GLSL Levels Shader for mpv&libplacebo

[Selur]

Using libplacebo in Vapoursynth, I thought I try to create an alternative to Levels() and ended up with:

//!HOOK MAIN
//!BIND HOOKED
//!DESC Levels

// Define levels adjustment parameters
#define input_low 16.0
#define input_high 235.0
#define gamma 1.0
#define output_low 0.0
#define output_high 255.0

/* A comprehensive Levels adjustment shader for mpv
   Similar to AviSynth Levels filter
   v0.1
*/

vec4 hook() {
    vec4 color = HOOKED_texOff(0);
    
    // Normalize input parameters to [0, 1] range
    float input_low_norm = input_low / 255.0;
    float input_high_norm = input_high / 255.0;
    float output_low_norm = output_low / 255.0;
    float output_high_norm = output_high / 255.0;

    // Apply input low/high levels
    color.rgb = (color.rgb - input_low_norm) / (input_high_norm - input_low_norm);
    color.rgb = clamp(color.rgb, 0.0, 1.0);
    
    // Apply gamma correction using 1/gamma
    color.rgb = pow(color.rgb, vec3(1.0 / gamma));
    
    // Apply output low/high levels
    color.rgb = color.rgb * (output_high_norm - output_low_norm) + output_low_norm;
    
    return color;
}

using:

clip = core.resize.Bicubic(clip=clip, format=vs.YUV444P16, range_s="limited")
with open("F:/Hybrid/64bit/vsfilters/GLSL/parameterized/Levels.glsl") as glslf:
  glsl = glslf.read()
glsl = glsl.replace('#define input_low 16.0', '#define input_low 16')
glsl = glsl.replace('#define input_high 235.0', '#define input_high 235')
glsl = glsl.replace('#define output_low 0.0', '#define output_low 16')
glsl = glsl.replace('#define output_high 255.0', '#define output_high 235')
glsl = glsl.replace('#define gamma 1.0', '#define gamma 1')
clip = core.placebo.Shader(clip=clip, shader_s=glsl, width=clip.width, height=clip.height)

which properly changes the defines to:

#define input_low 16
#define input_high 235
#define gamma 1
#define output_low 16
#define output_high 235

I expected the in- and output to be identical, but there seems to be a mistake, since the lower values all get clamped.


Does anyone see the mistake I made?
I suspect I made a logical mistake in the scaling of the input parameters,..

Cu Selur

[Selur]

Suspecting that the issue to early clamping, I used:

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

    // Normalize input and output parameters to [0, 1] range
    float input_low_norm = input_low / 255.0;
    float input_high_norm = input_high / 255.0;
    float output_low_norm = output_low / 255.0;
    float output_high_norm = output_high / 255.0;

    // Apply input low/high levels
    vec3 normalized = (color.rgb - input_low_norm) / (input_high_norm - input_low_norm);

    // Apply gamma correction using 1/gamma
    vec3 gamma_corrected = pow(normalized, vec3(1.0 / gamma));

    // Apply output low/high levels
    vec3 adjusted_output = gamma_corrected * (output_high_norm - output_low_norm) + output_low_norm;

    // Ensure values are within the [0, 1] range
    color.rgb = clamp(adjusted_output, 0.0, 1.0);

    return color;
}

=> that looks different, but still wrong.

Cu Selur