With this blog post I release a small script that helps in comparing quantization tables of JPEG images.
Quantization tables are stored in DQT segments of JPEG files.
I wrote a little script called dqtmd5 in order to ease the comparison of quantization tables from multiple JPEG files. The script parses the header of a JPEG file and extracts the DQT segments. At this it mostly relies upon the Image::MetaData::JPEG module. Then the script parses the matching segments, extracts the quantization tables and joins them in a canonic way. Hence the method is resilient against different ways of storing quantization tables. Finally the script calculates a MD5 checksum of the extracted data:
$ ./dqtmd5.pl sample.jpg 41e8fb65ebb533a82f392bed8988eb47 sample.jpg
In the same manner DQT hashes can be calculated from (unedited!) pictures that were taken by digital cameras. I provide some sample hashes for Canon. The make and model strings were extracted from the EXIF data of the sample images.
8ba82c6f0fe031d5b7234126b9521d34 Canon Canon DIGITAL IXUS 75 bc610f3b20f21ed06ce1f72b0ba0aeb1 Canon Canon DIGITAL IXUS 75 41e8fb65ebb533a82f392bed8988eb47 Canon Canon EOS 10D 71377799fea3469984b1a3fb2f0943c6 Canon Canon EOS 10D d2c2c9f7b759bd6ffa62f57a28381ac4 Canon Canon EOS 10D 55efbe121f05f17cde4ef656d8c9f4f3 Canon Canon PowerShot A410 981c174a64918a338d4b8ea3c59068bb Canon Canon PowerShot A410
While some manufacturers, like Canon, decided to go with a small set different quantization tables, others vary them frequently depending on the light situation and program settings. Again here is an example, taken from the FUJI FineFix F455.
Now one can compare DQT check sums from pictures of an unknown origin and compare them easily with reference data set. This procedure can hint at the camera make and model or at the graphics program that was used to edit the image.
