PremultiplyAlpha

This converts an image to using premultiplied alpha. The format and size are not changed.

Note that the conversion to premultipled alpha is not fully reversible. For example if alpha is 0, then all the color information becomes 0.

HRESULT PremultiplyAlpha( const Image& srcImage, DWORD flags,
    ScratchImage& image );

HRESULT PremultiplyAlpha( const Image* srcImages, size_t nimages,
    const TexMetadata& metadata, DWORD flags,
    ScratchImage& result );

Breaking Change notice: The flags parameter was not present in the August 2013 and previous versions of DirectXTex.

Parameters

flags: Combination of options to apply

PMALPHA_IGNORE_SRGB Ignore any sRGB colorspace conversions
TEX_PMALPHA_REVERSE If set to true, then the conversion goes from premultipled alpha to nonpremultipled alpha (a.k.a. straight alpha).

This controls color space transformation for conversions.

TEX_PMALPHA_SRGB_IN Indicates the input format is the sRGB format. This is implied if using a DXGI_FORMAT_*_SRGB format
TEX_PMALPHA_SRGB_OUT Indicates the output format is the sRGB format. This is implied if using a DXGI_FORMAT_*_SRGB format
TEX_PMALPHA_SRGB This is the same as setting both TEX_PMALPHA_SRGB_IN and TEX_PMALPHA_SRGB_OUT

Example

ScratchImage srcImage;

...

ScratchImage destImage;
hr = PremultiplyAlpha( srcImage.GetImages(), srcImage.GetImageCount(),
    srcImage.GetMetadata(), TEX_PMALPHA_DEFAULT, destImage );
if ( FAILED(hr) )
    ...

Remarks

This function does not operate directly on block compressed images. See Decompress and Compress.

This function cannot operate directly on a planar format image. See ConvertToSinglePlane for a method for converting planar data to a format that is supported by this routine.

This is most useful for working with rendering that relies on premultiplied alpha blending (see DirectXTK).