Artistic Photo Processing

Create a visual image directory (thumbnail card) of all images in a directory:

$ gm convert 'vid:*.jpg' directory.jpg
or
$ gm convert 'vid:*.jpg' directory.miff

To view .miff files:
$ gm display directory.miff

Create a charcoal drawing from a photo:

$ gm convert -charcoal FACTOR INPUT.xxx OUTPUT.xxx
or
$ gm mogrify -charcoal FACTOR FILE.xxx
or
$ convert INPUT.xxx -charcoal 5 OUTPUT.png

Create a colored-in outline image from a photo:

$ convert INPUT.xxx -colorspace gray \( +clone -blur 0x2 \) +swap -compose divide -composite -linear-stretch 5%x0%  OUTPUT.png

Create a pencil sketch from a photo:

$ convert INPUT.xxx -colorspace gray -sketch 0x20+120 OUTPUT.gif

Create an oil painting from a photo:

$ gm convert -paint 7 INPUT.xxx OUTPUT.xxx

Create an Embossed image from a photo:

use emboss script

$ ./emboss -m 1 -d 7 INPUT.xxx OUTPUT.xxx

Texturize a photo:

(this method requires an external texture file, INPUT.xxx)

$ composite texture_file.gif INPUT.xxx -tile -compose Hardlight OUTPUT.jpg

Make a photo look like a Cartoon/Comic output:

use cartoon script

$ ./cartoon INPUT.xxx OUTPUT.xxx

ASCII/ANSI Art utilities

jp2a CLI tool (image to ASCII/ANSI)

$ jp2a --size=320x240 input.jpg --output=output_ascii.txt
or
$ jp2a --width=320 --color --html --fill --background=light input.jpg --output=output.html

caca-utils package (image to ANSI)

$ img2txt -W 480 -f tga ./input.jpg > output_ansi.tga

Gnome ASCII art generator (GAAT), similar to jp2a but not as many output options.

cacaview (views images using ascii, only good for small resolution images like icons)

nfoview (NFO viewer that handles the ascii art code page properly)

python-aafigure (converts ascii to png or svg, works better with the ascii generated from GAAT):

$ aafigure input_ascii -t png -o output.png

acidview (need to run under Wine, for viewing animated ANSI)  Anyone know of a native Linux software that can view animated ANSI art files (and I mean a program that acutally shows the animations)?

PabloDraw This is acutally a "native Linux" via Mono  application that can view animated ANSI.  It also requires a very new version of Mono which rules out a lot of Debian distros, but works fine on my Slackware box.

a3b (ascii art viewer)

tetradraw/tetraview (ANSI art drawing and viewing)

ansi2gif  Creates animated GIFs from animated or blinking ANSI art.  Very cool!  Some examples of usage:
$ ./ansi2gif --gif --ysize auto INPUT.ANS OUTPUT.GIF
$ ./ansi2gif --animate --timedelay 2 --gif --ysize auto INPUT.ANS OUTPUT.GIF
$ ./ansi2gif --blink --gif INPUT.ANS OUTPUT.GIF

Animated GIFs from Videos 

Here are some methods for producing animated GIF from video/movie files.  In contrast to other webpages or blog posts you'll find if you search on this topic, these two methods actually work with contemporary ffmpeg and ImageMagick as of the year 2012.

Method 1 (basic method):
Specify an input file, the time to start (-ss), the duration in seconds (-t), and how many fps (-r).
$ ffmpeg -i INPUT.mp4 -r 10 -ss 00:14:31 -t 00:00:10 /home/user/temp/output%05d.png

$ convert /home/user/temp/output*.png /home/user/temp/output.gif

Optimize the output, some of these options may be redundant depending on the input file:
$ gifsicle --delay=50 --loop --optimize=2 --colors=256 --multifile /home/user/temp/output.gif > /home/user/temp/final_output.gif

Method 2 (produces smaller more refined GIFs), original source here:
Specify an input file, the time to start (-ss), how many fps (-r), and the total amount of frames (-vframes).
$ ffmpeg -ss 00:12:30 -i INPUT -r 8 -vframes 32 -vbsf remove_extra -an -vcodec pam -f rawvideo -y /home/user/temp/anim.pam

Adjust the -resize to your specific needs.
$ convert /home/user/temp/anim.pam -depth 32 -colorspace YCC -resize 300x169! -colorspace Lab -identify /home/user/temp/anim.miff

Use the output of the next command in order to adjust the dither levels and threshold pattern try to hit 255 colors on the output. The bare minimum dither should be 15,11,11. That gives 15 levels of luminance and 11*11=121 different colors, plus the dithering itself which fakes other hues. AB levels from 11 to 13 are best.

$ convert /home/user/temp/anim.miff -ordered-dither o8x8,15,11,11 -depth 8 -colorspace sRGB -append -format %k info:

Final conversion command, adjust -ordered-dither o8x8,25,12,12 to the values you determined hit 255 colors in the previous step:

$ convert /home/user/temp/anim.miff -ordered-dither o8x8,25,12,12 -depth 8 -colorspace sRGB +dither +remap -coalesce -deconstruct -layers RemoveDups -layers Optimize -delay 1x9 -identify/home/user/temp/OUTPUT.GIF

If the output is too large, try dropping the levels or go to o2x2 for a bigger size drop.

Also take a look at this for more tips.

Other

Gimp - I'm not a huge user of Gimp.  It does too much and I find the command line easier.
G'MIC gimp plugin - Many different artistic conversions, has preview function.
Gimp Plug-Ins Registry - If you're going to use Gimp, then plug-ins and scripts are a must have.
Image processing utilities
Image to ASCII Art
Another Image to ASCII Art generator
stereograph/xstereograph ("magic eye" stereogram tools for Linux! yay!)

Photomosaics

metapixel  (really nice!  Very professional results!)
pixelize (also very nice!)

Here is a quick tutorial on using metapixel:

First you need to prepare your source images.

$ metapixel-prepare -r /source_dir /destination_dir --width=24 --height=24

You can do that for each source directory full of images.  Note, this is a recursive operation, so if you have all your images under one directory with sub-directories, they all get processed.

$ metapixel --metapixel input.jpg output.png --library /destination_dir --scale=35 --distance=500

or try this

$ metapixel --metapixel input.jpg output.png -l /destination_dir1 -l /destination_dir1 -l /destination_dir1 --scale=15 --distance=3

The example above is using 3 directories of prepared tile images.

Play with scale and destination to suit your needs.  Read the man page before proceeding.