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24 lines
1.4 KiB
Markdown
24 lines
1.4 KiB
Markdown
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# Unsharp Mask performance
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Large-radius native Unsharp Mask uses separable FFT convolution with the original
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discrete Gaussian coefficients and the existing `rustfft` dependency. A streamed
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separable path preserves the original tap order for exact processing.
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FFT is eligible for radii 64–1000 with at least 65,536 output pixels, a nonzero
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amount and a zero or negative threshold. Positive thresholds, unstable samples,
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unsupported geometry and insufficient scratch use the exact path. WebAssembly
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retains exact processing. Alpha, selections, edges and cancellation are preserved.
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Smart Sharpen, High Pass and shared blur math are unchanged.
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FFT changes summation order and permits small rounding differences. The measured
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maximum was one 8-bit storage level, approximately one 16-bit level, and 8.762e-6
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for the HDR fixture. Exact-path comparisons were byte-identical. Transform axes
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are capped at 32,768; FFT scratch grouping has a 1 GiB budget, excluding document
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tiles. This is not a whole-process memory limit.
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Before/after numbers, benchmark methodology, numerical tolerances and preview
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measurement limits are recorded in [PR #2987](https://github.com/storytold/photocraft/pull/2987).
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The comparisons use synthetic images and measure CPU computation, not slider
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event-to-screen latency. No published performance budget or rendering-parity
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floor is raised by these measurements.
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