Working radial distortion

shader/texture/barrel.vert

@@ -4,14 +4,18 @@ layout (location = 1) in vec2 rad1;
 layout (location = 2) in vec2 rad2;
 layout (location = 3) in float factor;
 layout (std140, binding = 0) uniform TheMat { mat4 theMat; } ;
+out vec2 vCen;
 out vec2 vRad1;
 out vec2 vRad2;
 out float vFactor;
 void main()
 {
-  gl_Position = theMat * vec4(pos,0,1);
-  //gl_Position = vec4(1,1,0,1);
+  gl_Position = vec4(0,0,0,1);
+  vCen = (theMat * vec4(pos,0,1)).xy;
+  //vCen = (theMat * vec4(0,0,0,1)).xy;
   vRad1 = (theMat * vec4(rad1,0,1)).xy;
+  //vRad1 = (theMat * vec4(0,200,0,1)).xy;
   vRad2 = (theMat * vec4(rad2,0,1)).xy;
+  //vRad2 = (theMat * vec4(200,0,0,1)).xy;
   vFactor = factor;
 }