public IEnumerable <Blob> FindBlobs(Bitmap img)
{
var width = img.Width;
var height = img.Height;
var imgData = ImageUtils.ReadPixels(img, out int bytesPerPixel, out int stride);
// Copy image to two-dimensional array for easier and more efficient navigation through the image.
var imageMap = new byte[img.Width, img.Height];
for (int i = 0; i < imgData.Length; i++)
{
var x = (i % stride) / bytesPerPixel;
var y = (i / stride);
imageMap[x, y] = imgData[i];
}
// the set of pixels that we already processed
var visited = new HashSet <PixelLocation>(PixelLocation.EqualityComparer);
// Iterate column-wise.
for (int x = 0; x < width; x++)
{
for (int y = 0; y < height; y++)
{
var c = new PixelLocation(x, y);
if (visited.Contains(c))
{
continue;
}
visited.Add(c);
var pixelValue = imageMap[x, y];
if (pixelValue == _blobColorValue)
{
// Once we find a matching pixel, we start blob detection.
var blob = CollectContiguousBlob(imageMap, c, visited);
if (blob.Pixels.Count > _minBlobPixels)
{
yield return(blob);
}
}
}
}
}
private void AddNeighbouringPixels(
PixelLocation fromLocation,
Queue <PixelLocation> toQueue,
HashSet <PixelLocation> visited,
int minX, int minY, int maxX, int maxY)
{
void enqueueIfNotVisitedAndNotOutOfBounds(int x, int y)
{
if (x < minX || x > maxX || y < minY || y > maxY)
{
return;
}
var pxLocation = new PixelLocation(x, y);
if (!visited.Contains(pxLocation))
{
// Mark as visited already to prevent it from being enqueued again.
visited.Add(pxLocation);
toQueue.Enqueue(pxLocation);
}
};
// Right
enqueueIfNotVisitedAndNotOutOfBounds(fromLocation.X + 1, fromLocation.Y);
// Bottom Right
enqueueIfNotVisitedAndNotOutOfBounds(fromLocation.X + 1, fromLocation.Y + 1);
// Bottom
enqueueIfNotVisitedAndNotOutOfBounds(fromLocation.X, fromLocation.Y + 1);
// Bottom Left
enqueueIfNotVisitedAndNotOutOfBounds(fromLocation.X - 1, fromLocation.Y + 1);
// Top
enqueueIfNotVisitedAndNotOutOfBounds(fromLocation.X, fromLocation.Y - 1);
// Top Right
enqueueIfNotVisitedAndNotOutOfBounds(fromLocation.X + 1, fromLocation.Y - 1);
// Left and top left are never needing when traversing columnwise from the top left of the image
// to the bottom right.
}
/// <summary>
/// Starts at a position and finds all adjacent pixels of the same color. Search if performed in all directions (also diagonal).
/// </summary>
/// <param name="imageMap">The input image to collect blobs from.</param>
/// <param name="pxLocation">The initial pixel to start collecting blob pixels from.</param>
/// <param name="visited">The set of pixels that we already checked.</param>
/// <returns>A <see cref="Blob"/> of connected pixels of the same color.</returns>
private Blob CollectContiguousBlob(byte[,] imageMap, PixelLocation pxLocation, HashSet <PixelLocation> visited)
{
var width = imageMap.GetLength(0);
var height = imageMap.GetLength(1);
var connectedPixels = new List <PixelLocation>();
connectedPixels.Add(pxLocation);
var pending = new Queue <PixelLocation>();
AddNeighbouringPixels(pxLocation, pending, visited, 0, 0, width - 1, height - 1);
while (pending.Count > 0)
{
var p = pending.Dequeue();
if (imageMap[p.X, p.Y] == _blobColorValue)
{
connectedPixels.Add(p);
AddNeighbouringPixels(p, pending, visited, 0, 0, width - 1, height - 1);
}
}
return(new Blob(connectedPixels));
}
public bool Equals(PixelLocation other)
{
return(X == other.X && Y == other.Y);
}
private bool AreConnected(PixelLocation p1, PixelLocation p2)
{
return(AreClose(p1.X, p2.X) && AreClose(p1.Y, p2.Y));
}
