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I needed to make an object in my game transparent, but it wasn't working properly. So, after some research, I later found out how to properly do alpha blending in Direct3D9 and implemented some code to make the object finally transparent. However, while I have a basic idea of it, I am still a bit confused on how it all works. I have done lots of research, but I have only found very vague answers which just left me more confused. So, what do these two lines of code really mean and do? 

   d3ddev->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_SRCALPHA);
   d3ddev->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_INVSRCALPHA);
jacob
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1 Answers1

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Alpha blending in D3D is done according to this equation

Color = TexelColor x SourceBlend + CurrentPixelColor x DestBlend

The setting you quote sets the "SourceBlend" to the alpha value of the source texture. The value of DestBlend is set to one minus the alpha value of the source texture. This is a classic post-blending setting. This kind of mode is used to "anti-alias" an image, for example to blur the edges of a circle so that they do not look pixelated. It is quite good for things like smoke effects or for semi-transparent objects like plastics.
Why do have to specify it? Well if you set the DestBlend to always be one, you get a kind of "ghosting" effect, like when you partially see a reflection in a pane of glass. This effect is particularly good when used with an environment map. Pre-blending can also be useful, but requires you change the format of your inputs.

Tiger4Hire
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