Copying from BitmapSource to WritableBitmap

Question

I am trying to copy a part of a BitmapSource to a WritableBitmap.

This is my code so far:

var bmp = image.Source as BitmapSource;
var row = new WriteableBitmap(bmp.PixelWidth, bottom - top, bmp.DpiX, bmp.DpiY, bmp.Format, bmp.Palette);
row.Lock();
bmp.CopyPixels(new Int32Rect(top, 0, bmp.PixelWidth, bottom - top), row.BackBuffer, row.PixelHeight * row.BackBufferStride, row.BackBufferStride);
row.AddDirtyRect(new Int32Rect(0, 0, row.PixelWidth, row.PixelHeight));
row.Unlock();

I get "ArgumentException: Value does not fall within the expected range." in the line of CopyPixels.

I tried swapping row.PixelHeight * row.BackBufferStride with row.PixelHeight * row.PixelWidth, but then I get an error saying the value is too low.

I couldn't find a single code example using this overload of CopyPixels, so I'm asking for help.

Thanks!

Brandonbrandt

Answer 1

What part of the image are trying to copy? change the width and height in the target ctor, and the width and height in Int32Rect as well as the first two params (0,0) which are x & y offsets into the image. Or just leave if you want to copy the whole thing.

BitmapSource source = sourceImage.Source as BitmapSource;

// Calculate stride of source
int stride = source.PixelWidth * (source.Format.BitsPerPixel + 7) / 8;

// Create data array to hold source pixel data
byte[] data = new byte[stride * source.PixelHeight];

// Copy source image pixels to the data array
source.CopyPixels(data, stride, 0);

// Create WriteableBitmap to copy the pixel data to.      
WriteableBitmap target = new WriteableBitmap(
  source.PixelWidth, 
  source.PixelHeight, 
  source.DpiX, source.DpiY, 
  source.Format, null);

// Write the pixel data to the WriteableBitmap.
target.WritePixels(
  new Int32Rect(0, 0, source.PixelWidth, source.PixelHeight), 
  data, stride, 0);

// Set the WriteableBitmap as the source for the <Image> element 
// in XAML so you can see the result of the copy
targetImage.Source = target;

Coquito

Thanks! I kind of hoped that I could copy directly from the BitmapSource to WritableBitmap... Now I wonder what this overload of CopyPixels is really meant to do... – Brandonbrandt

The rectangle overload will copy the bitmap image to a Int32Rect so not so useful to pass that to WriteableBitmap. If you want something really short and you want to copy the whole image: WriteableBitmap target = new WriteableBitmap(Pic1.Source as BitmapSource); Pic2.Source = target; – Coquito

And if I want only a part of the BitmapSource (I need a rectangle with a relatively small height and same width)? – Brandonbrandt

If you're using a single byte per pixel, this will break. Correct stride calculation of "bytes per pixel" is (bitsPerPixel + 7) / 8 – Kashmiri

In the answer there is width * (bitsPerPixel + 7) / 8. Should it not instead be width * ((bitsPerPixel + 7) / 8) ? – Selby

To add to my confusion, learn.microsoft.com/en-us/dotnet/framework/wpf/… uses (width * pf.BitsPerPixel + 7) / 8 – Selby