[Metaballs] Do all the texture generation in the fragment shader

Metaballs/Shaders.metal

@@ -24,6 +24,13 @@ typedef struct {
     float2 textureCoordinate;
 } RasterizerData;
 
+typedef struct {
+    int2 size;
+    int numberOfBalls;
+} Parameters;
+
+typedef half3 Ball;
+
 vertex RasterizerData
 passthroughVertexShader(uint vid                    [[vertex_id]],
                         constant Vertex* vertexes   [[buffer(0)]])
@@ -35,19 +42,36 @@ passthroughVertexShader(uint vid                    [[vertex_id]],
     return out;
 }
 
+float sampleAtPoint(float2, constant Ball*, uint);
+
 fragment float4
-sampleToColorShader(RasterizerData in           [[stage_in]],
-                    texture2d<half> samples     [[texture(0)]])
+sampleToColorShader(RasterizerData in               [[stage_in]],
+                    constant Parameters& parameters [[buffer(0)]],
+                    constant Ball* balls            [[buffer(1)]])
 {
-    constexpr sampler textureSampler(mag_filter::linear, min_filter::linear);
-    const half4 sample = samples.sample(textureSampler, in.textureCoordinate);
+    const float sample = sampleAtPoint(in.textureCoordinate, balls, parameters.numberOfBalls);
 
     float4 out;
-    if (sample.r > 1.0) {
+    if (sample > 1.0) {
         out = float4(0.0, 1.0, 0.0, 0.0);
-    }
-    else {
+    } else {
         out = float4(0.0, 0.0, 0.0, 0.0);
     }
     return out;
 }
+
+float
+sampleAtPoint(float2 point,
+              constant Ball* balls,
+              uint count)
+{
+    float sample = 0.0;
+    for (uint i = 0; i < count; i++) {
+        constant Ball& ball = balls[i];
+        float r2 = ball.z * ball.z;     // Radius stored in z coordinate.
+        float xDiff = point.x - ball.x;
+        float yDiff = point.y - ball.y;
+        sample += r2 / ((xDiff * xDiff) + (yDiff * yDiff));
+    }
+    return sample;
+}