I downloaded libjpeg from http://www.ijg.org and crosscompiled to my camera (ARM9 @ 120MHz) and started playing with JPEG decoding (djpeg application). Initially it was delaying 2.8s to convert, then I tested some parameters to improve it a little bit:
# time ./djpeg -fast -dct fast -nosmooth -onepass -grayscale tigre-fractal.jpg > tigre3.bmp real 0m 0.88s user 0m 0.83s sys 0m 0.05s
More info about JPEG:
http://netghost.narod.ru/gff/graphics/book/ch09_06.htm
http://www.cs.sfu.ca/CourseCentral/365/mark/material/notes/Chap4/Chap4.2/Chap4.2.html
http://en.wikipedia.org/wiki/Huffman_coding
http://www.splashnology.com/article/how-to-optimize-png-and-jpeg-without-quality-loss-part-1/2071/
Alan C. Assis 