ImageMagick

ImageMagick is a command line program for mangling pictures. It is called via magick. It is very versatile and have a ton of flags and options. On this page you'll find some useful or interesting technics, you can find experimentations on this project page.

[x] indicate a value to be modified

Table of Content

Basics

Size

The -sample command will have a much more distinct tesselation. -scale and -resize tend to get more blurry.

Deformation

Documentation about warping

Other Useful Things

Dithering

Error Diffusion

-dither FloydSteinberg or Riemersa -colors [number of colors]
It seems that the colors reduction must take place after the dithering for it to work
Reducing size will lead to more obvious patterns and overall a more typical lo-fi dithering look.

Ordered Dithering

-ordered-dither [map]

Lists of maps :

Name Alias Description
threshold 1x1 Threshold 1x1 (non-dither)
checks 2x1 Checkerboard 2x1 (dither)
o2x2 2x2 Ordered 2x2 (dispersed)
o3x3 3x3 Ordered 3x3 (dispersed)
o4x4 4x4 Ordered 4x4 (dispersed)
o8x8 8x8 Ordered 8x8 (dispersed)
h4x4a 4x1 Halftone 4x4 (angled)
h6x6a 6x1 Halftone 6x6 (angled)
h8x8a 8x1 Halftone 8x8 (angled)
h4x4o Halftone 4x4 (orthogonal)
h6x6o Halftone 6x6 (orthogonal)
h8x8o Halftone 8x8 (orthogonal)
h16x16o Halftone 16x16 (orthogonal)
c5x5b c5x5 Circles 5x5 (black)
c5x5w Circles 5x5 (white)
c6x6b c6x6 Circles 6x6 (black)
c6x6w Circles 6x6 (white)
c7x7b c7x7 Circles 7x7 (black)
c7x7w Circles 7x7 (white)

Adding maps

All maps are stored in a file named thresholds.xml that you can find by using
magick identify -list threshold
Here is an example of map that you can append to the file (it creates a diagonal dithering): 
<threshold map="diag5x5" alias="diag">
    <description>Simple Diagonal Line Dither</description>
      <levels width="5" height="5" divisor="6">
      4 2 1 3 5
      2 1 3 5 4
      1 3 5 4 2
      3 5 4 2 1
      5 4 2 1 3
  </levels>
  </threshold>

Using Pictures as map

Each frame of gif will be used for a level of luminosity. This can be used to make ASCII version of a picture.

  1. Collect multiple pictures in a gif: (example with jpg) magick *.jpg out.gif(you may also add -scale [x]x[y] to be sure that all image will be the same size (and squared))
  2. magick input.png -alpha off -colorspace sRGB -grayscale Average \
-scale 1600% -negate \
symbols.gif -virtual-pixel tile -fx 'u[floor(15.9999*u)+1]'\
output.png
    Adjust 15.9999, here it is for a 16 frame gif

Note on Virtual Pixels
Virtual Pixels aim to ease edge processing for algorithm requiering pixel's neighboors for computation. It got various method to define those "virtual pixels" outside of the image, such as black which use black (duh) or tile which repeat the image For more info check the Virtual Pixel Documentation.

Generating a palette

To extract a palette from an image, you can use the code below, you can find a palette script here, with all my bash utilities. 

# speed up process by reducing image size
magick $INPUT -sample 10% temp.png

# find all the colors
COLOR_LIST=`magick temp.png -format %c -colorspace LAB -colors $COLORS histogram:info:-`
# format the list text
FORMATTED_LIST=`echo $COLOR_LIST | sort -n -r | tr ' ' '\n' | sed -n '/^srgb/p' | tr ' ' '\r'`
rm temp.png

# generate an array with the colors to be used in next command
images=()
for ((i=1; i<=COLORS; i++)); do
    color=$(echo "$FORMATTED_LIST" | sed -n "${i-1}p")
    images+=("xc:\"$color\"")
done

# create picture where each pixel is one of the colors
eval magick -size 60x60 "${images[@]}" +append $OUTPUT

Then you can call this command to apply the palette to any image. 

magick input.png -remap palette.png output.png

References

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