Flat shading quads

I'm trying to draw a voronoi diagram using a shader. I do this by creating a collection of quads, each centred at a point in the diagram. The shader below uses the the depth buffer to turn each quad into a cone, which creates the voronoi effect.

That much works but what I want to do next is to colour each voronoi cell by sampling the colour of a texture at that point in space. I tried doing this by setting gl_FrontColor in the vertex shader but the gl_Color variable in the fragment shader is interpolated between the four vertices of the quad which is not what I want. I want the whole cell to be flat shaded based on the colour of the centre point.

Ideally, I'd be using a point sprite for this but I understand they aren't working correctly in current versions of Unity (or has this been fixed?)

Any suggestions how to solve this problem?

Code (csharp):

1.  
2. Shader "Custom/Voronoi" {
3.     Properties {
4.          _MainTex ("RGBA Texture Image", 2D) = "white" {}
5.     }
6.     SubShader {
7.         Pass {
8.             Cull Off
9.             GLSLPROGRAM
10.            
11.             uniform sampler2D _MainTex;    
12.  
13.             #ifdef VERTEX
14.             void main() {
15.                 gl_TexCoord[0] = gl_MultiTexCoord0;
16.                 gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
17.                
18.                 // sample the texture at the vertex location
19.                 gl_FrontColor = texture2D(_MainTex, gl_Vertex.xy);
20.             }
21.  
22.             #endif
23.  
24.  
25.             #ifdef FRAGMENT
26.  
27.             void main() {
28.                 // find the distance to the middle of the quad
29.                 vec2 v = 2. * gl_TexCoord[0].xy - 1.;  
30.                 float dist = length(v);
31.                
32.                 // make the quad circular by discarding
33.                 // fragments more that 1 away from the centre
34.                 if (dist > 1.) {
35.                     discard;
36.                 }
37.  
38.                 // make a cone by setting the depth equal to the distance
39.                 gl_FragDepth = dist;
40.                
41.                 // set the colour -- this doesn't work how I want
42.                 gl_FragColor = gl_Color;
43.                
44.             }
45.            
46.             #endif
47.  
48.             ENDGLSL
49.         }
50.     }
51. }
52.  

 

Here is an example of the kind of output I am trying to achieve.

 

My hacky solution: set the normals of the quad to point towards the centre point. Then do

Code (csharp):

1.  
2.             #ifdef VERTEX
3.             void main() {
4.                 gl_TexCoord[0] = gl_MultiTexCoord0;
5.                 gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
6.                
7.                 // sample the texture at the centre of the quad
8.                 vec2 st = gl_Vertex.xy + gl_Normal.xy;
9.                 gl_FrontColor = texture2D(_MainTex, st);
10.             }
11.  
12.             #endif
13.  

That way all four vertices end up with the same colour, so interpolation has no effect.