB (Qwen2.5-VL) and 7.49 GiB (Z-Image) of a 87.6 GiB per-request read, worth a median 11.76 s and 4.10 s of load time (paired within five containers). The output is byte-identical -- the stored tensors are the encoder's own -- so this needs no provider-oracle re-verification.

Three properties are load-bearing:

• The key self-heals. A model bump or a prompt edit changes the key, so the next container recomputes rather than reading a stale embedding. _PROMPT_CACHE_VERSION covers a change to the stored shape itself.
• The write is atomic. A torn write must never be readable as a cache hit, so the payload lands in a temp file and is renamed into place.
• A miss after the encoder was dropped raises. require_cache records that the stack was built without a text encoder on the strength of the file; falling back would call a model that is not loaded, which surfaces as an opaque crash.

_model_cache_dir() prefers HF_HOME for the same reason: on a scale-to-zero runner that is the only persistently mounted path, and anything below it is re-derived per request. The YuNet download follows the same root.

Regression coverage:

• test_qwen_zimage_pipeline.py
• test_cpu_offload.py

Tiling

_internal/tiling.py contains pure tile planning, feather weights, tile orchestration, and region compositing.

Tiling engages only when requested and the long side exceeds the tile size. It avoids an explicit full-image downscale but does not make diffusion pixel-preserving. Each tile is still regenerated.

feather_region_composite changes only the requested box and leaves pixels outside it unchanged.

Regression coverage:

• test_tiling.py

Upscaling and postprocessing

upscaler.py is the optional Real-ESRGAN path used only when enlarging a small image to the minimum resolution floor. Failure or an absent extra falls back to Lanczos.

humanizer.py contains explicit grain, unsharp masking, and adaptive polish helpers.

Regression coverage:

• test_upscaler.py
• test_humanizer.py

Image input and output

image_io.py is the shared image codec boundary.

Contracts:

• All package OpenCV file reads and writes use image_io.imread and image_io.imwrite.
• to_bgr normalizes grayscale and alpha-bearing arrays.
• read_bgr_and_alpha and write_bgr_with_alpha preserve the alpha plane.
• imwrite returns a success flag; every caller must check it.
• HEIC, HEIF, and AVIF pixel reads fall back to Pillow plus pillow-heif from the independent heif extra. Metadata scanning does not require that plugin.
• A visible no-op can preserve the original file bytes.

Regression coverage:

• test_image_io.py
• test_cli_robustness.py

Adding or changing behavior

For a new visible mark:

1. create a synthetic detection silhouette;
2. add or extend a vendor engine;
3. add one registry entry;
4. test detection, false positives, localization, and actual pixel change;
5. update supported signals.

For a new metadata signal:

1. add the scanner;
2. add every supported removal placement;
3. verify the output through strip_and_verify;
4. add identification and removal tests;
5. update supported signals and, when relevant, the watermarking landscape.

For a diffusion change:

1. keep model-free logic in pure helpers where possible;
2. test option propagation and dispatch without downloading models;
3. run a real model smoke for the changed model path;
4. treat provider-verifier results as specific to the exact checked output;
5. update known limitations.