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<TITLE>JPEG image compression FAQ, part 1/2 - [22] How can my program extract image dimensions from a JPEG file?</TITLE>
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<H1>[22] How can my program extract image dimensions from a JPEG file?</H1>
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The header of a JPEG file consists of a series of blocks, called "markers".
The image height and width are stored in a marker of type SOFn (Start Of
Frame, type N). To find the SOFn you must skip over the preceding markers;
you don't have to know what's in the other types of markers, just use their
length words to skip over them. The minimum logic needed is perhaps a page
of C code. (Some people have recommended just searching for the byte pair
representing SOFn, without paying attention to the marker block structure.
This is unsafe because a prior marker might contain the SOFn pattern, either
by chance or because it contains a JPEG-compressed thumbnail image. If you
don't follow the marker structure you will retrieve the thumbnail's size
instead of the main image size.) A profusely commented example in C can be
found in rdjpgcom.c in the IJG distribution (see part 2, item 15). Perl
code can be found in wwwis, from <A NAME=id13 HREF="tppmsgs/msgs0.htm#83" tppabs="http://www.tardis.ed.ac.uk/%7eark/wwwis/">http://www.tardis.ed.ac.uk/~ark/wwwis/</A>.
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