Stride issue with WriteableBitmap.PixelBuffer.AsStream()

Question

I am seeing some strange effects when manipulating pixels directly using a WriteableBitmap when I am trying to modify pixels from a loaded image vs pixels from an empty created WriteableBitmap.

When I create an empty WriteableBitmap setting pixels via the usual pixels[ (x + width * y)<<2] method works just as expected, but when I am trying to modify a PixelBuffer of a loaded image instead which has identical dimensions the same pixel setting method does set the pixels with a sheared offset and I have to manually adjust the stride by a fixed value (which varies with each loaded image file) in order to get the same result: pixels[ (x + (width + adjustment) * y)<<2]. What makes this so strange is that the length of the pixel buffer is identical in both cases.

So I suspect there might be some kind of conversion or padding going on in the Stream.write() or Stream.read() methods, but I cannot really make sense of it.

I am using the following methods to access the data:

// Read PixelBuffer from a WriteableBitmap: 
Width = map.PixelHeight; 
Height= map.PixelWidth;
Stream stream = map.PixelBuffer.AsStream(); 
byte[] pixels = new byte[stream.Length];
stream.Seek(0, 0); 
stream.Read(pixels, 0, pixels.Length); 

[...]

//Write PixelBuffer back 
Stream stream = map.PixelBuffer.AsStream();
stream.Seek(0, 0); 
stream.Write(pixels, 0, pixels.Length);
map.Invalidate();

[...]

//Create an empty WriteableBitmap
pixels = new byte[ Width * Height << 2];
WriteableBitmap map = new WriteableBitmap(Width,Height);
Stream stream = map.PixelBuffer.AsStream();
stream.Seek(0, 0);
stream.Write(pixels, 0, pixels.Length);
map.Invalidate();

[...]

//set pixel 
public void setPixel(int x, int y, uint rgba) { 
int stride = Width; 
int index = (y * stride + x) << 2; 
pixels[index] = (byte)((rgba >> 24) & 0xff);
pixels[index+1] = (byte)((rgba >> 16) & 0xff);
pixels[index+2] = (byte)((rgba >> 8) & 0xff);
pixels[index+3] = (byte)(rgba & 0xff); 
}

Answer 1

It looks indeed like the problem is somewhere in Stream.write() - I a have switched to using BitmapDecoder to read the image file and the stride problem has gone away:

ImageProperties props = await file.Properties.GetImagePropertiesAsync();
BitmapDecoder decoder = await BitmapDecoder.CreateAsync(await file.OpenAsync(FileAccessMode.Read));
BitmapTransform transform = new BitmapTransform();
transform.Bounds = new BitmapBounds() { Width = props.Width, Height = props.Height };
PixelDataProvider pixelData = await decoder.GetPixelDataAsync(BitmapPixelFormat.Rgba8, decoder.BitmapAlphaMode, new BitmapTransform(), ExifOrientationMode.RespectExifOrientation, ColorManagementMode.DoNotColorManage);
byte[] pixels = flipRGBA( pixelData.DetachPixelData());
WriteableBitmap map = new WriteableBitmap(props.Width, props.Height);
Stream stream = map.PixelBuffer.AsStream();
stream.Seek(0, 0);
stream.Write(pixels, 0, pixels.Length);
map.Invalidate();            

Answer 2

Hello Quasimondo,

Do you feel that your stride question is answerd by using the BitmapDecoder? This certainly sounds like the way to go.

Thanks,

James

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Windows Media SDK Technologies - Microsoft Developer Services - http://blogs.msdn.com/mediasdkstuff/

Answer 3

Well, BitmapDecoder surely solves it for me, but just out of curiosity I still would like to know what happens under the hood with the Stream method and why it causes this effect.