SDF (Signed Distance Field) compute shader

sdf_gen.glsl

#version 430 core

layout(local_size_x = 16, local_size_y = 16) in;
layout(rgba8, binding = 0) uniform image2D inputImage;
layout(rgba8, binding = 1) uniform image2D outputTexture;

uniform int searchRadius = 32;

void main() {
    ivec2 pixelCoords = ivec2(gl_GlobalInvocationID.xy);
    ivec2 imageSize = imageSize(inputImage);

    // Read a pixel from the image
    vec4 inputPixel = imageLoad(inputImage, pixelCoords);

    // The input is BLACK (inside the glyph) and TRANSPARENT (outside the glyph)!
    // We need to calculate BOTH, and values 0.5 SHOULD be at the glyph's edge!
    bool isInsideGlyph = inputPixel.a == 1.0;
    float alphaToBeChecked = 1.0;
    if (isInsideGlyph)
        alphaToBeChecked = 0.0;

    // Search around us...
    float squaredDistance = 32 * 32;

    int startX = max(0, pixelCoords.x - searchRadius);
    int startY = max(0, pixelCoords.y - searchRadius);
    int endX = min(imageSize.x, pixelCoords.x + searchRadius + 1);
    int endY = min(imageSize.y, pixelCoords.y + searchRadius + 1);

    int ix = startX;
    int iy = startY;

    while (true) {
        // The second check is "No need to check self"
        if (ix == endX || (ix == pixelCoords.x && iy == pixelCoords.y)) {
            ix = startX;
            iy++;

            if (iy == endY)
                break;
        }

        vec4 distancePixel = imageLoad(inputImage, ivec2(ix, iy));
        if (distancePixel.a == alphaToBeChecked) {
            // We are inside the glyph!
            // But how far are we from the glyph?
            float dx = (ix - pixelCoords.x);
            float dy = (iy - pixelCoords.y);
            float thisSquaredDistance = (dx * dx) + (dy * dy);
            squaredDistance = min(squaredDistance, thisSquaredDistance);
        }

        ix++;
    }

    float distanceToEdge = sqrt(squaredDistance);

    // Value from 0.5 to 1.0
    float sdfValueAsRange = clamp(0.5 - (distanceToEdge / (2.0 * float(searchRadius))), 0.0, 1.0);

    if (isInsideGlyph) {
        // Don't ask me why do we need to use 0.98 instead of 1.0
        // I did it like this because there was a visible "jump" on the edges of the SDF
        imageStore(outputTexture, pixelCoords, vec4(0.98 - sdfValueAsRange, 0.98 - sdfValueAsRange, 0.98 - sdfValueAsRange, 1.0));
    } else {
        imageStore(outputTexture, pixelCoords, vec4(sdfValueAsRange, sdfValueAsRange, sdfValueAsRange, 1.0));
    }
}