Implement arc rendering, works for increasing angle difference < 2pi

shader/arc.frag

@@ -1,18 +1,33 @@
 #version 430 core
 in vec4 gColor;
 in vec2 cenPosT;
-in float gRad;
-in float gWdth;
+in float gRadIn;
+in float gRadOut;
+in vec2 angles;
 out vec4 fColor;
 out float gl_FragDepth;
 
 void main()
 {
   vec2 pos = gl_FragCoord.xy;
- // fColor = vec4( gColor.xyz , 0 );
- // fColor = vec4( gColor.xyz , 1 - step(distance(pos,cenPos.xy),gRad) );
- // fColor = vec4( gColor.xyz , distance(pos,cenPos.xy)/gRad );
-  fColor = vec4( gColor.rgb, gColor.a * (1-step(gRad/2,distance(pos,cenPosT))) );
-  gl_FragDepth = max(gl_FragCoord.z , step(gRad/2,distance(pos,cenPosT)));
-  //fColor = vec4( 0.5,0,0 , 1 );
+  float d = distance(pos,cenPosT);
+  float dTest = max ( step(gRadOut/2,d)
+                    , 1 - step(gRadIn/2,d)
+                    );
+
+  vec2 posDiff = pos - cenPosT;
+  float sa = angles.x;
+  vec2 sv = vec2 (-sin(sa), cos(sa));
+  float saTest = dot(sv,posDiff) >= 0 ? 0 : 1;
+  float ea = angles.y;
+  vec2 ev = vec2 (-sin(ea), cos(ea));
+  float eaTest = dot(ev,posDiff) <= 0 ? 0 : 1;
+  float aTest = ea - sa < radians(180) ? max (saTest,eaTest) 
+                                       : min (saTest,eaTest);
+
+  float onArcTest = max(dTest,aTest); 
+
+  //gl_FragDepth = max(gl_FragCoord.z , step(gRadOut/2,distance(pos,cenPosT)));
+  gl_FragDepth = max(gl_FragCoord.z , onArcTest);
+  fColor = gColor;
 }