Descale
VapourSynth plugin to undo upscaling.
Usage
The plugin itself supports every constant input format. If the format is subsampled, left-aligned chroma planes are always assumed.
The included python wrapper, contrary to using the plugin directly, doesn't descale the chroma planes but scales them normally with Spline36.
descale.Debilinear(clip src, int width, int height, float src_left=0.0, float src_top=0.0, float src_width=width, float src_height=height, int opt=0)
descale.Debicubic(clip src, int width, int height, float b=0.0, float c=0.5, float src_left=0.0, float src_top=0.0, float src_width=width, float src_height=height, int opt=0)
descale.Delanczos(clip src, int width, int height, int taps=3, float src_left=0.0, float src_top=0.0, float src_width=width, float src_height=height, int opt=0)
descale.Despline16(clip src, int width, int height, float src_left=0.0, float src_top=0.0, float src_width=width, float src_height=height, int opt=0)
descale.Despline36(clip src, int width, int height, float src_left=0.0, float src_top=0.0, float src_width=width, float src_height=height, int opt=0)
descale.Despline64(clip src, int width, int height, float src_left=0.0, float src_top=0.0, float src_width=width, float src_height=height, int opt=0)
descale.Descale(clip src, int width, int height, str kernel, int taps=3, float b=0.0, float c=0.0, float src_left=0.0, float src_top=0.0, float src_width=width, float src_height=height, int opt=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
$ meson build
$ ninja -C build
Cross-compilation for Windows
$ meson build --cross-file cross-mingw-x86_64.txt
$ ninja -C build