libpng.txtはこんなこといってる。
PNG files store 3-color pixels in red, green, blue order. This code
changes the storage of the pixels to blue, green, red:
if (color_type == PNG_COLOR_TYPE_RGB ||
color_type == PNG_COLOR_TYPE_RGB_ALPHA)
png_set_bgr(png_ptr);
PNG files store RGB pixels packed into 3 bytes. This code expands them
into 4 bytes for windowing systems that need them in this format:
if (bit_depth == 8 && color_type ==
PNG_COLOR_TYPE_RGB) png_set_filler(png_ptr,
filler, PNG_FILLER_BEFORE);
where "filler" is the number to fill with, and the location is
either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
you want the filler before the RGB or after. This transformation
does not affect images that already have full alpha channels.
If you are reading an image with an alpha channel, and you need the
data as ARGB instead of the normal PNG format RGBA:
if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
png_set_swap_alpha(png_ptr);
BMPのピクセル構成は前述の通りBGRAだから、png_set_swap_alpha()を呼ぶ必要は無い。