An accelerated graphics operation that copies a rectangular region of two bitmaps to a destination, using alpha blending values provided in a third bitmap to blend the corresponding entries in the first and second bitmaps.

The way alpha blending works is as follows: if the alpha value is the maximum, the pixel from the first source is opaque, in other words, the full colour of the pixel is written to the destination. If the alpha value is zero, the pixel from the first source is fully transparent, in other words, the full colour of the pixel in the second source is used. Values in-between cause blending with the following formula:

Destination = Source1*Alpha/max_Alpha + Source2*(max_Alpha-Alpha)/max_Alpha

Colour alpha bitmaps specify red, green and blue alpha values for each pixel, greyscale bitmaps specify a single alpha value for each pixel. The maximum alpha value depends on the bitmap's display mode. For example, 255 is the maximum for an EGray256 or EColor16M bitmap. The maximum is less for bitmaps which use fewer bits per colour component.

Supported bitmap formats than can be used as alpha bitmaps are given in TGraphicsAcceleratorCaps::iAlphaBitmap.

Objects of this class can be passed to a graphics accelerator's Operation() function either individually, or in a buffer.

TGraphicsAcceleratorCaps::iAlphaBitmap