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Images

Write the IPTC Digital Source Type into PNG, JPEG and WebP files, read it back, and how this relates to C2PA.

markImage writes an XMP packet into an image file. The packet carries:

Iptc4xmpExt:DigitalSourceType: the IPTC term for how the image was made, e.g. trainedAlgorithmicMedia for fully generated images or compositeWithTrainedAlgorithmicMedia for edited ones.
Iptc4xmpExt:AISystemUsed, Iptc4xmpExt:AISystemVersionUsed and xmp:CreatorTool: the AI system.
xmp:CreateDate, and optionally a description, your company and your AI page.

The pixels are not touched. The file is not re-encoded.

import { markImage, readImageMarking } from "@sweberdev/witness";

const result = markImage(bytes, {
  kind: "generated",
  generator: "gpt-image-2",
  provider: "Beispiel AG",
  disclosureUrl: "https://beispiel.ch/ki",
});

if (result.status === "marked") await save(result.bytes);

readImageMarking(result.bytes);
// { format: "png", aiGenerated: true, sourceType: "trainedAlgorithmicMedia",
//   generator: "gpt-image-2", witness: true, c2pa: false, xmp: "…" }

Format	Where the XMP goes
PNG	`iTXt` chunk `XML:com.adobe.xmp` after `IHDR`. Existing `iTXt` or `zTXt` XMP is merged and replaced
JPEG	`APP1` segment after JFIF and Exif
WebP	`XMP` chunk; simple files get a `VP8X` header

Existing XMP from other tools (rights, captions) is kept; Witness adds its own description block and replaces only that on later runs.

C2PA

C2PA Content Credentials are signed manifests and the format the Code of Practice points to for signed metadata. Many image generators already embed them. Rewriting such a file breaks the manifest's hash binding, so markImage returns status: "skipped-c2pa" and leaves the bytes untouched. Pass { c2pa: "overwrite" } only if you know you do not need the manifest.

readImageMarking reports c2pa: true when a manifest is present. It does not verify the signature.

Limits

XMP metadata is removed by many upload pipelines, messengers and social networks, and anyone can strip it. It is one layer, useful because it is widely read. Pair it with a visible label, and with C2PA or a provider watermark where you need robustness.

Batch and CI

See CLI for witness mark and witness inspect. Witness Pro adds a scanner that checks a whole build.