Add node for decoding invisible watermarks from images

[Copilot]

Summary

Adds a workflow node to decode invisible watermarks previously embedded with the existing Add Invisible Watermark node. Returns the watermark text as a string, or an empty string if no watermark is detected or decoding fails.

invokeai/backend/image_util/invisible_watermark.py

• Added decode_watermark(image, length=8) classmethod to InvisibleWatermark
• Uses vendored WatermarkDecoder("bytes", length * 8) with dwtDct — mirrors the encoding path
• Returns decoded UTF-8 string stripped of null bytes; catches all exceptions and returns ""

invokeai/app/invocations/image.py

• Added DecodeInvisibleWatermarkInvocation node ("decode_watermark")
• Inputs: image: ImageField, length: int (bytes, default 8 — matches the default "InvokeAI" watermark)
• Output: StringOutput with the decoded watermark text

The length parameter must match the byte length of the text used during encoding (e.g., 8 for "InvokeAI").

Related Issues / Discussions

QA Instructions

1. Use Add Invisible Watermark node with default text "InvokeAI" on an image ≥ 256×256.
2. Feed the output image into the new Decode Invisible Watermark node with length=8.
3. Verify the output string is "InvokeAI".
4. Connect a plain (non-watermarked) image — verify output is "".

Merge Plan

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

Original prompt

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This section details on the original issue you should resolve

<issue_title>[enhancement]: Create node for decoding invisible watermark</issue_title>
<issue_description>### Is there an existing issue for this?

• I have searched the existing issues

Contact Details

No response

What should this feature add?

I'd like a workflow node that accepts an Image as input, attempts to decode its invisible watermark, and outputs a string with the watermark. If no watermark is detected, return an empty string.

Alternatives

No response

Additional Content

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [enhancement]: Create node for decoding invisible watermark #128

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[lstein]

Works as advertised.