VapourSynth plugin to undo upscaling
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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.

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)

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.

Compilation

Linux

g++ -std=c++11 -shared -fPIC -O2 descale.cpp -o libdescale.so

Cross-compilation for Windows

x86_64-w64-mingw32-g++ -std=c++11 -shared -fPIC -O2 descale.cpp -static-libgcc -static-libstdc++ -Wl,-Bstatic -lstdc++ -lpthread -Wl,-Bdynamic -s -o libdescale.dll