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VapourSynth plugin to undo upscaling
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README.md improved fork explanation Feb 5, 2018
descale.cpp
descale.py moved matrix caching to global scope and did all the related changes … Oct 17, 2017

README.md

Descale

VapourSynth plugin to undo upscaling.

Usage

The plugin itself only supports GrayS, RGBS, and YUV444PS input. The included python wrapper supports YUV (every subsampling), Gray, and RGB of every bitdepth.

Descalers:
descale.Debilinear(clip src, int width, int height, float src_left=0.0, float src_top=0.0)

descale.Debicubic(clip src, int width, int height, float b=1/3, float c=1/3, float src_left=0.0, float src_top=0.0)

descale.Delanczos(clip src, int width, int height, int taps=3, float src_left=0.0, float src_top=0.0)

descale.Despline16(clip src, int width, int height, float src_left=0.0, float src_top=0.0)

descale.Despline36(clip src, int width, int height, float src_left=0.0, float src_top=0.0)

descale.Despline36(clip src, int width, int height, int kernel="bilinear" float src_left=0.0, float src_top=0.0)
CacheSize:

descale.CacheSize(int size=-1) changes the internal size of the matrix lru cache. -1 (default) means no limit.

How does this work?

Resampling can be described as A x = b.

A is an n x m matrix with m being the input dimension and n the output dimension. x is the original vector with m elements, b is the vector after resampling with n elements. We want to solve this equation for x.

To do this, we extend the equation with the transpose of A: A' A x = A' b.

A' A is now a banded symmetrical m x m matrix and A' b is a vector with m elements.

This enables us to use LDLT decomposition on A' A to get LD L' = A' A. LD and L are both triangular matrices.

Then we solve LD y = A' b with forward substitution, and finally L' x = y with back substitution.

We now have the original vector x.

Differences of this Fork to the Original

This fork generates the above-described matrices lazily (or reuses them from cache) when needed and caches them globally (instead of generating them during instantiation).
This has three main advantages.

  • Matrices can be reused in case of multiple instances of descale inside the same vapoursynth context.
  • The Matrices for multiple filter instances with different parameters in the same context will be generated in the vapoursynth thread-pool whereas the original matrix creation happens on filter-tree generation itself and is therefore forced to run inside the single-threaded python (or whatever generates the filter-tree) context.
  • Matrices now can be used for either orientation which means that for example for an aspect ratio of 1:1 only 1 matrix would be needed.

Getnative profits from the second and third point and the getnative functionality within the Tsuzuru discord Bot additionally profits from the first point tremendously.

I've made some very crude benchmarks without spotting any performance differences in a normal use-case so there seems to be no obvious disadvantage to the original.

Compilation

Linux

g++ -std=c++17 -shared -fPIC -O2 descale.cpp -o libdescale.so

Cross-compilation for Windows

x86_64-w64-mingw32-g++ -std=c++17 -shared -O2 descale.cpp -static-libgcc -Wl,-Bstatic -lstdc++ -lpthread -s -o libdescale.dll

I couldn't spot any problems with higher optimization settings but it seems to actually perform worse on some systems.

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