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

Basics

Size

-sample [x]%
-scale [x]%
-resize [x]%

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

Deformation

Documentation about warping

-rotate [degree]
-shear [val]x[val]
-swirl [degree]

Other Useful Things

-alpha off : Deactivate the transparency layer
-threshold [x]% : Basic black and white by comparison
-quality [value] : Value between 0 and 10
-edge [thikness] : Keep only the edges of the image
-blur [radius]x[sigma] : For the calculation of a pixel, radius determine the area around that is involved, sigma the amount of contribution
-modulate [hue]x[sat]x[lightness] : HSL modulation, 100 = no modification
+level-colors [replace black with],[replace white with] : easy manipulation of colors. List of all imagemagick colors can be found on this webpage
magick in.source out.destination : convert from any format to any other

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.

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))

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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