BLENDFUNCTION

A version of this page is also available for

Windows Embedded CE 6.0 R3

4/8/2010

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

Syntax

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. When you only want to use per-pixel alpha values, set the SourceConstantAlpha value to 255 (opaque) .

• AlphaFormat
  This member controls the way the source and destination bitmaps are interpreted. The following table shows the value for AlphaFormat.

  Value	Description
  AC_SRC_ALPHA	This flag is set when the bitmap has an Alpha channel (that is, per-pixel alpha). Because this API uses premultiplied alpha, the red, green and blue channel values in the bitmap must be premultiplied with the alpha channel value. For example, if the alpha channel value is x, the red, green and blue channels must be multiplied by x and divided by 0xff before the call.

Remarks

When the AlphaFormat parameter is AC_SRC_ALPHA, the source bitmap must be 32 bpp. If it is not, the AlphaBlend function will fail.

When the BlendOp parameter is AC_SRC_OVER , 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 (that is, AC_SRC_ALPHA is not set), the SourceConstantAlpha value determines the blend of the source and destination bitmaps, as shown in the following table. Note that SCA is used for SourceConstantAlpha here. Also, SCA is divided by 255 because it has a value that ranges from 0 to 255.

Dst.Red   = Src.Red   * (SCA/255.0) + Dst.Red   * (1.0 - (SCA/255.0)) 
Dst.Green = Src.Green * (SCA/255.0) + Dst.Green * (1.0 - (SCA/255.0)) 
Dst.Blue  = Src.Blue  * (SCA/255.0) + Dst.Blue  * (1.0 - (SCA/255.0)) 

If the destination bitmap has an alpha channel, then the blend is as follows.

Dst.Alpha = Src.Alpha * (SCA/255.0) + Dst.Alpha * (1.0 - (SCA/255.0)) 

If the source bitmap does not use SourceConstantAlpha (that is, it equals 0xFF), the per-pixel alpha determines the blend of the source and destination bitmaps, as shown by the following equations.

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 the blend is as follows.

Dest.alpha = Src.Alpha + (1 - SrcAlpha) * Dst.Alpha 

If the source has both the SourceConstantAlpha (that is, it is not 0xFF) and per-pixel alpha, the source is pre-multiplied by the SourceConstantAlpha and then the blend is based on the per-pixel alpha. The following equations show this. Note that SourceConstantAlpha is divided by 255 because it has a value that ranges from 0 to 255.

Src.Red   = Src.Red   * SourceConstantAlpha / 255.0; 
Src.Green = Src.Green * SourceConstantAlpha / 255.0; 
Src.Blue  = Src.Blue  * SourceConstantAlpha / 255.0; 
Src.Alpha = Src.Alpha * SourceConstantAlpha / 255.0; 
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 
Dst.Alpha = Src.Alpha + (1 - Src.Alpha) * Dst.Alpha 

Requirements

See Also

Reference

GDI Structures
AlphaBlend

Concepts

Creating Bitmaps