AlphaBlend function (wingdi.h)

The AlphaBlend function displays bitmaps that have transparent or semitransparent pixels.

Syntax

BOOL AlphaBlend(
  [in] HDC           hdcDest,
  [in] int           xoriginDest,
  [in] int           yoriginDest,
  [in] int           wDest,
  [in] int           hDest,
  [in] HDC           hdcSrc,
  [in] int           xoriginSrc,
  [in] int           yoriginSrc,
  [in] int           wSrc,
  [in] int           hSrc,
  [in] BLENDFUNCTION ftn
);

Parameters

[in] hdcDest

A handle to the destination device context.

[in] xoriginDest

The x-coordinate, in logical units, of the upper-left corner of the destination rectangle.

[in] yoriginDest

The y-coordinate, in logical units, of the upper-left corner of the destination rectangle.

[in] wDest

The width, in logical units, of the destination rectangle.

[in] hDest

The height, in logical units, of the destination rectangle.

[in] hdcSrc

A handle to the source device context.

[in] xoriginSrc

The x-coordinate, in logical units, of the upper-left corner of the source rectangle.

[in] yoriginSrc

The y-coordinate, in logical units, of the upper-left corner of the source rectangle.

[in] wSrc

The width, in logical units, of the source rectangle.

[in] hSrc

The height, in logical units, of the source rectangle.

[in] ftn

The alpha-blending function for source and destination bitmaps, a global alpha value to be applied to the entire source bitmap, and format information for the source bitmap. The source and destination blend functions are currently limited to AC_SRC_OVER. See the BLENDFUNCTION and EMRALPHABLEND structures.

Return value

If the function succeeds, the return value is TRUE.

If the function fails, the return value is FALSE.

Remarks

If the source rectangle and destination rectangle are not the same size, the source bitmap is stretched to match the destination rectangle. If the SetStretchBltMode function is used, the iStretchMode value is automatically converted to COLORONCOLOR for this function (that is, BLACKONWHITE, WHITEONBLACK, and HALFTONE are changed to COLORONCOLOR).

The destination coordinates are transformed by using the transformation currently specified for the destination device context. The source coordinates are transformed by using the transformation currently specified for the source device context.

An error occurs (and the function returns FALSE) if the source device context identifies an enhanced metafile device context.

If destination and source bitmaps do not have the same color format, AlphaBlend converts the source bitmap to match the destination bitmap.

AlphaBlend does not support mirroring. If either the width or height of the source or destination is negative, this call will fail.

When rendering to a printer, first call GetDeviceCaps with SHADEBLENDCAPS to determine if the printer supports blending with AlphaBlend. Note that, for a display DC, all blending operations are supported and these flags represent whether the operations are accelerated.

If the source and destination are the same surface, that is, they are both the screen or the same memory bitmap and the source and destination rectangles overlap, an error occurs and the function returns FALSE.

The source rectangle must lie completely within the source surface, otherwise an error occurs and the function returns FALSE.

AlphaBlend fails if the width or height of the source or destination is negative.

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

Requirements

See also

BLENDFUNCTION

Bitmap Functions

Bitmaps Overview

EMRALPHABLEND

GetDeviceCaps

SetStretchBltMode