Question

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?

Answer 1

The interpretation of the resulting byte array is dependent upon the pixel format of the source bitmap, but in the simplest case of a 32 bit, ARGB image, each pixel will be composed of four bytes in the byte array. The first pixel would be interpreted thusly:

alpha = pixelByteArray[0];
red   = pixelByteArray[1];
green = pixelByteArray[2];
blue  = pixelByteArray[3];

To process each pixel in the image, you would probably want to create nested loops to walk the rows and the columns, incrementing an index variable by the number of bytes in each pixel.

Some bitmap types combine multiple pixels into a single byte. For instance, a monochrome image packs eight pixels into each byte. If you need to deal with images other than 24/32 bit per pixels (the simple ones), then I would suggest finding a good book that covers the underlying binary structure of bitmaps.