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Compressed and Exact Result Transfer

Distributed Collector (DistributedCollector) collects images and/or audio. Its transfer_format and transfer_quality widgets control remote image returns only; they do not re-encode master-local images or change audio transport. The saved defaults remain legacy_png and quality 90.

transfer_format Fidelity Direct-worker transport
legacy_png (default) 8-bit pixels Original behaviour: one JSON request per frame, uncompressed PNG in base64. Compatible with legacy masters.
png Lossless 8-bit pixels Compressed PNG, binary, several frames per request.
ffv1 Lossless 8-bit pixels FFV1 video chunks; useful for frame sequences.
jpeg, webp Lossy 8-bit images Per-frame stills.
h264, h265 (experimental), av1 Lossy 8-bit images Video chunks; useful for similar consecutive frames.
exact (opt-in) Original floating-point tensors Binary multipart chunks with one raw tensor payload and tensor-le-v1 descriptor per frame. No 8-bit quantization.

“Lossless” PNG/FFV1 means lossless after 8-bit quantization, not preservation of the source float tensor. Direct non-exact returns decode to RGB float32. exact preserves supported floating-point dtype, shape, channels and values: float16, bfloat16, float32 or float64, finite dense BHWC tensors with 1, 3 or 4 channels. It generally costs substantially more bandwidth.

transfer_quality (1–100, default 90) applies to lossy formats, not exact, PNG or FFV1. JPEG/WebP use it directly; video codecs map it to CRF (90 → CRF 13 for h264/h265, CRF 16 for av1; 100 → CRF 0). A quality of 100 is not a substitute for exact float transport.

Serverless profiles

  • comfy-media-v1: the same Collector supplies the endpoint's image-return policy. The saved legacy_png default is mapped to compressed png with the same 8-bit pixel precision; normal serverless returns are not exact by default. All other formats retain their selection, including opt-in exact and experimental h265. Missing codecs or rejected formats fail instead of selecting another codec. Typed image returns preserve channel count and carry alpha separately where needed. See serverless images and audio.
  • legacy-image-v1: retains its image-file/SaveImage contract, not the typed return-format controls. Selecting a legacy pool when any Collector requests exact is rejected before dispatch; there is no 8-bit fallback. Audio requires direct workers or comfy-media-v1.

Direct-worker compatibility and limits

  • Choose the format on the master; the value is sent to workers with the workflow.
  • exact requires support on both master and worker. If an older master rejects an exact chunk, the worker raises an error and never resends it as legacy_png or another 8-bit format. An exact Collector also rejects direct-worker image returns labelled with a non-exact format. Update both sides or explicitly select a non-exact format.
  • For non-exact compressed formats only, the specific first-request older-master rejection (400 with Invalid JSON payload) causes a retry using legacy_png. Other rejections, including HTTP 413, remain errors.
  • For direct video returns only, an unavailable encoder falls back to h264 for lossy video if available, otherwise png; unavailable ffv1 falls back to png. The worker logs this. This compatibility fallback does not apply to exact or typed serverless returns.
  • Multipart requests use a 32 MiB chunk target (COMFYUI_DISTRIBUTED_TRANSFER_CHUNK_BYTES) and a 96 MiB request budget (COMFYUI_DISTRIBUTED_TRANSFER_MAX_REQUEST_BYTES), reserving space for the envelope and accounting separately for audio. A single large frame may exceed the chunk target but must fit the request budget. Keep the request budget below the master's --max-upload-size (default 100 MB).
  • Non-exact compressed chunks are all encoded before sending. Exact transfer instead plans frame sizes from shape/dtype before sending and encodes one chunk at a time. An oversized individual frame is rejected before any chunks are sent; choose another format or raise the worker and master limits together.
  • Audio is attached to the final image chunk when it fits, otherwise sent separately. Audio-only completions use the canonical JSON envelope. Direct audio uses float32 waveform bytes regardless of the image format; exact is an IMAGE option, not a new audio codec.
  • Each worker logs the format and bytes sent. These request limits are for direct multipart transfers; serverless request/artifact limits are documented separately.

Previously documented size example

The following measurements are retained from a 121-frame 1280×720 slow-pan batch (one worker's result); they are not new validation of the current implementation or exact transport.

Format Sent vs legacy PSNR
legacy_png 446 MB 1× lossless at 8-bit precision
png 163 MB 2.7× lossless at 8-bit precision
ffv1 128 MB 3.5× lossless at 8-bit precision
jpeg q90 27 MB 17× 42.7 dB
webp q90 13 MB 33× 40.6 dB
h264 q90 2.4 MB 186× 40.2 dB
h265 q90 (experimental) 1.0 MB 434× 40.1 dB
av1 q90 0.4 MB 1031× 40.5 dB

Video codecs compress this slow pan especially well; gains depend on content and motion.