FlowDocument loses ClearType in child elements when placed in window with custom glass

Question

So the new WPF 4 text rendering looks great, but enabling the aero glass effect on a window requires that you change the background to transparent, which of course disables ClearType rendering.

Using the provided RenderOptions.ClearTypeHint=Enabled allows you to designate child elements to reenable ClearType rendering from that point in the tree. I've found a few other topics that talk about doing this for the ScrollViewer used internally inside RichTextBox and FlowDocumentScrollViewer, and creating a custom style does indeed fix it so that my FlowDocument gets ClearType rendering again.

However, this only applies to top level paragraphs in the FlowDocument. If I add floaters or figures, or a table, any text inside them is inexplicably grayscale again. I know that the glass effect is to blame, since disabling it reenables the ClearType rendering.

I looked through the visual tree with Snoop, but both the main content (which renders properly) and the sub-content (which is grayscale) have similar element hierarchies without anything to which I can attach RenderOptions.ClearTypeHint.

Has anyone run into this problem? Is there a workaround or a solution? I checked Connect but there isn't any bug filed about this. It's quite an annoying problem.

Discomfort

so you still have not found out a better way to do this? I'm kinda lost with this one and your accepted solution seems cumbersome. – Mott

Answer 1

After doing a lot more research, and comparing the way different controls work on and off of aero glass, I've found a few answers. The TextBox control doesn't work properly either, but portions of FlowDocument and things like TextBlock do, which prompted me to explore why.

After digging around in reflector for a while, I found that when using the advanced text formatting APIs to get text and render it onto a drawing context, the RenderOption flags essentially go ignored, since the drawing system knows from the root visual (the window) that transparency was enabled. Once that happens, all the RenderOptions flags in the world aren't going to get ClearType back.

I did happen to stumble on a work-around though. If you have access to the DrawingContext and are doing the low-level text rendering yourself, you can do a DrawRectangle behind the text with a fill, and ClearType gets reenabled. I assume that this is the only way for the renderer to be sure that it has a proper background to draw on.

So in summary, you need to do your own text drawing, and additionally you need to explicitly draw a background using the same DrawingContext behind your text in order for ClearType to get rendered properly.

Discomfort

Answer 2

I know this could get tedious, but have you tried setting general styles like so:

    <Style TargetType="Paragraph">
        <Setter Property="RenderOptions.ClearTypeHint" Value="Enabled"/>
    </Style>

Orpheus

It doesn't work. The internal flow document renderer ignores the attribute. – Discomfort