Image Resize Aliasing in WPF v4 but not under v3.5

Question

I am using WPF for an image resizing pipeline which has been working beautifully under .NET v3.5. I just upgraded the project to target v4.0 and now all of my resized images are heavily aliased. None of the image pipeline code has changed.

1. Has a default WPF setting changed between v3.5 and v4.0?

2. How do I control the dithering of my resized bitmap images in WPF?

I'm using BitmapImage, DrawingVisual, DrawingContext, RenderTargetBitmap, BitmapEncoder, and BitmapFrame but I'm not seeing any properties related to dithering. GDI+ had a bunch of settings, so I'm guessing that I'm missing something.

Update: it appears that all of the solutions I've seen assume a Window object or XAML environment. This runs inside a Windows Service which has no UI. I need a way to programmatically affect this setting.

I specifically switched from GDI+ to WPF because GDI+ has memory leaks in long running processes like services & web apps.

Answer 1

The only way I've been able to affect the setting of BitmapScalingMode is to inherit from the DrawingVisual class and set it via its protected accessor:

// exposes BitmapScalingMode (also works for other protected properties)
public class MyDrawingVisual : DrawingVisual
{
    public BitmapScalingMode BitmapScalingMode
    {
        get { return this.VisualBitmapScalingMode; }
        set { this.VisualBitmapScalingMode = value; }
    }
}

If anyone else knows of a better way to set this, I would be excited to hear about it.

It seems that this would work:

RenderOptions.SetBitmapScalingMode(myDrawingVisual, BitmapScalingMode.HighQuality);

...but it does not. Apparently being outside of the XAML windowing runtime must mean that it cannot set the appropriate values.

Answer 2

The default BitmapScalingMode was Fant in 3.0 but in 4.0 it is now BiLinear. You can change the default a few different ways. A couple described here.