Render Target Alpha

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The frame buffer blender can now blend alpha channels independently of color-channel blending on render targets. This control is enabled with a new render state, RenderStateManager.SeparateAlphaBlendEnabled.

When RenderStateManager.SeparateAlphaBlendEnabled is set to false (which is the default condition), the render-target blending factors and operations applied to alpha are the same as those defined for blending color channels. A driver needs to set the MiscCaps.SupportsSeparateAlphaBlend cap to indicate that it can support render-target alpha blending. Be sure to set RenderStateManager.AlphaBlendEnable to tell the pipeline that alpha blending is needed.

To control the factors in the alpha channel of the render-target blenders, the following new render states are defined.

• RenderStateManager.AlphaDestinationBlend
• RenderStateManager.AlphaSourceBlend

Like RenderStateManager.SourceBlend and RenderStateManager.DestinationBlend, these states can be set to one of the values in the Blend enumeration. The source and destination blend settings can be combined in several ways, depending on the settings of the BlendCaps object set in Caps.DestinationBlendCaps and Caps.SourceBlendCaps.

The alpha blending is done as follows:

renderTargetAlpha = (alpha_in* srcBlendOp) BlendOp (alpha_rt* destBlendOp)

Where:

• alpha_in is the input alpha value.
• srcBlendOp is one of the blend factors in Blend.
• BlendOp is one of the blend factors in Blend.
• alpha_rt is the render-target alpha value.
• destBlendOp is one of the blend factors in Blend.
• renderTargetAlpha is the final blended alpha value.

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