Alpha Blending (Windows GDI)

Alpha blending is used to display an alpha bitmap, which is a bitmap that has transparent or semi-transparent pixels. In addition to a red, green, and blue color channel, each pixel in an alpha bitmap has a transparency component known as its alpha channel. The alpha channel typically contains as many bits as a color channel. For example, an 8-bit alpha channel can represent 256 levels of transparency, from 0 (the entire bitmap is transparent) to 255 (the entire bitmap is opaque).

Alpha blending mechanisms are invoked by calling AlphaBlend, which references the BLENDFUNCTION structure.

Alpha values per pixel are only supported for 32-bpp BI_RGB. This formula is defined as:

typedef struct {
  BYTE   Blue;
  BYTE   Green;
  BYTE   Red;
  BYTE   Alpha;
};

This is represented in memory as shown in the following table.

Bitmaps may also be displayed with a transparency factor applied to the entire bitmap. Any bitmap format can be displayed with a global constant alpha value by setting SourceConstantAlpha in the BLENDFUNCTION structure. The global constant alpha value has 256 levels of transparency, from 0 (entire bitmap is completely transparent) to 255 (entire bitmap is completely opaque). The global constant alpha value is combined with the per-pixel alpha value.

For an example, see Alpha Blending a Bitmap.