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Dylan M. Taylor
7d4d2f4648
The format half of PhotoCraft's OpenRaster import and export (crates/io/src/ora.rs at abeb16c96): the archive and its mimetype, stack.xml as a tree of layers and stacks with the attributes the file has, the 0.0.6 group isolation rule, and a writer. PNG coding, blend-mode mapping, rendering and warnings stay in the app. stack.xml is read with craft-xml and the archive with the zip crate.
38 lines
1.9 KiB
Markdown
38 lines
1.9 KiB
Markdown
# craft-ora
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OpenRaster documents (`.ora`), read and written. Written from the public specification
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(<https://www.openraster.org/>, `stack.xml` 0.0.x).
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An OpenRaster document is a ZIP whose first, stored entry is `mimetype` = `image/openraster`. It
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holds `stack.xml` (the layer tree), one PNG per layer, `mergedimage.png` (the whole image) and
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`Thumbnails/thumbnail.png`.
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- `is_ora`, `zip_mimetype`: recognise an archive by its first entry, without unzipping.
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- `Archive::open`: checks the mimetype and reads `stack.xml` into an `Image`: `Node::Layer`,
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`Node::Stack` and `Node::Other`, topmost first, with the attributes the file has (`name`,
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`visibility`, `opacity`, `composite-op`, `edit-locked`, `alpha-preserve`, `x`, `y`, `src`,
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`isolation`). `Archive::entry` reads a layer image or any other entry under a size limit.
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- `Image::explicit_group_isolation`: whether the file's version (0.0.6 and later) makes
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`isolation` explicit, or leaves pass-through to be inferred as older files do.
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- `Image::to_xml` and `Writer`: write `stack.xml` and the archive, mimetype first, entries
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stored.
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Layer images are PNG; decoding and encoding them is the caller's, as is what a `composite-op`
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means. `SVG_COMPOSITE_OPS` lists the names the specification defines.
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## Limits
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| Limit | Value |
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| --- | --- |
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| Canvas side | 300,000 px (`MAX_SIDE`); layer offsets up to four times that |
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| Canvas area | 1 GiB at 8 bytes per pixel (`MAX_PIXEL_BYTES`) |
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| Elements under the root stack | 8192 (`MAX_NODES`) |
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| `stack.xml` | 16 MiB, 512 levels of nesting, no document type declaration |
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| An entry | the caller's limit; `MAX_ENTRY` (512 MiB) is the suggested one for a layer image |
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## ZIP
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The archive is read and written with the `zip` crate directly. `zip_mimetype` (the leading stored
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`mimetype` entry), the bounded entry read and the mimetype-first writer are the same in every
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mimetype-first ZIP format and belong in a shared ZIP crate once there is one.
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