BLENDFUNCTION

[This is preliminary documentation and subject to change.]

The BLENDFUNCTION structure controls blending by specifying the blending functions for source and destination bitmaps.

typedef struct _BLENDFUNCTION
{
    BYTE     BlendOp;
    BYTE     BlendFlags;
    BYTE     SourceConstantAlpha;
    BYTE     AlphaFormat;
}BLENDFUNCTION, *PBLENDFUNCTION, *LPBLENDFUNCTION;

Members

BlendOp

Specifies the source blend operation. Currently, the only source and destination blend operation that has been defined is AC_SRC_OVER. For details, see the following Remarks section.

BlendFlags

Must be zero.

SourceConstantAlpha

Specifies an alpha transparency value to be used on the entire source bitmap. The SourceConstantAlpha value is combined with any per-pixel alpha values in the source bitmap. If you set SourceConstantAlpha to 0, it is assumed that your image is transparent. Set the SourceConstantAlpha value to 255 (opaque) when you only want to use per-pixel alpha values.

AlphaFormat

This member controls the way the source and destination bitmaps are interpreted. AlphaFormat has the following value:

Value	Meaning
AC_SRC_ALPHA	This flag is set when the bitmap has an Alpha channel.

Remarks

When the AC_SRC_OVER operation is used, the source bitmap is placed over the destination bitmap based on the alpha values of the source pixels.

If the source bitmap has no per-pixel alpha value, the blend is based on a constant source alpha value as shown in the following table.

Dst.Red

= Src.Red

* ConstantAlpha + (1 - ConstantAlpha) * Dst.Red

Dst.Green

= Src.Green

* ConstantAlpha + (1 - ConstantAlpha) * Dst.Green

Dst.Blue

= Src.Blue

* ConstantAlpha + (1 - ConstantAlpha) * Dst.Blue

If the source bitmap has per-pixel alpha, the constant alpha is not used (0xff) as shown in the following table.

`Dst.Red`	`= Src.Red`	`+ (1 - Src.Alpha) * Dst.Red`
`Dst.Green`	`= Src.Green`	`+ (1 - Src.Alpha) * Dst.Green`
`Dst.Blue`	`= Src.Blue`	`+ (1 - Src.Alpha) * Dst.Blue`

If the destination bitmap has an alpha channel, then:

Dest.alpha

= Src.Alpha

+ (1 - SrcAlpha) * Dst.Alpha

If the source has pixel per alpha, the constant alpha is not 0xff.

`Dst.Red`	`= Src.Red`	`- Src.Red = Src.Red * ConstantAlpha;`
`Dst.Red`	`= Src.Red`	`- Src.Green = Src.Green * ConstantAlpha ;`
`Dst.Red`	`= Src.Red`	`- Src.Blue = Src.Blue * ConstantAlpha ;`
`Dst.Red`	`= Src.Red`	`+ (1 - Src.Alpha) * Dst.Red`
`Dst.Red`	`= Src.Green`	`+ (1 - Src.Alpha) * Dst.Green`
`Dst.Red`	`= Src.Blue`	`+ (1 - Src.Alpha) * Dst.Blue`

QuickInfo

  Windows NT: Requires version 5.0 or later.
  Windows: Requires Windows 98 or later.
  Windows CE: Unsupported.
  Header: Declared in wingdi.h.

BLENDFUNCTION

Members

Remarks

QuickInfo

See Also