Basically it's an accurate sharpener + antiringing. Usage: glsl-shader="~~/SSimSuperRes.glsl"

SSimSuperRes.glsl

// SSimSuperRes by Shiandow
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 3.0 of the License, or (at your option) any later version.
// 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.
// 
// You should have received a copy of the GNU Lesser General Public
// License along with this library.

//!HOOK POSTKERNEL
//!BIND HOOKED
//!SAVE LOWRES
//!HEIGHT NATIVE_CROPPED.h
//!WHEN NATIVE_CROPPED.h OUTPUT.h <
//!COMPONENTS 4
//!DESC SSSR Downscaling I

#define axis        1

#define offset      vec2(0,0)

#define MN(B,C,x)   (x < 1.0 ? ((2.-1.5*B-(C))*x + (-3.+2.*B+C))*x*x + (1.-(B)/3.) : (((-(B)/6.-(C))*x + (B+5.*C))*x + (-2.*B-8.*C))*x+((4./3.)*B+4.*C))
#define Kernel(x)   MN(0.334, 0.333, abs(x))
#define taps        2.0

#define Luma(rgb)   dot(rgb*rgb, vec3(0.2126, 0.7152, 0.0722))

vec4 hook() {
    float low  = ceil((HOOKED_pos - taps/input_size) * HOOKED_size - offset - 0.5)[axis];
    float high = floor((HOOKED_pos + taps/input_size) * HOOKED_size - offset - 0.5)[axis];

    float W = 0.0;
    vec4 avg = vec4(0);
    vec2 pos = HOOKED_pos;
    vec4 tex;

    for (float k = low; k <= high; k++) {
        pos[axis] = HOOKED_pt[axis] * (k - offset[axis] + 0.5);
        float rel = (pos[axis] - HOOKED_pos[axis])*input_size[axis];
        float w = Kernel(rel);

        tex.rgb = textureLod(HOOKED_raw, pos, 0.0).rgb * HOOKED_mul;
        tex.a = Luma(tex.rgb);
        avg += w * tex;
        W += w;
    }
    avg /= W;

    return vec4(avg.rgb, max(abs(avg.a - Luma(avg.rgb)), 5e-7));
}

//!HOOK POSTKERNEL
//!BIND LOWRES
//!SAVE LOWRES
//!WIDTH NATIVE_CROPPED.w
//!HEIGHT NATIVE_CROPPED.h
//!WHEN NATIVE_CROPPED.w OUTPUT.w <
//!COMPONENTS 4
//!DESC SSSR Downscaling II

#define axis        0

#define offset      vec2(0,0)

#define MN(B,C,x)   (x < 1.0 ? ((2.-1.5*B-(C))*x + (-3.+2.*B+C))*x*x + (1.-(B)/3.) : (((-(B)/6.-(C))*x + (B+5.*C))*x + (-2.*B-8.*C))*x+((4./3.)*B+4.*C))
#define Kernel(x)   MN(0.334, 0.333, abs(x))
#define taps        2.0

#define Luma(rgb)   dot(rgb*rgb, vec3(0.2126, 0.7152, 0.0722))

vec4 hook() {
    float low  = ceil((LOWRES_pos - taps/input_size) * LOWRES_size - offset - 0.5)[axis];
    float high = floor((LOWRES_pos + taps/input_size) * LOWRES_size - offset - 0.5)[axis];

    float W = 0.0;
    vec4 avg = vec4(0);
    vec2 pos = LOWRES_pos;
    vec4 tex;

    for (float k = low; k <= high; k++) {
        pos[axis] = LOWRES_pt[axis] * (k - offset[axis] + 0.5);
        float rel = (pos[axis] - LOWRES_pos[axis])*input_size[axis];
        float w = Kernel(rel);

        tex.rgb = textureLod(LOWRES_raw, pos, 0.0).rgb * LOWRES_mul;
        tex.a = Luma(tex.rgb);
        avg += w * tex;
        W += w;
    }
    avg /= W;

    return vec4(avg.rgb, max(abs(avg.a - Luma(avg.rgb)), 5e-7) + LOWRES_texOff(0).a);
}

//!HOOK POSTKERNEL
//!BIND PREKERNEL
//!BIND LOWRES
//!SAVE var
//!WIDTH NATIVE_CROPPED.w
//!HEIGHT NATIVE_CROPPED.h
//!WHEN NATIVE_CROPPED.h OUTPUT.h <
//!COMPONENTS 2
//!DESC SSSR var

#define spread      1.0 / 4.0

#define GetL(x,y)   PREKERNEL_tex(PREKERNEL_pt * (PREKERNEL_pos * input_size + tex_offset + vec2(x,y))).rgb
#define GetH(x,y)   LOWRES_texOff(vec2(x,y)).rgb

#define Luma(rgb)   dot(rgb*rgb, vec3(0.2126, 0.7152, 0.0722))
#define diff(x,y)   vec2(Luma((GetL(x,y) - meanL)), Luma((GetH(x,y) - meanH)))

vec4 hook() {
    vec3 meanL = GetL(0,0);
    vec3 meanH = GetH(0,0);
    for (int X=-1; X<=1; X+=2) {
        meanL += GetL(X,0) * spread;
        meanH += GetH(X,0) * spread;
    }
    for (int Y=-1; Y<=1; Y+=2) {
        meanL += GetL(0,Y) * spread;
        meanH += GetH(0,Y) * spread;
    }
    meanL /= (1.0 + 4.0*spread);
    meanH /= (1.0 + 4.0*spread);

    vec2 var = diff(0,0);
    for (int X=-1; X<=1; X+=2)
        var += diff(X,0) * spread;

    for (int Y=-1; Y<=1; Y+=2)
        var += diff(0,Y) * spread;

    return vec4(max(var / (1.0 + 4.0*spread), vec2(1e-6)), 0, 0);
}

//!HOOK POSTKERNEL
//!BIND HOOKED
//!BIND PREKERNEL
//!BIND LOWRES
//!BIND var
//!WHEN NATIVE_CROPPED.h OUTPUT.h <
//!DESC SSSR final pass

#define oversharp   0.5

// -- Window Size --
#define taps        3.0
#define even        (taps - 2.0 * floor(taps / 2.0) == 0.0)
#define minX        int(1.0-ceil(taps/2.0))
#define maxX        int(floor(taps/2.0))

#define Kernel(x)   cos(acos(-1.0)*(x)/taps) // Hann kernel

// -- Input processing --
#define var(x,y)    var_tex(var_pt * (pos + vec2(x,y) + 0.5)).rg
#define GetL(x,y)   PREKERNEL_tex(PREKERNEL_pt * (pos + tex_offset + vec2(x,y) + 0.5)).rgb
#define GetH(x,y)   LOWRES_tex(LOWRES_pt * (pos + vec2(x,y) + 0.5))

#define Luma(rgb)   dot(rgb*rgb, vec3(0.2126, 0.7152, 0.0722))

vec4 hook() {
    vec4 c0 = HOOKED_texOff(0);

    vec2 pos = HOOKED_pos * LOWRES_size - vec2(0.5);
    vec2 offset = pos - (even ? floor(pos) : round(pos));
    pos -= offset;

    vec2 mVar = vec2(0.0);
    for (int X=-1; X<=1; X++)
    for (int Y=-1; Y<=1; Y++) {
        vec2 w = clamp(1.5 - abs(vec2(X,Y)), 0.0, 1.0);
        mVar += w.r * w.g * vec2(GetH(X,Y).a, 1.0);
    }
    mVar.r /= mVar.g;

    // Calculate faithfulness force
    float weightSum = 0.0;
    vec3 diff = vec3(0);

    for (int X = minX; X <= maxX; X++)
    for (int Y = minX; Y <= maxX; Y++)
    {
        float R = (-1.0 - oversharp) * sqrt(var(X,Y).r / (var(X,Y).g + mVar.r));

        vec2 krnl = Kernel(vec2(X,Y) - offset);
        float weight = krnl.r * krnl.g / (Luma((c0.rgb - GetH(X,Y).rgb)) + GetH(X,Y).a);

        diff += weight * (GetL(X,Y) + GetH(X,Y).rgb * R + (-1.0 - R) * (c0.rgb));
        weightSum += weight;
    }
    diff /= weightSum;

    c0.rgb = ((c0.rgb) + diff);

    return c0;
}

For scaling factors >1 ... <=1.3 - only SSimSuperRes (together with ewa_lanczossharp) is in use.
>1.3 ... <2 - FSRCNNX + SSimDownscaler.
>2 - FSRCNNX + SSimSuperRes (together with ewa_lanczossharp)

In artoriuz's tests Mathematically Evaluating mpv's Upscaling Algorithms,lanczos seems to have higher scores in every upscale tests.Does this mean we should just use scale=lanczos with SSR since it has higher test scores and use less resources?

yes.

@igv Would you be interested in adding this as a shader for mpv? I'm mostly interested in debilinear & debicubic. Would be useful for bluray sources from older animes.

It's a "VapourSynth plugin to undo upscaling"

Maybe not suitable for normal downscaling?

It's a "VapourSynth plugin to undo upscaling"

Maybe not suitable for normal downscaling?

Yeah. Not suitable for normal downscaling. But I don't see a reason why you couldn't do a chain like "scale down with debilinear shader > scale up with spline36 from mpv".

Undoing the bilinear/bicubic upscaling companies did for the bluray release and using a better upscaling filter would result in a sharper image.

Sadly almost no production has all elements at the same resolution, and a lot of them have to be descaled per-scene if a descale is even possible.
Descaling without masking isn't a great idea, and that's where you lose enough performance that your filtering may not be realtime viable anymore.
You can use mpv's vapoursynth filtering feature btw.

your filtering may not be realtime viable anymore.

That's why this kind of process(if it was needed at all) was usually done by a BDrip release group. For example "GA-REI -zero" by VCB-Studio.

Sadly almost no production has all elements at the same resolution, and a lot of them have to be descaled per-scene if a descale is even possible. Descaling without masking isn't a great idea, and that's where you lose enough performance that your filtering may not be realtime viable anymore. You can use mpv's vapoursynth filtering feature btw.

True. If you'd want perfection you'd add masking. But undoing bilinear/bicubic is enough for my needs.

And yeah, I guess I'll see if I can make the vapoursynth plugin work. But a shader would be more flexible.

@lextra2 No, not interested.

I followed the steps but I can't seem to get it working. It is not listed when I press Shift+I+2. Please help.

Placed the glsl in C:\Users_user_\AppData\Roaming\mpv\shaders\

Tried either of both usage parameters in mpv.config;

glsl-shader="C:\Users\_user_\AppData\Roaming\mpv\shaders\SSimSuperRes.glsl"
glsl-shader="~~/SSimSuperRes.glsl"

@igv how does this shader compares to adaptive-sharpen.glsl? Would those be mutually exclusive or does that make sense to use both at the same time?

SSSR is probably better than AS when upscaling.

I've seen this on MadVR. There might be differences between madvr and this shader's implementation but what/how does it do its stuff exactly and what term should I use so I can Wikipedia/Google Scholar this? I've found the madvr explanation lacking/confusing and I wasn't sure if what I googled was the Superres I was looking for.

Is this designed for linear light or gamma light? Can I correct-downscaling = yes?

如果您谈论暗线，那是因为您的动画在伽马光下缩小了比例。 SSSR 没有考虑到这一点，ravu 和 fsrcnnx 可以。

Can linear-downscaling = yes solve this problem?