Thank your answers. :) For the problem 2, I mean I need a raw image buffer **with png format** including header and not just pixels. png_io's png_write_view only writes png format to file. I must pass that data to other library to do something. If I write png to file than read it to memory then pass it, it might be too slow. How to do that? For the problem 3, If I can make sure that two images **always** have the same format (png), the same size, the same layout of the channels, and the same channel depth...etc. Is there any good performance way to do that? I am not sure that to compute Mean Absolute Error is the best way to do that. I just simply follow the recommendation of ImageMagicks (http://www.imagemagick.org/Usage/compare/). But after this review, I found MAE is used to compute "how different are two images"... oh, actually, I don't need to know how difference, I just want to know two images are different. :( I think that simply pixel by pixel comparison might be the fastest way to do that? Thank you very much. -- View this message in context: http://www.nabble.com/Some-problems-about-GIL-tp20526156p20533158.html Sent from the Boost - Users mailing list archive at Nabble.com.