SnapsToDevicePixels doesn't work with images?

Question

I've encountered a problem with bitmap images in WPF. When the image container starts on a position which is not a whole number, the image seems to not respect the value of SnapsToDevicePixels.

Example code:

<Window x:Class="BlurryImage.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Height="110" Width="200">
    <StackPanel Orientation="Horizontal" VerticalAlignment="Center">
        <Button SnapsToDevicePixels="True">
            <Image SnapsToDevicePixels="True" Source="i16.png" Stretch="None"/>
        </Button>
        <Button SnapsToDevicePixels="True" Margin="10.333333,0,0,0">
            <Image SnapsToDevicePixels="True" Source="i16.png" Stretch="None"/>
        </Button>
    </StackPanel>
</Window>

(Note the value of the left margin: 10.333333.)

Here the image i16.png is a simple 16x16 bitmap in 96 DPI resolution with thin vertical lines: . (My system resolution is 96 DPI, Windows XP, .NET 4)

When I run the program, the first image is sharp, whereas the second one is blurry:

Different sources, including some here on stackoverflow, suggest different workarounds. (For example, these posts: [1], [2] and [3].) I tried the workarounds, and them seem to work. Using UseLayoutRounding="true" on the main window makes both images sharp. Using RenderOptions.BitmapScalingMode="NearestNeighbor" on the image makes it sharp, too.

The question is, why doesn't SnapsToDevicePixels="True" work without workarounds? Is it a bug in WPF or I am using it in a wrong way?

Answer 1

From this blog entry:

SnapsToDevicePixels

WPF anticipated that there would be cases where people wanted to align with the pixel grid instead of using sub-pixel precision. You can set the SnapsToDevicePixels property on any UIElement. This will cause us to try and render to the pixel grid, but there are quite a few cases that don't work - including images. We will be looking to improve this in the future.

So it's just a known limitation of what SnapsToDevicePixels can do.