WriteableBitmap not invalidating and rendering in Windows Phone but working in silverlight5? - windows-phone-7

Question

I am having a property which returns WriteableBitmap. when i am binding that property to the silverlight5 Image it showing the image, but when i am doing that in the WP it is not showing the image. when i am googling around this issue i saw that in WP the raw pixel values did not have alpha bits is set. But the same is working with silverlight. I don't know what is happening. Anybody have the similar issue or any round about?
(Imageproperty as WriteableBitmap).Invalidate();
<Image Source="{Binding Imageproperty}"/> (this is working in silverlight not in WP(7.1))

Answer 1

I had a similar sort of issue when trying to port some of the FaceLight code across from Silverlight to Windows Phone. The easiest way around this would be to manually set the Alpha value to 255 / opaque. So say if you had your WriteableBitmap that you wanted to display, result:
//convert to byte array
int stride = result.PixelWidth * 4; //brga32 is 32
int bytes = Math.Abs(stride) * result.PixelHeight;
byte[] data = result.ToByteArray();
int dataIndex = 0;
int nOffset = stride - result.PixelWidth * 4;
for (int y = 0; y < result.PixelHeight; ++y)
{
for (int x = 0; x < result.PixelWidth; ++x)
{
data[dataIndex + 3] = 0xFF; //set alpha to 255
dataIndex += 4; //skip to next pixel
}
dataIndex += nOffset;
}
WriteableBitmap finalImg = new WriteableBitmap(input.PixelWidth, input.PixelHeight);
return finalImg.FromByteArray(data);
Displaying the result from this method (the finalImg.FromByteArray(data) call) should display properly on the phone.
An alternative to this method above as well, is writing the WriteableBitmap to a .jpeg and then display the .jpeg instead - that seemed to work for me also but I didn't investigate that too thoroughly.

Answer 2

If the writeablebitmap has its pixeldata set or changed after the binding is established you need to call Invalidate to cause an update of the screen.
This goes for both Silverlight and Phone but you might have a race condition here that runs differently on both platforms.

Answer 3

I know it's an old post, but today I encountered the same problem on Windows Phone 8.1 Silverlight and I found nice solution, so I have decided to left it for people with similar problem. It was posted by Charles Petzold on MSDN as Video Feeds on Windows Phone 7 (it is shown on VideoSink class example, but it shouldn't be a problem to reproduce it with other case). He created a simple class that derives from VideoSink:
SimpleVideoSink C# code:
public class SimpleVideoSink : VideoSink
{
VideoFormat videoFormat;
WriteableBitmap writeableBitmap;
Action<WriteableBitmap> action;
public SimpleVideoSink(Action<WriteableBitmap> action)
{
this.action = action;
}
protected override void OnCaptureStarted() { }
protected override void OnCaptureStopped() { }
protected override void OnFormatChange(VideoFormat videoFormat)
{
this.videoFormat = videoFormat;
System.Windows.Deployment.Current.Dispatcher.BeginInvoke(() =>
{
writeableBitmap = new WriteableBitmap(videoFormat.PixelWidth,
videoFormat.PixelHeight);
action(writeableBitmap);
});
}
protected override void OnSample(long sampleTimeInHundredNanoseconds,
long frameDurationInHundredNanoseconds, byte[] sampleData)
{
if (writeableBitmap == null)
return;
int baseIndex = 0;
for (int row = 0; row < writeableBitmap.PixelHeight; row++)
{
for (int col = 0; col < writeableBitmap.PixelWidth; col++)
{
int pixel = 0;
if (videoFormat.PixelFormat == PixelFormatType.Format8bppGrayscale)
{
byte grayShade = sampleData[baseIndex + col];
pixel = (int)grayShade | (grayShade << 8) |
(grayShade << 16) | (0xFF << 24);
}
else
{
int index = baseIndex + 4 * col;
pixel = (int)sampleData[index + 0] | (sampleData[index + 1] << 8) |
(sampleData[index + 2] << 16) | (sampleData[index + 3] << 24);
}
writeableBitmap.Pixels[row * writeableBitmap.PixelWidth + col] = pixel;
}
baseIndex += videoFormat.Stride;
}
writeableBitmap.Dispatcher.BeginInvoke(() =>
{
writeableBitmap.Invalidate();
});
}
}
However, this code needs some modification - VideoSink.CaptureSource must be provided with our CaptureSource (I just passed it into constructor):
public SimpleVideoSink(CaptureSource captureSource, Action<WriteableBitmap> action)
{
base.CaptureSource = captureSource;
this.action = action;
}
When we initialize SimpleVideoSink class in a ViewModel, we have to provide it an Action parameter. In my case it was enough to provide ViewModel with initialized field writeableBitmap:
ViewModel C# code:
private WriteableBitmap videoWriteableBitmap;
public WriteableBitmap VideoWriteableBitmap
{
get
{
return videoWriteableBitmap;
}
set
{
videoWriteableBitmap = value;
RaisePropertyChanged("VideoWriteableBitmap");
}
}
private void OnWriteableBitmapChange(WriteableBitmap writeableBitmap)
{
VideoWriteableBitmap = writeableBitmap;
}
//this part goes to constructor/method
SimpleVideoSink videoFrameHandler = new SimpleVideoSink(captureSource, OnWriteableBitmapChange);
Then all we have to do is to bind it to View:
View XAML code:
<Image Source="{Binding VideoWriteableBitmap}" />
In this example Image source is refreshed on every OnSample method invocation, and is dispatched to main thread through WriteableBitmap.Dispatcher.
This solution generates proper image with no blank pixel (alpha channel is also filled), and Invalidate() method works as it should.