EsoErik

When converting to formats without an alpha channel, the Boost Generic Image Library interprets RGBA images as using premultiplied alpha. An application that I am writing stores non-graphical data in the alpha channel. Converting from such images to formats without an alpha channel requires simply discarding alpha channel data, rather than using it to reverse premultiplication of the RGB channels. To do this in a generic manner that overrides only from-RGBA conversion, I wrote the following custom conversion functor:

// Default to color converters provided by GIL
template<typename TSrcColorSpace, typename TDstColorSpace>
struct ConvertRGBA_impl
  : public default_color_converter_impl<TSrcColorSpace, TDstColorSpace> {};

// Override GIL's from-RGBA color converter.  GIL's from-RGBA color converter assumes premultiplied alpha, which we do not
// use.
template<typename TDstColorSpace>
struct ConvertRGBA_impl<rgba_t, TDstColorSpace>
{
    template<typename TSrcPixel, typename TDstPixel>
    void operator () (const TSrcPixel& srcPixel, TDstPixel& dstPixel) const;
};

struct ConvertRGBA
{
    template<typename TSrcPixel, typename TDstPixel>
    void operator () (const TSrcPixel& srcPixel, TDstPixel& dstPixel) const;
};

template<typename TDstColorSpace>
template<typename TSrcPixel, typename TDstPixel>
void ConvertRGBA_impl<rgba_t, TDstColorSpace>::operator () (const TSrcPixel& srcPixel, TDstPixel& dstPixel) const
{
    default_color_converter_impl<rgb_t, TDstColorSpace>()
    (
        pixel<typename channel_type<TSrcPixel>::type, rgb_layout_t>
        (
            get_color(srcPixel, red_t()),
            get_color(srcPixel, green_t()),
            get_color(srcPixel, blue_t())
        ),
        dstPixel
    );
}

template<typename TSrcPixel, typename TDstPixel>
void ConvertRGBA::operator () (const TSrcPixel& srcPixel, TDstPixel& dstPixel) const
{
    typedef typename color_space_type<TSrcPixel>::type TSrcColorSpace;
    typedef typename color_space_type<TDstPixel>::type TDstColorSpace;
    ConvertRGBA_impl<TSrcColorSpace, TDstColorSpace>()(srcPixel, dstPixel);
}

This may be used, for example, by calling copy_and_convert_pixels with ConvertRGBA() supplied as the third parameter.

Labels: coding