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Barn Door

Texture/Alpha blending pipeline

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Hi, Can somebody tell me if the texture blending and alpha blending pipelines are independent? In other words, do the results from the alpha operations feed into the colour operations and vica-verca? Or, is it that the final computed alpha value is only used after the pipeline has finished and the pixel being calculated is being copied to the back buffer and blended with what is already there? Thanks, BD.

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The alpha pipeline is part of the texture blending pipeline.

However _conceptually_ you can view the alpha (aka scalar) pipeline as being separate from the colour (aka vector) pipeline.

The values stay separate until the frame buffer blending stage which is a totally different thing from the texture blending cascade. So your second statement is much closer to the way it works.

There are some exceptions to this though:

1) The D3DTA_ALPHAREPLICATE argument modifier which copies the contents of the alpha channel to the colour channel components (R,G,B).

2) D3DTOP_DOTPRODUCT3 which copies the result of the operation to R,G,B *and* A.

Also some colour operations such as D3DTOP_MODULATEINVALPHA_ADDCOLOR read values from the alpha pipe as well as the colour pipe.

Simon O''Connor
Creative Asylum Ltd

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