desktopRuntime/resources/assets/minecraft/shaders/post/box_blur.fsh

#version 150

uniform sampler2D InSampler;

in vec2 texCoord;
in vec2 sampleStep;

uniform float Radius;

out vec4 fragColor;

// This shader relies on GL_LINEAR sampling to reduce the amount of texture samples in half.
// Instead of sampling each pixel position with a step of 1 we sample between pixels with a step of 2.
// In the end we sample the last pixel with a half weight, since the amount of pixels to sample is always odd (actualRadius * 2 + 1).
void main() {
    vec4 blurred = vec4(0.0);
    float actualRadius = round(Radius);
    for (float a = -actualRadius + 0.5; a <= actualRadius; a += 2.0) {
        blurred += texture(InSampler, texCoord + sampleStep * a);
    }
    blurred += texture(InSampler, texCoord + sampleStep * actualRadius) / 2.0;
    fragColor = blurred / (actualRadius + 0.5);
}