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71 lines
4.2 KiB
Markdown
71 lines
4.2 KiB
Markdown
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# OpenRaster documents
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PhotoCraft opens and saves OpenRaster (`.ora`), the layered format Krita, MyPaint, GIMP, Pinta
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and others share. File › Open, recent files, drops, Save and Save As, the CLI (`convert`, `info`,
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`batch`) and automation (`doc.open`, `doc.save`, MCP) all use the same reader and writer in
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`photocraft-io`. The browser build accepts `.ora` too. A file is recognised by its `mimetype`
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entry, so an OpenRaster file with another extension still opens as one.
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## What is kept
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| | Open | Save |
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|---|---|---|
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| Pixel layers (any size and position, also off-canvas) | yes | yes, cropped to their pixels |
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| Groups, nested | yes | yes |
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| Pass-through groups | `isolation="auto"` without a blend mode | `isolation="auto"` |
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| Names, visibility, opacity | yes | yes |
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| Blend modes | the `svg:*` names, `svg:plus`, and Krita's `krita:*` names | `svg:*` where the spec has the mode, else `krita:*` (read back by Krita, Normal elsewhere) |
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| Locks | `edit-locked`, `alpha-preserve` | the same |
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| 8 and 16-bit | yes (the deepest layer sets the depth) | 8-bit documents as 8-bit PNGs, 16 and 32-bit as 16-bit PNGs |
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| Resolution | `xres` | `xres` and `yres` |
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| Merged image and thumbnail | not read (we render our own) | written (`mergedimage.png`, `Thumbnails/thumbnail.png`) |
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Saving reports what OpenRaster can't hold, layer by layer: pixel masks are applied to the
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layer's pixels, fill opacity is folded into the pixels, text, shape, fill and smart object layers
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are written as their rendered pixels, and adjustment layers, layer styles, vector masks,
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clipping, alpha channels, metadata and the colour profile are left out. Xor and Paint.NET's
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Color Burn and Color Dodge variants have no OpenRaster name and are saved as Normal. CMYK, Lab
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and other modes are refused with a message to convert first; Grayscale saves as RGB.
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OpenRaster elements other than `stack` and `layer` (MyPaint's text, for example), missing
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layer images and unknown composite operations open with a warning instead of failing the file.
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Krita's W3C Soft Light (`svg:soft-light`, `krita:soft_light_svg`) and its own Hard Mix open as
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Photoshop's Soft Light and Hard Mix, which differ slightly, also with a warning.
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## Limits and safety
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The reader takes the archive with the bounded ZIP reader of `photocraft-format` (no ZIP64,
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CRC-checked), parses `stack.xml` with `roxmltree` (16 MiB cap), and decodes each layer image
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through `photocraft-codecs` with size limits. Canvases are at most 300,000 px a side, at most
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8,192 layers and groups, nesting at most `MAX_GROUP_DEPTH` deep, and 1 GiB of decoded pixels in
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total. Malformed archives, XML, numbers and images return errors; truncated and corrupted files
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are covered by tests.
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## Verification
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Unit tests (`crates/io/src/ora/tests.rs`) round-trip layers, groups, pass-through, offsets,
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opacity, visibility, locks and every blend mode at 8 and 16 bit, check masks, rendered layer
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kinds, the archive layout and malformed input. `crates/automation/tests/openraster.rs` saves,
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reopens, edits and saves in place through the agent surface; the CLI test converts both ways.
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Real-file checks use Krita-authored oracles from
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[photocraft-corpus](https://github.com/storytold/photocraft-corpus) (`krita/`, made by
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`tools/krita-oracles/generate.py`). Each Krita file carries Krita's rendering of itself in
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`mergedimage.png`:
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```sh
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KRITA_TEST_DIR=/path/to/photocraft-corpus/krita cargo test -p photocraft-io --test krita_corpus -- --ignored --nocapture
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```
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2026-10-09, Krita 5.2.9: 30 of 34 `.ora` files render within 3/255 of Krita's merged image; the
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other four differ for known reasons listed in `crates/io/tests/krita_corpus.rs` (Krita's
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tie-break in Darker and Lighter Color, Hard Mix at a channel sum of exactly 1, and a
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linear-light 16-bit document that Krita blends in linear light). In the other direction,
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Krita opened every PhotoCraft re-export of those files and rendered 33 of them identically to
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its original; the exception is Photoshop's Soft Light, which is written as `svg:soft-light`.
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MyPaint and GIMP files have not been checked yet.
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Format reference: the [OpenRaster specification](https://www.openraster.org/). The reader and
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writer are written from the specification and from files Krita writes; no application code
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was used.
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