Qt Quick 3D - Stencil Outline Extension Example

#version 450
layout(location = 0) in vec2 qt_TexCoord0;
layout(location = 0) out vec4 fragColor;
layout(std140, binding = 0) uniform buf {
    mat4 qt_Matrix;
    float qt_Opacity;
    float value;
};
vec3 hsv2rgb(vec3 c)
{
    vec4 K = vec4(1.0, 2.0 / 3.0, 1.0 / 3.0, 3.0);
    vec3 p = abs(fract(c.xxx + K.xyz) * 6.0 - K.www);
    return c.z * mix(K.xxx, clamp(p - K.xxx, 0.0, 1.0), c.y);
}
#define TWO_PI 6.28318530718
void main()
{
    vec2 st = qt_TexCoord0;
    vec4 color = vec4(1.0);
    // Use polar coordinates instead of cartesian
    vec2 toCenter = vec2(0.5)-st;
    float angle = atan(toCenter.y,toCenter.x);
    float radius = length(toCenter)*2.0;
    if (radius <= 1.0) {
       color.rgb = hsv2rgb(vec3((angle/TWO_PI)+0.5,radius,value));
    } else {
        discard;
    }
    fragColor = color * qt_Opacity;
}