WPF render performance with BitmapSource

Question

I\'ve created a WPF control (inheriting from FrameworkElement) that displays a tiled graphic that can be panned. Each tile is 256x256 pixels at 24bpp. I\'ve overridden OnRender.

Answer 1

It's not just with a large number of images. Just one large image is enough to hold up rendering until it has been loaded in, and that can be quite noticable when your image dimensions start getting up in the thousands.

I do agree with you that it's probably the render thread: I did a test and the UI thread was still happily dispatching messages while this render delay was taking place from trying to display a fully pre-cached BitmapImage.

It must be doing some sort of conversion or preparation on the image, like you were speculating. I've tried to mitigate this in my app by "rendering" but hiding the image, then revealing it when I need to show it. However this is less than ideal because the rendering freezes happen anyway.

(Edit)

Some followup: After a discussion on the MS WPF alias I found what was causing the delays. On my Server 2008 machine it was a combination of old video drivers that don't support the new WDDM driver model and a delay for resizing the image.

If the source image size is different from the display size, that will delay the render thread before the image shows up. By default an image is set to the highest quality, but you can change the scaling options for rendering by calling RenderOptions.SetBitmapScalingMode(uiImage, BitmapScalingMode.LowQuality); . Once I did that, the mysterious freeze before displaying an image went away. An alternative, if you don't like the quality drop in scaling, is to load the BitmapImage with DecodePixelWidth/Height equal to the size it will be displayed at. Then if you load the BitmapImage on a background thread, you should have no delay in displaying it.