589    png_byte alpha; /* for alpha channel files */  member
604    png_uint_16 alpha;  member
2571 #  define png_composite(composite, fg, alpha, bg)         \  argument
2573            * (png_uint_16)(alpha)                         \
2575            - (png_uint_16)(alpha)) + 128);                \
2578 #  define png_composite_16(composite, fg, alpha, bg)       \  argument
2580            * (png_uint_32)(alpha)                          \
2582            - (png_uint_32)(alpha)) + 32768);               \
2587 #  define png_composite(composite, fg, alpha, bg)                          \  argument
2588      (composite) = (png_byte)(((png_uint_16)(fg) * (png_uint_16)(alpha) +  \
2589      (png_uint_16)(bg) * (png_uint_16)(255 - (png_uint_16)(alpha)) +       \
2592 #  define png_composite_16(composite, fg, alpha, bg)                         \  argument
2593      (composite) = (png_uint_16)(((png_uint_32)(fg) * (png_uint_32)(alpha) + \
2594      (png_uint_32)(bg)*(png_uint_32)(65535 - (png_uint_32)(alpha)) +         \