composing an image from multiple channels data

Nobody has claimed this yet.

What are you trying to achieve?
Extracting channels from an image, and re-composing them into a different setup

Have you searched for similar questions?
yes

Are you able to provide a standalone code sample that demonstrates this question?
yes

Are you able to provide a sample image that helps explain the question?
no

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I'm not sure if this is something that would warrant a new feature being added, or just some clarification to the documentation, but it took me quite some time (actually reading the test suite, rather than the documentation) to figure out that joinChannel keeps adding new channels to a file, despite the documentation appearing to suggest that if you give it an array form, it might replace/overwrite the channels, since you can specify [ red, green, blue, alpha ] format if your image is RGB.

That is clearly not how it works after reading the test suite.

Perhaps a "replaceChannel" function, or just explaining in joinChannel, that if you want to replace channels, you'll need to compose them together starting from a single channel image.

Start by reading the joinChannel documentation and the related test suite, which the issue identifies as showing the actual behavior. Compare the documented array form with the tests, then clarify whether channels are appended and document the single-channel composition workaround; a separate replaceChannel feature would need an explicit decision.

Written by the indexing model from the issue text.