I've been disecting the CPAN Image::Size module, and have found out some useful stuff
that I wanted to share. The beginners list seemed an appropriate place.
Here's the algorithm for finding the height and width of a JPEG (all nambers are hex):
1. Skip the first two bytes of the file.
2. Read the next four bytes
first byte->$marker
second byte->$code
byte 3 and 4->$length
3. If ($marker != FF) Bomb! It's not a JPEG!
4. If ($code>=C0) and ($code<=C3) GOTO 6
5. Skip the next ($length-2) bytes. GOTO 2.
6. Read the next 5 bytes. Discard the first byte.
The height is in the second and third byte
The width is in the fourth and fifth byte
Happy hacking!
--Nigel
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