Many thanks to all the regulars on #gimp who answered my questions, gave me advice, and bugged me to write this stuff up. The GIMP is my friend. It makes me happy. :-) Background ---------- I've recently been trying to reproduce really odd film grain to hide image editing. This process has three steps: 1) Extract the old grain. 2) Make it into a tileable pattern. 3) Merge the pattern into a new image. Step (1): To extract the old grain, make a copy of the image, blur it, and compare it to the original. The "flat" areas of the image should now contain some kind of characteristic noise--grains, vertical lines, and so on. (The trick here is the comparsion; more details below.) You may--or may not--want to desaturate your noise! Step (2): To make a pattern tileable, use "Make Seamless", the Resynthesizer & Homogenizer, hand-hacking, or your favorite tricks. Step (3): To merge the pattern back into the image, create a channel of solid noise, mask it as needed, and add it to the underlying channel. Again, the addition is tricky. Addition & Subtraction ----------------------
