Lens distortion for Dolphin emulator (for VR viewers) as custom shader

cardboard.glsl

/*
[configuration]

[OptionRangeInteger]
GUIName = Distortion amount
OptionName = DISTORTION_FACTOR
MinValue = 1
MaxValue = 10
StepAmount = 1
DefaultValue = 4

[OptionRangeInteger]
GUIName = Eye Distance Offset
OptionName = EYE_OFFSET
MinValue = 0
MaxValue = 10
StepAmount = 1
DefaultValue = 5

[OptionRangeFloat]
GUIName = Zoom adjustment
OptionName = SIZE_ADJUST
MinValue = 0.0
MaxValue = 1.0
StepAmount = 0.1
DefaultValue = 0.5

[OptionRangeFloat]
GUIName = Aspect Ratio adjustment
OptionName = ASPECT_ADJUST
MinValue = 0.0
MaxValue = 1.0
StepAmount = 0.1
DefaultValue = 0.5

[/configuration]
*/


void main() {
    // Base Cardboard distortion parameters
    float factor = GetOption(DISTORTION_FACTOR) * 0.01f;
    float ka = factor * 3.0f;
    float kb = factor * 5.0f;

    float sizeAdjust = 1.0f - GetOption(SIZE_ADJUST) + 0.5f;

    float aspectAdjustment = 1.25f - GetOption(ASPECT_ADJUST);

    // offset centering per eye
    float stereoOffset = GetOption(EYE_OFFSET) * 0.01f;
    float offsetAdd;

    // layer0 = left eye, layer1 = right eye
    if (layer == 1) {
      offsetAdd = stereoOffset;
    } else {
      offsetAdd = 0 - stereoOffset;
    }

    // convert coordinates to NDC space
    float2 fragPos = (GetCoordinates() - 0.5f - vec2(offsetAdd, 0.0f)) * 2.0f;

    // Calculate the source location "radius" (distance from the centre of the viewport)
    float destR = length(fragPos);
    float srcR = destR * sizeAdjust + ( ka * pow(destR,2) + kb * pow(destR,4));

    // Calculate the source vector (radial)
    vec2 correctedRadial = normalize(fragPos) * srcR;

    // fix aspect ratio
    vec2 widenedRadial = correctedRadial * vec2(aspectAdjustment, 1.0f);

    // Transform the coordinates (from [-1,1]^2 to [0, 1]^2)
    vec2 uv = (widenedRadial/2.0f) + vec2(0.5f) + vec2(offsetAdd, 0.0f);

    // Sample the texture at the source location
    if (uv[0] > 1.0 || uv[1] > 1.0 || uv[0] < 0.0 || uv[1] < 0.0) {
      // black if beyond bounds
      SetOutput(float4(0,0,0,0));
    } else {
      SetOutput(SampleLocation(uv));
    }

}

Please note that my code has now been merged into the official Dolphin project:
dolphin-emu/dolphin#4209

As of this moment, you'll have to download the latest development release here:
https://dolphin-emu.org/download/

The shader is now named lens_distortion in the Post-Processing dropdown.

Hello,
I have a problem with this shader.
When initializing a game at startup, I get a error :
error C1503 undefined variable "layer"

I note that this refers to line 56 :
"if (layer == 1)"
But this variable is never defined higher in the code...how to fix this?
I read that your shader is now integrated in Dophin under the name lens_distortion.glsl ...but the shader codes are strictly identical, and the result is unfortunately also strictly identical.... C1503 error :/

Thanks in advance for your answer, I really want to test your shader ;)