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I am using XNA 4.0, hence DX9 and HLSL. I have a RenderTarget2D "MAIN" that I want to add semi-transparent objects "TRANS" to. I want them to blend with MAIN using the TRANS's alpha and inverseAlpha BUT I want to preserve information kept in the MAIN's alpha channel.

If I simply use BlendState.AlphaBlend, MAIN's alpha channel becomes 1 everywhere where TRANS is written. I know there is a property in the BlendState class that prevents writing to color channels Is there an option that prevents writing to alpha?

Thanks.

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Reset to default
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BlendState.ColorWriteChannels is a bitfield with a bit for each of the four channels. You can make it write to RGB only by setting

blendState.ColorWriteChannels = ColorWriteChannels.Red | ColorWriteChannels.Green | ColorWriteChannels.Blue;
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