RenderTargetBitmap : Impossible to free/dispose handle

Question

Sorry for my bad english.

I want to render a DrawingVisual to a bitmap so I can create a DiffuseMaterial. I do this:

public DiffuseMaterial GetDiffuseMaterial(string text, string font, double fontSize, Brush brush)
{
    FormattedText formattedText = new FormattedText(text, CultureInfo.InvariantCulture, FlowDirection.LeftToRight, new Typeface(font), fontSize, brush);
    DrawingVisual drawingVisual = new DrawingVisual();
    using (DrawingContext drawingContext = drawingVisual.RenderOpen())
        drawingContext.DrawText(formattedText, new Point(0, 0));

    RenderTargetBitmap bmp = new RenderTargetBitmap((int)(text.Length * fontSize), (int)fontSize, 96, 96, PixelFormats.Pbgra32);
    bmp.Render(drawingVisual);

    DiffuseMaterial mat = new DiffuseMaterial(new ImageBrush(bmp));
    mat.Freeze();
    return mat;
}

But...

• If I use the Method GetDiffuseMaterial 10000 times, the application stop. This is "normal" because every application can have maximum 10000 GDI handles and RenderTargetBitmap use 1 GDI handle each time it is created.

• A workaround to this is to free RenderTargetBitmap object. But I didn't get to do it. I tried this (between mat.Freeze() and return mat;) :

         bmp.Freeze();  
         bmp = null;  
         GC.Collect();  
         GC.WaitForPendingFinalizers();  
         GC.Collect();  
  

• Another way to do it is by having only one RenderTargetBitmap and give it as a parameter to GetDiffuseMaterial.
  But if I call two times GetDiffuseMaterial, I will have two identic Materials because the handle is shared and the Materials take the last modification. (so even if the text parameter is not the same !)

I don't see another way to do what I want with RenderTargetBitmap.
NB : GetDiffuseMaterial can be called 1 million times...

RenderTargetBitmap is the fastest way I found to create a DiffuseMaterial from a String text. Other technics are really slow : using a VisualBrush with TextBlock, using a DrawingBrush, ... e.g : 5000 calls to GetDiffuseMaterial is at least 10x faster using RenderTargetBitmap than the other technics.

Thank you for your help.