It is very easy to determine 555 vs 565 mode in FreeImage:
unsigned red_mask, green_mask, blue_mask;
red_mask = FreeImage_GetRedMask(dib);
green_mask = FreeImage_GetGreenMask(dib);
blue_mask = FreeImage_GetBlueMask(dib);
if (FreeImage_GetBPP(dib) == 16)
{
if ((red_mask == FI16_565_RED_MASK) &&
(green_mask == FI16_565_GREEN_MASK) &&
(blue_mask == FI16_565_BLUE_MASK))
{
// We are in RGB16 565 mode
}
else
{
// We are in RGB16 555 mode
}
}
And you can always use FreeImage_ConvertTo16Bits555()
and FreeImage_ConvertTo16Bits565()
functions to convert to the format you desire.