Tiff: Support decoding f16 images - #3015
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Jul 16, 2026
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Thank you for this PR! I was about to send one with pretty much the same changes, because I somehow missed this one initially. Just leaving minor comments on where we diverged, feel free to disregard them all. 😃
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| for (half, out) in v.iter().zip(buf.as_chunks_mut::<4>().0.iter_mut()) { | ||
| *out = half.to_f32().to_ne_bytes(); | ||
| } |
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Maybe a bit more intuitive:
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| for (half, out) in v.iter().zip(buf.as_chunks_mut::<4>().0.iter_mut()) { | |
| *out = half.to_f32().to_ne_bytes(); | |
| } | |
| for (out, f) in bytemuck::cast_slice_mut::<u8, f32>(buf).iter_mut().zip(v) { | |
| *out = f.to_f32(); | |
| } |
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This is wrong, unfortunately. A slice of f32 must have an alignment requirement of 4, but the slice of u8 we have doesn't guarantee that. So bytemuck::cast_slice_mut will panic if the slice doesn't happen to be aligned to a multiple of 4. Users are allowed to give us slices of any alignment, so we have to be careful.
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Resolves #2795
Based on #3014
Changes:
ExtendedColorType::{L16F, La16F, Rgb16F, Rgba16F}.I added support for decoding f16 TIFF images by converting them to f32. This is a lossless conversion, if a bit inefficient. Importantly, this does not add new dependencies. I used the methods on
half::f16exposed by thetiffcrate to avoid addinghalfto our direct dependencies.This correctly decodes the f16 test images from image-rs/image-tiff#257. (Interestingly, a lot of programs don't seem to decode f16 correctly and interpret it as u16. From the programs I tested, only GIMP and Photoshop decode them correctly.)
TODO:
ExtendedColorType, because I probably messed up the order of variants, which breaks serde backwards compat.halfcrate has optimized methods for converting a slice of f16s to a slice of f32s, but that requires takinghalfas a direct dependency. We need to decide whether we want that.