What error variant are encoders supposed to return when they don't support the given color type?

[RunDevelopment]

I'm working on integrating the new DDS de/encoder, so I have to handle the case where the encoder doesn't support the ExtendedColorType given by the user. I have to return an error, but which one?

My question is: what error variant am I supposed to return?

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I tried looking at what other encoders are doing, but it's a colorful mix. There are 3 main categories:

1. avif, jpeg, tga, webp, farbfeld, gif, and tiff return ImageError::Unsupported with UnsupportedErrorKind::Color.
2. hdr, openexr, and qoi return ImageError::Encoding with a simple string error (message).
3. And the weird ones...
   • bmp returns ImageError::IoError with io::ErrorKind::InvalidInput. (It even creates a new std::io::Error for it...)
   • pnm returns either ImageError::Unsupported with UnsupportedErrorKind::Color OR ImageError::Parameter.
   • png returns either ImageError::Unsupported with UnsupportedErrorKind::Color OR ImageError::Encoding with its custom BadPngRepresentation::ColorType error.

(As for the formats not included above: ico uses the PNG encoder under the hood, and dds and pcx didn't have encoders.)

If I had to guess how this came to be, I'd say that ImageError::Unsupported with UnsupportedErrorKind::Color was the indented error, but then PNG came along with its BadPngRepresentation and that influenced the formats that came after it to use ImageError::Encoding. Oh, and BMP is just weird.

If ImageError::Unsupported with UnsupportedErrorKind::Color really is the intended error, I'm willing to make a PR that (1) documents this fact and (2) changes all encoders to use this error. Just let me know.

[fintelia]

I think that UnsupportedErrorKind::Color is probably best. Though I've also been meaning to double check what the actual printed message from that looks like. I think I recall the output being a bit confusing (not specifying whether encoding or decoding was happening, whether the encoder or the format itself is to blame for lack of support, etc.)