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Dylan M. TaylorandTed Mao
305bc3c971
* Add craft-gainmap: ISO 21496-1 and hdrgm gain map metadata, compute and apply, from LightCraft Moved from lightcraft-codecs' gainmap.rs at 8ef35987 with its tests: the metadata reader and writer and the arithmetic. The JPEG container (MPF index, two encoded images) stays in the app. * craft-gainmap: Apple legacy gain-map reconstruction Apple's legacy single-channel gain maps interpolate a linear gain ratio, not the log2 gain of ISO 21496-1: apple::apply rebuilds the HDR rendition from a decoded base and the map, with the headroom taken from the maker-note pair or the HDRGainMapHeadroom tag, checked inputs and pixel-centre sampling. Ported from storytold/lightcraft#3760 (which carries #1978). --------- Co-authored-by: Ted Mao <tedmao@gmail.com>
37 lines
2.0 KiB
Markdown
37 lines
2.0 KiB
Markdown
# craft-gainmap
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HDR gain maps for the Craft apps, from LightCraft's `lightcraft-codecs`: an SDR base image plus a
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small map of gains that rebuilds the HDR rendition on a display with headroom. This crate is the
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metadata and the arithmetic. It does not encode or decode images and does not know a container.
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No `unsafe`, no panics on malformed input; the only dependency is `thiserror`.
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```toml
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craft-gainmap = { git = "https://github.com/storytold/craft-libs", rev = "<commit>" }
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```
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| `GainMapMeta::from_iso` / `to_iso` | ISO 21496-1 binary metadata (the JPEG APP2 payload after `ISO_URN`; the same bytes HEIF and AVIF carry) |
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| `GainMapMeta::from_hdrgm_xmp` / `to_gain_map_xmp` | Adobe `hdrgm` XMP (Ultra HDR), attributes or `rdf:Seq` lists; the writer adds Apple's description of the same map |
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| `compute` | measures a 1 or 3-channel 8-bit gain map from SDR and HDR renditions in linear light |
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| `apply` | rebuilds the HDR rendition from a base and its map, at a weight for the display's headroom |
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| `primary_iso_marker`, `primary_xmp` | what the primary image carries to say the file has a gain map |
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| `apple::apply`, `apple::Metadata` | Apple's legacy single-channel gain maps (a linear ratio, headroom from the maker note) |
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```rust
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use craft_gainmap::{GainMapOptions, apply, compute};
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// `sdr` and `hdr`: the same picture, linear RGB in the same primaries.
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let (map, meta) = compute(&sdr, &hdr, width, height, [0.2126, 0.7152, 0.0722], &GainMapOptions::default())?;
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let mut rendition = sdr.clone();
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apply(&mut rendition, width, height, &map, &meta, 1.0)?;
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```
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Metadata readers return `None` for anything a decoder could not use: unknown versions, an HDR
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base, zero denominators, non-finite values, gammas that are not positive. `compute` and `apply`
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check every size (with overflow) against the buffers they are given and return `Error` instead
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of indexing past them.
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What stays with the caller: encoding the base and the map, the container (for a JPEG, the MPF
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index that locates the second image), and colour management of the base.
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