Finding the Dimensions of a JPEG file in Python
For a project I am working on I had to quickly throw together a Python function to grab the width and height of JPEG files on disk . There is nothing in the python standard library to do this and although some third party projects that read JPEG files (micro-jpeg-visulaizer, PIL) they seem like overkill if all you want are the dimensions.
A quick trip to the JPEG standards document provided the information I needed. In case anyone else needs something similar, here is what I came up with:
from struct import unpack def GetWidthAndHeight(stream): """ Extract the dimensions from a jpeg file as a tuple (width, height) Keyword arguments: stream -- a stream (eg: open()ed). Must be opened in binary mode Author: Andrew Stephens http://sandfly.net.nz/ License: use in any way you wish """ while (True): data = stream.read(2) marker, = unpack(">H", data) # some markers stand alone if (marker == 0xffd8): # start of image continue if (marker == 0xffd9): # end of image return; if ((marker >= 0xffd0) and (marker <= 0xffd7)): # restart markers continue if (marker == 0xff01): # private marker: continue # all other markers specify chunks with lengths data = stream.read(2) length, = unpack(">H", data) if (marker == 0xffc0): # baseline DCT chunk, has the info we want data = stream.read(5) t, height, width = unpack(">BHH", data) return (width, height) # not the chunk we want, skip it stream.seek(length - 2, 1)
This should be quicker than other solutions since it skips over parts of the stream it doesn't need and is flexible enough to work on anything that looks like a seekable file. Beware, it may misbehave on malformed files.
Usage is simple:
f = open("somepicture.jpg", "rb") # need to open the stream in binary mode w, h = GetWidthAndHeight(f) f.close()