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Descale

Video/Image filter to undo upscaling.

Includes a VapourSynth and AviSynth+ plugin

Usage

The VapourSynth 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, bool force, bool force_h, bool force_v, 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, bool force, bool force_h, bool force_v, 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, bool force, bool force_h, bool force_v, 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, bool force, bool force_h, bool force_v, 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, bool force, bool force_h, bool force_v, 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, bool force, bool force_h, bool force_v, int opt=0)

descale.Descale(clip src, int width, int height, str kernel, func custom_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, bool force, bool force_h, bool force_v, int opt=0)

The AviSynth+ plugin is used similarly, but without the descale namespace. Custom kernels are only supported in the VapourSynth plugin.

Custom kernels

# Debilinear
core.descale.Descale(src, w, h, custom_kernel=lambda x: 1.0 - x, taps=1)

# Delanczos
import math
def sinc(x):
	return 1.0 if x == 0 else math.sin(x * math.pi) / (x * math.pi)
taps = 3
core.descale.Descale(src, w, h, custom_kernel=lambda x: sinc(x) * sinc(x / taps), taps=taps)

# You can also use the python wrapper instead of calling the plugin functions directly
import descale
descale.Decustom(src, w, h, lambda x: 1.0 - x, taps=1)

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

By default only the VapourSynth plugin is compiled To build the AviSynth+ plugin, add -Dlibtype=avisynth or -Dlibtype=both to the meson command below.

Linux

$ meson build
$ ninja -C build

Cross-compilation for Windows

$ meson build --cross-file cross-mingw-x86_64.txt
$ ninja -C build