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copied!<p>The Bitmap class contains a GetPixel method that returns the Color of a pixel in the bitmap, given its X and Y coordinates. One technique might be to walk the rows and columns of data to determine the lowest X and Y coordinates at which a non-white pixel exists. This technique is adequate if the images are small and/or performance is not a prime consideration since calling GetPixel is rather slow.</p> <p>Another approach would be to get the Bitmap image data into a byte[] and then walk the bytes in the array to determine the location of the non-white pixels. This approach requires some knowledge of the way bytes are laid out for a given bitmap type (e.g. 32 bit, 24 bit, 1 bit, etc.).</p> <p>To get the bytes for a bitmap, you call the LockBits method on the bitmap to lock a region of the bitmap and obtain a BitmapData object. You then use the Stride and Height properties of the BitmapData object to determine the size of the byte array needed to contain the bitmap.</p> <p>I whipped up and tested the following method, which seems to work speedily on a couple test images I created to detect the location of an ellipse.</p> <pre><code>private Point DetectLocation(Bitmap original) { Bitmap source = null; // If original bitmap is not already in 32 BPP, ARGB format, then convert if (original.PixelFormat != PixelFormat.Format32bppArgb) { source = new Bitmap(original.Width, original.Height, PixelFormat.Format32bppArgb); source.SetResolution(original.HorizontalResolution, original.VerticalResolution); using (Graphics g = Graphics.FromImage(source)) { g.DrawImageUnscaled(original, 0, 0); } } else { source = original; } // Lock source bitmap in memory BitmapData sourceData = source.LockBits(new Rectangle(0, 0, source.Width, source.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb); // Copy image data to binary array int imageSize = sourceData.Stride * sourceData.Height; byte[] sourceBuffer = new byte[imageSize]; Marshal.Copy(sourceData.Scan0, sourceBuffer, 0, imageSize); // Unlock source bitmap source.UnlockBits(sourceData); int sourceIndex = 0; int pixelTotal = 0; int height = source.Height; int width = source.Width; int threshold = 255 * 3; int minX = width; int minY = height; // Iterate lines for (int y = 0; y < height; y++) { sourceIndex = y * sourceData.Stride; // Iterate pixels for (int x = 0; x < width; x++) { // Compute pixel brightness (i.e. total of Red, Green, and Blue values) pixelTotal = sourceBuffer[sourceIndex + 1] + sourceBuffer[sourceIndex + 2] + sourceBuffer[sourceIndex + 3]; if (pixelTotal < threshold) { minX = Math.Min(minX, x); minY = Math.Min(minY, y); } sourceIndex += 4; } } return new Point(minX, minY); } </code></pre>

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