Fade image to grayscale in silverlight

I'm开发者_C百科 using expression blend and can't figure out how to go about fading a color image to grayscale. Can I do it in the xaml or do I need to handle this in my C#?

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Couple ways to do it.

1) pre-compute gray-scale image; place it on top of the color image; animate opacity of the gray scale image from 0 to 1 to get the desired effect.

2) use pixel shader. wpf shader library might have one