WriteableBitmap.SaveJpeg renders a black image (WP7)

Question

I'm trying to render some text and an image to a writeable bitmap to make 1 larger image, and this method has worked in other locations for creating or manipulating images, but for some reason, this instance is only creating a black image. If I just set the image source to the original WriteableBitmap, it shows just fine, but when I call SaveJpeg and then LoadJpeg, it shows as a black image (and yes, I need to call SaveJpeg since this is actually getting passed up to a server). The following is how I'm trying to render the elements:

NoteViewModel note = Instance.Note;
var grid = new Grid()
{
    Height = 929,
    Width = 929
};
grid.RowDefinitions.Add(new RowDefinition() { Height = new GridLength(679) });
grid.RowDefinitions.Add(new RowDefinition() { Height = new GridLength(1, GridUnitType.Star) });
var noteText = new TextBlock()
{
    Text = note.Text,
    FontFamily = note.FontFamily,
    Foreground = note.FontColor,
    TextWrapping = System.Windows.TextWrapping.Wrap,
    Width = 929,
    Height = 679
};
Grid.SetRow(noteText, 0);
grid.Children.Add(noteText);

WriteableBitmap sigImage = Instance.Signature.SignatureImage;
var sig = new Image()
{
    Source = sigImage,
    Height = 250,
    Width = (sigImage.PixelWidth / sigImage.PixelHeight) * 250,
    Margin = new Thickness(929 - ((sigImage.PixelWidth / sigImage.PixelHeight) * 250), 0, 0, 0)
};
Grid.SetRow(sig, 1);
grid.Children.Add(sig);

var messagePicture = new WriteableBitmap(grid, null);

var stream = new MemoryStream();

messagePicture.SaveJpeg(stream, messagePicture.PixelWidth, messagePicture.PixelHeight, 0, 100); //Save to a temp stream

stream.Position = 0;

var test = new WriteableBitmap(929,929); //Load the picture back up to see it
test.LoadJpeg(stream);

img.Source = test; //Show the image on screen (img is an Image element)

How big (resolution wise) is your jpg? Can you change the quality from 100 to 95 and tell if it still happens (only asking because I never use 100, always use 95 and I have similar code in numerous places, working) — Shahar Prish, Apr 09 '12 at 20:10
It's 929 x 929, and changing the quality doesn't seem to fix anything. — Daniel, Apr 09 '12 at 20:18

score 1 · Accepted Answer · answered Apr 10 '12 at 22:08

1

So apparently WriteableBitmap will render a transparent background as black when calling SaveJpeg, so I solved this by rendering a white canvas as well, like so:

var background = new Canvas()
{
    Width = 929,
    Height = 929,
    Background = new SolidColorBrush(Colors.White)
};

messagePicture.Render(background, new TranslateTransform());

answered Apr 10 '12 at 22:08

Daniel

73
2
10

any idea how i can achieve this in WPF . – eran otzap Feb 13 '14 at 19:35

WriteableBitmap.SaveJpeg renders a black image (WP7)

1 Answers1