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;
 }