BGD_DECLARE(int) gdAlphaBlend (int dst, int src)
{
	int src_alpha = gdTrueColorGetAlpha(src);
	int dst_alpha, alpha, red, green, blue;
	int src_weight, dst_weight, tot_weight;

	/* -------------------------------------------------------------------- */
	/*      Simple cases we want to handle fast.                            */
	/* -------------------------------------------------------------------- */
	if( src_alpha == gdAlphaOpaque )
		return src;

	dst_alpha = gdTrueColorGetAlpha(dst);
	if( src_alpha == gdAlphaTransparent )
		return dst;
	if( dst_alpha == gdAlphaTransparent )
		return src;

	/* -------------------------------------------------------------------- */
	/*      What will the source and destination alphas be?  Note that      */
	/*      the destination weighting is substantially reduced as the       */
	/*      overlay becomes quite opaque.                                   */
	/* -------------------------------------------------------------------- */
	src_weight = gdAlphaTransparent - src_alpha;
	dst_weight = (gdAlphaTransparent - dst_alpha) * src_alpha / gdAlphaMax;
	tot_weight = src_weight + dst_weight;

	/* -------------------------------------------------------------------- */
	/*      What red, green and blue result values will we use?             */
	/* -------------------------------------------------------------------- */
	alpha = src_alpha * dst_alpha / gdAlphaMax;

	red = (gdTrueColorGetRed(src) * src_weight
	       + gdTrueColorGetRed(dst) * dst_weight) / tot_weight;
	green = (gdTrueColorGetGreen(src) * src_weight
	         + gdTrueColorGetGreen(dst) * dst_weight) / tot_weight;
	blue = (gdTrueColorGetBlue(src) * src_weight
	        + gdTrueColorGetBlue(dst) * dst_weight) / tot_weight;

	/* -------------------------------------------------------------------- */
	/*      Return merged result.                                           */
	/* -------------------------------------------------------------------- */
	return ((alpha << 24) + (red << 16) + (green << 8) + blue);
}
Unknown