Skybox darkening shader?

#include "ReShade.fxh"

// I hereby decree this shader to be version 1

// Spectral power distribution
static const float3 SPD = float3(0.412656, 0.715158, 0.072186);

static const float SPD_MAG = 1.73205;

uniform float amount <
    ui_category = "General";
    ui_label = "Amount";
    ui_max = 4;
    ui_min = -4;
    ui_step = 0.0000001;
    ui_tooltip = "Effect intensity";
    ui_type = "slider";
> = 1;

uniform float skyboxDepth <
    ui_category = "General";
    ui_label = "Skybox Depth";
    ui_max = 1;
    ui_min = -1;
    ui_step = 0.0001;
    ui_tooltip = "Objects at or past this threshold will be treated as part of the skybox. Use negative values if the skybox has a zero depth.";
    ui_type = "slider";
> = 1;

uniform float skyboxKnee <
    ui_category = "General";
    ui_label = "Skybox Depth Knee";
    ui_max = 1;
    ui_min = 0;
    ui_step = 0.0001;
    ui_tooltip = "I guess this might prevent the threshold from being visible? Leave at 1 if not seeing any issues";
    ui_type = "slider";
> = 1;

uniform float darkenAmount <
    ui_category = "Darkening";
    ui_label = "Strength";
    ui_max = 4;
    ui_min = -4;
    ui_step = 0.0000001;
    ui_tooltip = "How much darker would dark areas be";
    ui_type = "slider";
> = 0;

uniform float darkenKnee <
    ui_category = "Darkening";
    ui_label = "Knee";
    ui_max = 4;
    ui_min = 0;
    ui_step = 0.0000001;
    ui_tooltip = "How smooth will the transition between affected and unaffected areas be";
    ui_type = "slider";
> = 1;

uniform float saturateAmount <
    ui_category = "Saturation";
    ui_label = "Strength";
    ui_max = 4;
    ui_min = -4;
    ui_step = 0.0000001;
    ui_tooltip = "Darker colors appear more saturated to a human eye; use this slider to account for that";
    ui_type = "slider";
> = 1;

uniform float saturateKnee <
    ui_category = "Saturation";
    ui_label = "Knee";
    ui_max = 4;
    ui_min = 0;
    ui_step = 0.0000001;
    ui_tooltip = "How smooth will the transition between affected and unaffected areas be";
    ui_type = "slider";
> = 1;

// PD80 Base Effects
float getLuma(in float3 col)
{
    return dot(col, SPD);
}

// PD80 Base Effects
float3 sat(float3 col, float fac)
{
    return saturate(lerp(getLuma(col.rgb), col.rgb, fac));
}

float3 exposure(float3 col, float fac)
{
    // Why? Hell if I know, I'm just not gonna touch that
    fac = fac < 0.0f ? fac * 0.333f : fac;

    float3 preexp = col * exp2(fac);
    float3 blowout = (max(0, preexp - 1));
    float3 adjust = float3(blowout.g + blowout.b, blowout.r + blowout.b, blowout.r + blowout.g);

    return preexp + adjust;
}

float3 PS_SkyboxDarkenFXmain(float4 vpos : SV_Position, float2 texcoord : TexCoord) : SV_Target
{
    float3 input = tex2D(ReShade::BackBuffer, texcoord).rgb;
    float fragDepth = pow(ReShade::GetLinearizedDepth(texcoord), skyboxKnee);

    float3 result = input;

    if(skyboxDepth <= 0.0)
    {
        skyboxDepth = -skyboxDepth;
        fragDepth = 1.0 - fragDepth;
    }

    if(fragDepth >= skyboxDepth)
    {
        float shadow = 1.0 - getLuma(input) / SPD_MAG;
        result = exposure(result, pow(shadow, darkenKnee) * -darkenAmount);
        result = lerp(result, sat(result, saturateAmount), pow(shadow, saturateKnee));
    }

    return lerp(input, result, amount);
}

technique SkyboxDarken
{
    pass draw
    {
        VertexShader = PostProcessVS;
        PixelShader = PS_SkyboxDarkenFXmain;
    }
}