Split PNM ImageFormat into subformats for PBM, PGM, etc.#2841
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mstoeckl wants to merge 3 commits intoimage-rs:mainfrom
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Split PNM ImageFormat into subformats for PBM, PGM, etc.#2841mstoeckl wants to merge 3 commits intoimage-rs:mainfrom
mstoeckl wants to merge 3 commits intoimage-rs:mainfrom
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The test images are easier to compare for correctness with other decoders. Note that some test images have been modified to have unusual but valid headers and whitespace.
The new ImageFormats ensure that methods like DynamicImage::save only write image data which is compatible with the file extension. (Previously PAM image data was written for any path with a NetPBM extension, producing images that some other programs couldn't read.) The PnmDecoder continues to accept any NetPBM format. This commit also fixes a few match expression cases and parameterizes error messages.
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This PR tries to fix #2810.
It splits ImageFormat::Pnm into five different formats, so that each of the NetPBM formats has its own enum entry. This makes it possible for methods like DynamicImage::save to set the exact format based on the file extension, so that saving to a ".pgm" file will only try to save a PGM image.
This behavioral change may surprise anyone who previously was writing Rgba8 images to a ".pnm" file (actually producing PAM); this will now fail because PNM does not support alpha.
Other than this change, I have tried to avoid changing the behavior of the PnmDecoder, so that anyone using it directly will continue to see the same output. The only addition is a
with_dynamic_pnm_header()method to allow autoselecting a PNM header.I've also added some test images that cover all the main netpbm format variations; some of them are also used as reference images for updated PNM encoding tests in
tests/save_pnm.rs.The test images were all made by me, and licensed Apache-2.0 OR MIT OR CC-0. I've checked their references against NetPBM's conversion (using
pamdepth 65535 <$in | pamtopng -verbose >$out.png; the test image maxvals are chosen to be exactly convertible to 16 bit without rounding.).