Question:
What is a fast way to scale and/or crop a bitmap provided from a WritableBitmap for display in the UI?
Requirements:
- Have Low CPU usage
- Handle large images (5 Megapixel, abt 2500x2000 pixels)
- Resize and/or crop to the same resolution/area as the UI element the bitmap is displayed in.
- Use WPF
Specifically, it must allow a 14FPS 5 Megapixel camera image stream to be displayed in a WPF UI element at full speed.
Update:
I have been able to speed up the drawing quite a bit by painting to a Canvas control using an ImageBrush as follows, where m_bitmap is my WriteableBitmap:
ImageBrush brush = new ImageBrush();
brush.ImageSource = m_bitmap;
brush.Stretch = Stretch.Uniform;
canvas.Background = brush;
I'm now able to get the full 14FPS, though it still using about 20% CPU, so I'm not sure how well it preform if I add another camera or two (the plan is to have 4 or so running).
Another thing I think might be slowing down the drawing is the images are in a mono, Gray8, format, not the standard RGB32 (or is it bgra32 for WPF?) format. If I understand correctly, the image has to be converted to the standard format to be displayed, which would add significant overhead to each frame's drawing time.
Some background:
I'm currently working with a 5 Megapixel, 14 FPS, video camera and am trying to get the frames to render to the screen at full speed. I would like to do this using WPF.
I currently have an example in WinForms that runs full speed for an unscaled image, but (as I would expect) it has major trouble if I set the pictureBox.SizeMode = Zoom;. The example reads raw data directly from the camera stream to a buffer and then copies the data from the buffer into the bitmap set to the PictureBox control. The copy algorithm uses LockBits to speed things up.
I converted that example into WPF, rewriting the parts using Bitmap objects to instead use WritableBitmap objects and an Image control instead of PictureBox. Unforunately this is not able to render the stream to the screen at any decent rate, scaled or unscaled. Both have significant CPU load and very slow updates.
The performance when rendering to the screen is turned off is great. It is able to process the image stream at full speed and resolution while using around 3% CPU and less than 100MB memory.
Note: when I say rendering to the screen is turned off, the WritableBitmap is still being continuously updated, only is not set to the Image control.
I've seen a lot of discussion about getting fast bitmap updating in WPF, but have been unsuccessful in getting it to work at an reasonable speed/cpu load. Also I would like to have the image scaled in such a way that I can see the whole image.
I imagine the key will lie in some sort of scaling/crop combination that needs to be done so that WPF will not try to render(cache?) all 5 million pixels, but only those on the screen, and only at the current screen resolution. I imagine/hope this can be done fairly easily and without too much hit to memory or CPU, but currently have no idea how to do so. I have found the DecodePixelWidth and DecodePixelHeight properties, but those are only applicable when loading an image from a file to a BitmapImage.
