shader/ellipsis_frag.glsl


layout(location = 0) out vec4 diffuse;
in vec2 uvcoord;

uniform vec2 radii;
#ifdef FILL
uniform vec4 color;
#endif
#ifdef BORDER
uniform float thickness;
uniform vec4 border_color;
#endif

void main(void) {
    // Calculate center of the ellipse
    vec2 center = radii;

    // Calculate position relative to center
    vec2 pos = uvcoord - center;

    // For an ellipse, we need to calculate normalized distance based on the formula:
    // (x/a)² + (y/b)² = 1 where a and b are the semi-axes
    vec2 normalized = pos / center;

    // Calculate squared distance in ellipse space
    // Value = 1.0 exactly at the ellipse boundary
    float dist_squared = dot(normalized, normalized);

    #ifndef BORDER
    // For filled ellipse
    if (dist_squared > 1.0) {
        discard; // Outside the ellipse
    }
    diffuse = color;
    #else // BORDER
    // For outlined ellipse
    // Calculate inner border threshold based on thickness
    // We need to determine the inner ellipse boundary
    float border_ratio = thickness / min(center.x, center.y);

    // The inner ellipse has a smaller radius but follows the same equation
    // We need to calculate what normalized value corresponds to the inner ellipse
    float inner_threshold = 1.0 - border_ratio;
    inner_threshold = inner_threshold * inner_threshold; // Square it because we're comparing to dist_squared

    if (dist_squared > 1.0) {
        discard; // Outside the ellipse
    } else if (dist_squared > inner_threshold) {
        // In the border region
        diffuse = border_color;
    } else {
        // Inside the outline
        #ifdef FILL
        diffuse = color;
        #else
        discard;
        #endif
    }
    #endif // BORDER
}