remove-ai-watermarks/docs/known-limitations.md

Known limitations

This page describes current product limits. Historical measurements and superseded experiments live in the research archive listed in the documentation index.

Visible removal

Fill quality depends on the background

Visible removal changes only the selected mask, but the hidden pixels still have to be reconstructed.

• OpenCV is fast and requires no model download. It works well on flat backgrounds but can smear texture or repeated structure.
• MI-GAN is a lighter learned backend. It can improve natural texture but may ghost or invent structure.
• LaMa is the heaviest learned backend and is generally the strongest option for difficult backgrounds.

--backend auto selects LaMa when available, then MI-GAN, then OpenCV.

No backend can recover detail that is completely hidden by an opaque mark. A successful detection therefore does not guarantee a visually perfect fill.

Automatic detection covers registered variants only

The registry contains vendor and locale specific templates. A redesigned mark, an unsupported locale, a different position, or a crop may be missed.

Known examples:

• Samsung detection is calibrated for the Italian Contenuti generati dall'AI text variant.
• The Jimeng top-left pill has a weak visual detector and is intentionally subject to additional product and background checks.
• Kling support covers the calibrated variants rather than every Kling label.

Use erase --region when you can see and select an unsupported or missed mark.

Strict and automatic sensitivity trade recall for precision

--sensitivity strict uses the visual gate alone. The default auto mode may relax a mark only when metadata or a confidently detected sibling mark corroborates the same product.

There is no blanket "this image is AI" relaxation. That information does not identify the vendor, mark, or location and caused unacceptable false detections in the removed experimental mode.

Invisible removal

Regeneration is lossy

Invisible removal does not decode and delete a payload. It regenerates the image through a diffusion pipeline. Faces, text, colors, and fine detail can change even when the watermark is successfully disrupted.

qwen-zimage is the default profile and sdxl-zimage the only alternative. Both are CUDA only and differ only in the global regeneration model: each conditions that stage on a canny edge map, which preserves structure but not identity or exact texture, and each then runs the same face stage. qwen-zimage is the higher fidelity of the two. Both are large, slow, and may still alter small text or difficult faces.

Removal cannot be verified locally for proprietary SynthID

The project has no public local SynthID pixel decoder. It can infer likely presence from supported provenance metadata, but a missing metadata proxy is not a negative pixel verdict.

For important outputs:

1. preserve the original;
2. process a copy;
3. verify with the matching provider tool when available;
4. do not assume one provider's verifier covers another provider's payload.

Provider systems can change, so a result verified on one file, seed, or version is not a permanent certification.

Video SynthID removal is lossy and content-dependent

The video invisible command and remove_video_invisible API regenerate video pixels through a VAE. The shipped noise_std=0.15 profile is oracle-certified, but Google does not publish a local decoder for arbitrary runtime outputs. A quiet metadata scan, paired PSNR, and the temporal-residual metric are fidelity measurements, not independent SynthID verdicts.

The control must use the same clip, frame rate, dimensions, and final codec as the candidates. The separate scripts/video_synthid_sweep.py harness produces that matched control. If the control is not detected by the matching provider oracle, the experiment cannot attribute a quiet candidate to regeneration.

The 2026-07-29 two-clip calibration used Gemini's built-in content verifier: both matched controls were SynthID-positive, the stronger candidate was negative on both carriers, and a weaker candidate was negative on one. A 2026-07-30 adversarial follow-up incorrectly asked the ordinary chat model to reinterpret the verifier while excluding every other input; its UNAVAILABLE answer was not another detector run and does not invalidate the original built-in results. The calibrated default remains content-dependent; a fresh source-positive, output-negative pair is an optional audit for unusually important files or after provider changes.

The 2026-07-31 full-clip check used the public eight-second Veo off-road sample. The source was detected across the full clip, the complete product path at noise_std=0.10 remained detected, and 0.15 returned no SynthID detection. The default was raised to 0.15. At 512 px / 12 fps, the accepted candidate measured 25.39 dB paired PSNR and a 1.058 motion-compensated temporal-residual ratio. This is one carrier, not a universal guarantee; hashes and exact verdicts are tracked in data/evaluations/video-synthid-oracle.csv.

The shipped engine streams sampled frames in bounded batches, computes its fidelity metrics incrementally, and pipes regenerated pixels directly to ffmpeg. Its frame and latent memory is therefore bounded by --batch-size rather than clip duration. Runtime still grows linearly with duration, and the separate multi-candidate research sweep deliberately retains its short sampled prefix so it can reuse identical latents across candidate strengths.

Strength is content and seed dependent

The two profiles resolve an unset strength differently, because different things were measured for each.

qwen-zimage reads it from image area, through the resolution-adaptive denoise curve. The vendor is deliberately ignored: the curve, not the issuer, is what was calibrated.

sdxl-zimage reads it from the C2PA issuer, on a flat ladder:

• OpenAI: 0.15;
• Google: 0.25;
• unknown: 0.25, following the stricter of the two.

An SDXL global pass needs more denoise than Qwen at the same fidelity, and the values are flat rather than a curve because flat values are what was measured: each verdict came from a fixed strength at one size, and no size dependence has been established for that stage.

An explicit --strength overrides both. The defaults are operating points, not universal guarantees. Near a removal threshold, different content or a different random seed may change the verifier result, which is why both profiles are certified at a fixed seed. The live resolver is watermark_profiles.py.

Pipelines have different quality tradeoffs

Pipeline	Main limit
qwen-zimage	CUDA only, large model stack, and limited broad certification across seeds and content.
sdxl-zimage	CUDA only. Its strength ladder is flat per vendor, not a resolution curve, because flat values are what was measured.

The controlnet, sdxl, qwen and default profiles were removed, not aliased onward: a retired name is rejected at parse time rather than routed into a profile the caller never chose. There is no --model, --steps, --guidance-scale, --device or --auto option either; each profile pins its model stack, its per-stage schedule, CFG 1.0 and CUDA.

Resolution and memory

Small images are processed at their native size

There is no minimum-resolution floor. It existed to enlarge small inputs toward SDXL's ~1024 training resolution and was removed with the SDXL profiles, which never applied it anyway. Both surviving profiles run at native geometry, so a small input is neither enlarged before diffusion nor restored afterward.

--max-resolution still caps very large inputs, and only ever scales down.

Large images stay at native resolution unless capped

--max-resolution 0 means no explicit downscale cap. A positive value caps the long side before diffusion and restores the result afterward. This reduces memory use but introduces a downscale and upscale round trip.

--tile preserves the input dimensions while running the diffusion stage in overlapping tiles. It avoids the explicit downscale, but it is not pixel lossless: each tile is independently regenerated. With qwen-zimage, only the global Qwen stage is tiled; the face stage runs after tile blending.

CPU offload trades speed for VRAM

--cpu-offload forces both stacks of the two-stage profile out of automatic device residency, streaming weights instead of pinning them. It reduces CUDA memory pressure at the cost of speed.

Residency is otherwise chosen from the card's total VRAM. On a card large enough to hold a stack, offloading is pure waste: DiffSynth drops weights to the meta device and re-reads every parameter from disk on each stage transition.

There is no CPU or MPS fallback

Invisible-watermark removal refuses any device but CUDA at construction. There is no MPS out-of-memory fallback that continues on CPU, and no lighter profile to drop to: both remaining profiles need an NVIDIA GPU.

When a run does not fit, the levers are --tile (native geometry, tiled diffusion), --max-resolution (an explicit downscale), and --cpu-offload. Memory needs depend on the profile, input size, dtype, and card.

Metadata and formats

Missing metadata does not mean clean

Screenshots, social platforms, and re-encoding can remove metadata while a pixel watermark remains. identify therefore reports unknown rather than clean when no supported signal is found.

JPEG XL is metadata only

The metadata path recognizes JPEG XL containers, but the visible and diffusion image paths do not list .jxl as a supported pixel format because the package does not include a JPEG XL pixel decoder.

HEIC, HEIF, and AVIF pixel decoding uses an optional Pillow fallback

OpenCV does not decode these formats in the project. image_io.imread falls back to Pillow with pillow-heif when the heif extra is installed alongside a pixel feature. The default metadata path scans these containers without that plugin. A corrupt or truncated file may still fail to decode.

Some metadata removal requires ffmpeg

WebM, Matroska, MP3, WAV, FLAC, OGG, Opus, and AAC container metadata is stripped through ffmpeg with stream copying. The operation fails if ffmpeg is absent or cannot parse the input.

Video pixel removal is provider-specific

The video metadata command and high level video API inspect and strip supported AI provenance metadata without transcoding streams.

video visible and remove_video_visible additionally support the moving Sora 2 mascot and wordmark, the current Veo four-point diamond, the legacy Veo text, the Seedance boxed AI label, the fixed Dola AI text, the Hailuo MINIMAX/Hailuo composite label, and the bottom-right Kling label with its version suffix. Detection requires a recurring visual candidate across adjacent frames. Fixed-mark candidates must remain anchored rather than drifting with a scene object. Kling also requires a bright low-saturation candidate near the expected frame edge. Provider provenance can recover low-contrast runs only after visual evidence exists for the marks that define a provenance prior, so metadata alone does not erase a clean API export. The default auto-router evaluates all detectors in one decode pass but does not rank their raw confidence values. Those scores are provider-specific and known to cross-match in some layouts, so the router applies the independent temporal policies and selects the first stable result in specificity order. Use an explicit mark when the provider is already known. Historical Sora Turbo exports use a small OpenAI swirl in the corner rather than the moving mascot-and-wordmark design; that earlier variant is not detected by the sora video mark. Hailuo and Kling coverage is specific to the verified lower-edge layouts; a new provider layout needs a separate calibrated silhouette. Other provider video labels are not supported yet. Google video SynthID has an oracle-certified VAE removal path, while other proprietary invisible video watermarks have no registered attack.

Visible removal transcodes the video stream and copies the complete audio stream without shortening an audio tail. Completed visible and invisible encodes are published atomically, so an encode failure preserves an existing output. Visible removal now applies a guarded motion-compensated blend after the per-frame fill. It uses adjacent optical flow only when the warped prior mask covers the current mask and a source-context ring agrees; scene cuts and disjoint marks keep the independent fill. This reduces measured paired temporal error, but it is not a generative video-inpainting model and cannot recover structure that no frame exposes. OpenCV can still leave a visible smear where the mark overlaps a hard edge or structured texture. MI-GAN improves difficult individual frames and is the practical learned CPU tier. LaMa remains an offline quality option: a full real sequence confirmed its multi-GB memory use and CPU throughput unsuitable for an online worker. The Veo diamond uses a shape mask to limit damage outside the symbol. Seedance fills the full localized box because a synthetic outline mask left part of the real translucent border visible in an end-to-end check. OpenCV may therefore soften texture inside that small box; use MI-GAN or LaMa when reconstruction quality matters. Relative variable-frame timestamps are preserved through a timestamped NUT bridge. A non-zero absolute video start PTS and the corresponding copied audio offset are preserved through ffmpeg timestamp passthrough. The encoder is source-aware for common 8-bit inputs: it probes and preserves supported chroma sampling, recognized color metadata, encoder time base, and MP4/MOV track timescale instead of accepting ffmpeg's implicit yuv444p raw-BGR output. OpenCV still decodes through 8-bit BGR, so HDR/high-bit-depth inputs are rejected before encoding rather than silently falling back to yuv420p. The synthetic Sora/OpenCV full-clip CI gate covers complete removal, untouched-region PSNR, frame count, frame rate, duration, copied-audio identity, source stream properties, paired temporal deltas against an independently encoded frame-local baseline inside the filled region, and metadata stripping through real ffmpeg. A second synthetic VFR clip verifies all display timestamps to the source time-base tick, including a non-zero source start, and checks both video and audio stream offsets. A separate constant-rate case guards the non-zero-start routing independently of VFR detection. A local full-sequence audit over all six providers passed both OpenCV and MI-GAN for complete-frame removal, quiet second detection, stream starts, duration, and copied audio. A full LaMa sequence passed the same checks but established that the backend belongs in the offline tier on CPU. These bounded local checks are still not universal evidence for every provider layout or source.

Native TC260 metadata in MP4/MOV is supported at its normative moov.udta.meta.keys/ilst placement, including non-faststart files whose moov follows a large media payload. MKV/WebM is supported at the normative Segment.Tags.Tag.SimpleTag placement and uses ffmpeg for stream-copy removal. AVI is supported at LIST/INFO/AIGC, and FLV at script.onMetaData.AIGC; both use ffmpeg stream-copy removal. Stock ffmpeg can write and strip the FLV form, but writing the nonstandard AVI child for fixture generation requires dedicated muxer support, so the AVI reader is verified against an exact synthetic RIFF structure.

MP4/MOV/M4V metadata removal now stream-copies the container in bounded chunks, keeps every box size and media offset fixed, and publishes atomically. The large-mdat regression rejects a full-source read_bytes() call and verifies that the encoded payload is byte-identical. HEIF/AVIF/JPEG-XL image metadata still uses the in-memory path because their XMP/EXIF items may live inside mdat/idat and require bounded format-item parsing before that path can stream safely.

Metadata transformation is fail safe

remove_ai_metadata may copy an undecodable file through unchanged instead of raising. User facing callers must use strip_and_verify and inspect its surviving marker mapping before reporting success. strip_and_verify recovers when image_io can still decode the raster by normalizing the container and checking again. A truly undecodable file still reports the surviving markers. The CLI uses this verified path.

Sixteen bit PNG output is not preserved

The Pillow based PNG metadata rewrite uses the normal image save path and may reduce a sixteen bit PNG to eight bits. A byte-level PNG metadata stripper would be required to preserve that bit depth.

Detection extras

The detect extra decodes an open DWT-DCT watermark used in some Stable Diffusion, SDXL, and FLUX workflows. That decoder is sensitive to the carrier and transformations. A negative result is not a universal negative.

The trustmark extra adds Adobe TrustMark decoding. The implementation retains an additional JPEG re-encode gate because isolated decoder hits can otherwise be content noise.

External AI versus real image classifiers are out of scope. The project identifies concrete local provenance signals instead of shipping a generic statistical classifier.

Output and traceability

Removing file-local signals does not remove:

• provider account history;
• server side copies or provenance stores;
• perceptual fingerprints;
• evidence that an image passed through a removal pipeline;
• legal disclosure duties.

See scope, safety, and legal notes.