Finding specific pixel colors of a BitmapImage

I have a WPF BitmapImage which I loaded from a .JPG file, as follows:

this.m_image1.Source = new BitmapImage(new Uri(path));

I want to query as to what the colour is at specific points. For example, what is the RGB value at pixel (65,32)?

How do I go about this? I was taking this approach:

ImageSource ims = m_image1.Source;
BitmapImage bitmapImage = (BitmapImage)ims;
int height = bitmapImage.PixelHeight;
int width = bitmapImage.PixelWidth;
int nStride = (bitmapImage.PixelWidth * bitmapImage.Format.BitsPerPixel + 7) / 8;
byte[] pixelByteArray = new byte[bitmapImage.PixelHeight * nStride];
bitmapImage.CopyPixels(pixelByteArray, nStride, 0);

Though I will confess there's a bit of monkey-see, monkey do going on with this code. Anyway, is there a straightforward way to process this array of bytes to convert to RGB values?

Here is how I would manipulate pixels in C# using multidimensional arrays:

[StructLayout(LayoutKind.Sequential)]
public struct PixelColor
{
  public byte Blue;
  public byte Green;
  public byte Red;
  public byte Alpha;
}

public PixelColor[,] GetPixels(BitmapSource source)
{
  if(source.Format!=PixelFormats.Bgra32)
    source = new FormatConvertedBitmap(source, PixelFormats.Bgra32, null, 0);

  int width = source.PixelWidth;
  int height = source.PixelHeight;
  PixelColor[,] result = new PixelColor[width, height];

  source.CopyPixels(result, width * 4, 0);
  return result;
}

usage:

var pixels = GetPixels(image);
if(pixels[7, 3].Red > 4)
{
  ...
}

If you want to update pixels, very similar code works except you will create a WriteableBitmap, and use this:

public void PutPixels(WriteableBitmap bitmap, PixelColor[,] pixels, int x, int y)
{
  int width = pixels.GetLength(0);
  int height = pixels.GetLength(1);
  bitmap.WritePixels(new Int32Rect(0, 0, width, height), pixels, width*4, x, y);
}

thusly:

var pixels = new PixelColor[4, 3];
pixels[2,2] = new PixelColor { Red=128, Blue=0, Green=255, Alpha=255 };

PutPixels(bitmap, pixels, 7, 7);

Note that this code converts bitmaps to Bgra32 if they arrive in a different format. This is generally fast, but in some cases may be a performance bottleneck, in which case this technique would be modified to match the underlying input format more closely.

Update

Since BitmapSource.CopyPixels doesn't accept a two-dimensional array it is necessary to convert the array between one-dimensional and two-dimensional. The following extension method should do the trick:

public static class BitmapSourceHelper
{
#if UNSAFE
  public unsafe static void CopyPixels(this BitmapSource source, PixelColor[,] pixels, int stride, int offset)
  {
    fixed(PixelColor* buffer = &pixels[0, 0])
      source.CopyPixels(
        new Int32Rect(0, 0, source.PixelWidth, source.PixelHeight),
        (IntPtr)(buffer + offset),
        pixels.GetLength(0) * pixels.GetLength(1) * sizeof(PixelColor),
        stride);
  }
#else
  public static void CopyPixels(this BitmapSource source, PixelColor[,] pixels, int stride, int offset)
  {
    var height = source.PixelHeight;
    var width = source.PixelWidth;
    var pixelBytes = new byte[height * width * 4];
    source.CopyPixels(pixelBytes, stride, 0);
    int y0 = offset / width;
    int x0 = offset - width * y0;
    for(int y=0; y<height; y++)
      for(int x=0; x<width; x++)
        pixels[x+x0, y+y0] = new PixelColor
        {
          Blue  = pixelBytes[(y*width + x) * 4 + 0],
          Green = pixelBytes[(y*width + x) * 4 + 1],
          Red   = pixelBytes[(y*width + x) * 4 + 2],
          Alpha = pixelBytes[(y*width + x) * 4 + 3],
        };
  }
#endif
}

There are two implementations here: The first one is fast but uses unsafe code to get an IntPtr to an array (must compile with /unsafe option). The second one is slower but does not require unsafe code. I use the unsafe version in my code.

WritePixels accepts two-dimensional arrays, so no extension method is required.

Edit: As Jerry pointed out in the comments, because of the memory layout, the two-dimensional array has the vertical coordinate first, in other words it must be dimensioned as Pixels[Height,Width] not Pixels[Width,Height] and addressed as Pixels[y,x].

I'd like to add to Ray´s answer that you can also declare PixelColor struct as a union:

[StructLayout(LayoutKind.Explicit)]
public struct PixelColor
{
    // 32 bit BGRA 
    [FieldOffset(0)] public UInt32 ColorBGRA;
    // 8 bit components
    [FieldOffset(0)] public byte Blue;
    [FieldOffset(1)] public byte Green;
    [FieldOffset(2)] public byte Red;
    [FieldOffset(3)] public byte Alpha;
}

And that way you'll also have access to the UInit32 BGRA (for fast pixel access or copy), besides the individual byte components.