Color Blending problem (DirectX Windows Mobile)

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C#

Mobile

DirectX

Hi there,

I've just started using directx for my windows mobile application and i ran into a little problem that i could not solve so far.
I'm trying to draw a polygon with complex coloring (like linear gradient) and i thought that if i create a rectangle 'behind' the polygon with the wanted texture, i could blend them together somehow.
I tried using alpha blending with SourceBlend, DestinationBlend and BlendOperation but i just cannot figure it out how it works, or what is the right combination for these three.
I made an image of what i'd like to achieve, the first two shows the rectangle and the polygon, and the third is what i want. The background is not neccesseraly black, it can be any other color (transparent) or image.

The image

Please give me some help or a hint for another way to do this,
Thanks in advance
Bali from Hungary