diff --git a/MetaballsKit/MarchingSquares.swift b/MetaballsKit/MarchingSquares.swift
index 2b0ec6b..ab17cec 100644
--- a/MetaballsKit/MarchingSquares.swift
+++ b/MetaballsKit/MarchingSquares.swift
@@ -35,11 +35,15 @@ class MarchingSquares {
     private(set) var gridGeometry: MTLBuffer?
 
     private var xSamples: Int {
-        return Int(field.size.x / sampleGridSize.x)
+        let xSize = field.size.x / sampleGridSize.x
+        let xRem = field.size.x % sampleGridSize.x
+        return Int(xSize + (sampleGridSize.x - xRem))
     }
 
     private var ySamples: Int {
-        return Int(field.size.y / sampleGridSize.y)
+        let ySize = field.size.y / sampleGridSize.y
+        let yRem = field.size.y % sampleGridSize.y
+        return Int(ySize + (sampleGridSize.y - yRem))
     }
 
     private var lastSamplesCount = 0
@@ -49,9 +53,12 @@ class MarchingSquares {
     }
 
     var contourIndexesCount: Int {
-        return (xSamples - 1) * (ySamples - 1)
+        return samplesCount
     }
 
+    /// Threadgroup size for the compute kernels.
+    private let threadgroupSize = MTLSize(width: 16, height: 16, depth: 1)
+
     init(field: Field) {
         self.field = field
         semaphore = DispatchSemaphore(value: 1)
@@ -136,6 +143,7 @@ class MarchingSquares {
 
     func populateGrid(withDevice device: MTLDevice) {
         guard lastSamplesCount != samplesCount else {
+            print("Populate requested, but lastSampleCount(\(lastSamplesCount) == samplesCount(\(samplesCount))")
             return
         }
 
@@ -181,9 +189,7 @@ class MarchingSquares {
         encoder.setBuffer(samplesBuffer, offset: 0, index: 2)
 
         // Dispatch!
-        // TODO: Kernel threadgroup size limit is 256. Figure out how to make this work.
-        let gridSize = MTLSize(width: xSamples, height: ySamples, depth: 1)
-        let threadgroupSize = MTLSize(width: xSamples, height: 1, depth: 1)
+        let gridSize = computeGridSize(forCellGridSize: Size(x: UInt32(xSamples), y: UInt32(ySamples)))
         encoder.dispatchThreads(gridSize, threadsPerThreadgroup: threadgroupSize)
 
         encoder.endEncoding()
@@ -206,12 +212,24 @@ class MarchingSquares {
         encoder.setBuffer(contourIndexesBuffer, offset: 0, index: 2)
 
         // Dispatch!
-        let gridSize = MTLSize(width: contourIndexesCount, height: 1, depth: 1)
-        let threadgroupSize = MTLSize(width: xSamples - 1, height: 1, depth: 1)
+        let gridSize = computeGridSize(forCellGridSize: Size(x: UInt32(xSamples - 1), y: UInt32(ySamples - 1)))
         encoder.dispatchThreads(gridSize, threadsPerThreadgroup: threadgroupSize)
 
         encoder.endEncoding()
     }
+
+    /// Grid size for the compute kernels.
+    func computeGridSize(forCellGridSize gridSize: Size) -> MTLSize {
+        let xs = Int(gridSize.x)
+        let ys = Int(gridSize.y)
+        let xrem = xs % threadgroupSize.width
+        let yrem = ys % threadgroupSize.height
+        // Our compute grid size is the next multiple of threadgroupSize larger than the current cell grid size.
+        let gridSize = MTLSize(width: xs + (threadgroupSize.width - xrem),
+                               height: ys + (threadgroupSize.height - yrem),
+                               depth: 1)
+        return gridSize
+    }
 }
 
 struct Variants {
diff --git a/MetaballsKit/Renderer.swift b/MetaballsKit/Renderer.swift
index 9cd65ea..59cd27d 100644
--- a/MetaballsKit/Renderer.swift
+++ b/MetaballsKit/Renderer.swift
@@ -206,7 +206,7 @@ public class Renderer: NSObject, MTKViewDelegate {
         if self.pixelGeometry == nil {
             self.pixelGeometry = self.pixelGeometry(forViewSize: view.drawableSize)
         }
-        let pixelGeometry = self.pixelGeometry!
+//        let pixelGeometry = self.pixelGeometry!
 
         if let renderPass = view.currentRenderPassDescriptor {
             // Render the per-pixel metaballs
@@ -240,7 +240,7 @@ public class Renderer: NSObject, MTKViewDelegate {
                     encoder.setVertexBuffer(marchingSquares.gridGeometry, offset: 0, index: 1)
                     encoder.setVertexBuffer(parametersBuffer, offset: 0, index: 2)
                     encoder.setFragmentBuffer(marchingSquares.contourIndexesBuffer, offset: 0, index: 0)
-                    encoder.drawPrimitives(type: .triangle, vertexStart: 0, vertexCount: Rect.geometry.count, instanceCount: marchingSquares.samplesCount)
+                    encoder.drawPrimitives(type: .triangle, vertexStart: 0, vertexCount: Rect.geometry.count, instanceCount: marchingSquares.contourIndexesCount)
                     encoder.endEncoding()
                     didEncode = true
                 }
diff --git a/MetaballsKit/Shaders/MarchingSquares.metal b/MetaballsKit/Shaders/MarchingSquares.metal
index 59af943..77ed09e 100644
--- a/MetaballsKit/Shaders/MarchingSquares.metal
+++ b/MetaballsKit/Shaders/MarchingSquares.metal
@@ -45,6 +45,11 @@ samplingKernel(constant MarchingSquaresParameters &parameters [[buffer(0)]],
                device float *samples [[buffer(2)]],
                uint2 position [[thread_position_in_grid]])
 {
+    if (position.x >= parameters.gridSize.x || position.y >= parameters.gridSize.y)
+    {
+        return;
+    }
+
     // Find the midpoint of this grid cell.
     const float2 point = float2(position.x * parameters.cellSize.x + (parameters.cellSize.x / 2.0),
                                 position.y * parameters.cellSize.y + (parameters.cellSize.y / 2.0));
@@ -68,38 +73,43 @@ kernel void
 contouringKernel(constant MarchingSquaresParameters &parameters [[buffer(0)]],
                  constant float *samples [[buffer(1)]],
                  device ushort *contourIndexes [[buffer(2)]],
-                 uint position [[thread_position_in_grid]])
+                 uint2 position [[thread_position_in_grid]])
 {
+    if (position.x >= (parameters.gridSize.x - 1) || position.y >= (parameters.gridSize.y - 1)) {
+        return;
+    }
+
     // Calculate an index based on the samples at the four points around this cell.
     // If the point is above the threshold, adjust the value accordingly.
     //      d--c    8--4
     //      |  | -> |  |
     //      a--b    1--2
-    uint a = position + parameters.gridSize.x;
-    uint b = position + parameters.gridSize.x + 1;
-    uint c = position + 1;
-    uint d = position;
+    uint rowSize = parameters.gridSize.x - 1;
+    uint d = position.y * rowSize + position.x;
+    uint c = d + 1;
+    uint b = d + rowSize + 1;
+    uint a = d + rowSize;
     uint index = (samples[d] >= 1.0 ? 0b1000 : 0) +
                  (samples[c] >= 1.0 ? 0b0100 : 0) +
                  (samples[b] >= 1.0 ? 0b0010 : 0) +
                  (samples[a] >= 1.0 ? 0b0001 : 0);
-    contourIndexes[position] = index;
+    contourIndexes[d] = index;
 }
 
 vertex RasterizerData
 gridVertexShader(constant Vertex *vertexes [[buffer(0)]],
-                 constant Rect *rects [[buffer(1)]],
+                 constant Rect *cells [[buffer(1)]],
                  constant RenderParameters &renderParameters [[buffer(2)]],
                  uint vid [[vertex_id]],
                  uint instid [[instance_id]])
 {
     Vertex v = vertexes[vid];
 
-    Rect rect = rects[instid];
+    Rect cell = cells[instid];
 
     RasterizerData out;
-    out.position = renderParameters.projection * rect.transform * float4(v.position.xy, 0, 1);
-    out.color = rect.color;
+    out.position = renderParameters.projection * cell.transform * float4(v.position.xy, 0, 1);
+    out.color = cell.color;
     out.textureCoordinate = v.textureCoordinate;
     out.instance = instid;
     return out;