qskinny/src/nodes/shaders/arcshadow.frag

uniform lowp float qt_Opacity;
// arc
uniform lowp vec4 rect;        // arc's rectangle on screen in pixel x,y,w,h
uniform lowp float radius;     // arc's radius [0.0, 1.0]
uniform lowp float thickness;  // arc's thickness
uniform lowp float startAngle; // arc's start angle
uniform lowp float spanAngle;  // arc's span angle
// shadow
uniform lowp vec4 color;       // shadow's color
uniform lowp vec2 offset;      // shadow's offset (x,y) : [-1.0, +1.0]x[-1.0,+1.0]
uniform lowp float extend;     // shadow length around

float sdRing( in vec2 p, in vec2 n, in float r, in float th )
{
    p.x = abs(p.x);
    
    p = mat2(n.x,n.y,-n.y,n.x)*p;

    return max( abs(length(p)-r)-th*0.5,
                length(vec2(p.x,max(0.0,abs(r-p.y)-th*0.5)))*sign(p.x) );
}

void main()
{
    // uniforms
    float shadowSize = extend;           // px >= 0.0
    vec2 iResolution = rect.zw;       

    // normalized pixel coordinates
    vec2 p = (2.0 * gl_FragCoord.xy - iResolution.xy) / iResolution.xy; // xy for ellipse
    float shadowSizeUv = shadowSize / min(iResolution.x, iResolution.y);
             
    float t = radians(abs(spanAngle) / 2.0);
    vec2 cs = vec2(cos(t),sin(t));
  
    // rotation
    float ra = radians(startAngle + spanAngle / 2.0);
    {        
        p = p + offset;

        float sin_ra = sin(ra);
        float cos_ra = cos(ra);
        mat2 transform = mat2
        ( 
            cos_ra, -sin_ra,
            sin_ra,  cos_ra
        );

        p = transform * p;
    }
  
    // distance
    float d = sdRing(p, cs, radius, thickness);
    float e = 1.0 / shadowSizeUv;
        
    // coloring
    float v = 1.0 - abs(d) * e;
    float a = d >= 0.0 && abs(d) < e ? v : 1.0; // alpha
    gl_FragColor = vec4(color.rgb, 1.0) * a * qt_Opacity;
}