How can I alphablend only certain parts of a texture in DX 9?

How can I alphablend only certain parts of a texture in DX 9?

For example, layers in Photoshop (or any other photo editing program开发者_JAVA技巧 that supports layers).

You can draw something in a layer (background filled with alpha), then place the layer over the original image (draw the texture on the screen) which leads to the original image + ONLY the things I drew in the layer.

Yes, I know my english is not very "shiny".

Thank you very much, in advance!

P.S. The background of my texture IS filled with alpha.

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So you have setup the alpha on the texture you wish to overlay such that 0 is transparent (ie shows whats underneath) and 1 is opaque (ie shows the overlay texture)?

If so then you just need to set up a a simple blend mode:

pDevice->SetRenderState( D3DRS_SRCBLEND, D3DBLEND_SRCALPHA );
pDevice->SetRenderState( D3DRS_DESTBLEND, D3DBLEND_INVSRCALPHA );
pDevice->SetRenderState( D3DRS_ALPHABLENDENABLE, TRUE );

Make sure you draw the background first! Also note that values between 0 and 1 represent a linear interpolation between background and the overlay texture.