Making seasonal cards with Snowglobe

You may remember Snowglobe, a screensaver alike graphical demo of generative fractal snowflakes interacting in a particle system. This week I uploaded two new versions to Hackage, snowglobe-2.0.0.2 which is compatible with the latest Haskell OpenGL libraries, and snowglobe-3 which removes an awkward dependency on hmatrix (replaced with simple small matrix maths copied from an old project of mine) and adds a new feature: saving each generated snowflake to a file (hit shift-S to activate and deactivate saving the files to the current working directory). Which makes snowglobe-2.0.0.2 obsolete already. You can get the latest (when you have ghc and cabal-install available) by:

cabal update && cabal install snowglobe

Having the flake images makes it possible to do all kinds of other fun stuff. They look like this:

flake

I thought it would be nice to send all my neighbours a unique home made seasonal card, using these flake images. I got some card blanks from a local shop, they are 5x7 inches when folded, and from experience my printer doesn't print right to the edge (and margins are good too for aesthetic reasons), so I thought a 4x6 grid of flakes each 1 inch square would be nice.

The flake images are saved as PGM, part of the NetPBM family of formats, also known as PNM. There's a large suite of command line tools to do all kinds of image processing, here's the one's I'll be using:

pngtopnm: convert PNG to PNM, fairly self explanatory.
pnmcat: stack images next to each other, either from top to bottom (-tb) or left to right (-lr).
pnmsplit: takes a stream of PNM images and writes each to a separate file.
pgmtoppm: I use it to invert the colours, but it gives PPM (RGB) output, so...
ppmtopgm: ...this converts RGB back to greyscale.
pnmgamma: gamma adjustment, 0.25 makes it darker.
pnmscale: the flakes are 1024px square, so we've been working at 1024dpi - but the output should be 300dpi, with fewer pixels
pnmtopng: converts to PNG, -force makes the PNG greyscale instead of possibly indexed palette, -interlace makes the PNG interlaced (useful for previewing over a slow network connection, mainly), and the -phys flag sets the DPI (300dpi is 11811 pixels per metre).
xargs: not a NetPBM package at all, but I use it here to group same-sized batches of input lines into the pnmcat command

Here's the whole process, maybe run it one line at a time in case something explodes, you need a fair bit of free disk space too (all the NetPBM formats are uncompressed, each flake is 1MB and there are multiple copies in the temporary files):

#!/bin/bash
mkdir cards
cd cards
gimp
# make 5120x512  black bakcground greyscale image with no alpha > edge.png
# make 1500x2100 white background greyscale image with no alpha > back.png
snowglobe
# hit shift-S to start saving images to current directory, q to quit
# in another window keep track of how many files there are
# you need 24 per card and a few extra because:
geeqie
# delete any you find ugly, there's always a few huge dense ones...
pngtopnm < edge.png > edge.pgm
pngtopnm < back.png > back.pgm
mkdir 4x1
ls snowglobe-*.pgm | xargs -n 4 pnmcat -lr | ( cd 4x1 ; pnmsplit )
mkdir 4x6
ls 4x1/* | xargs -n 6 pnmcat -tb | ( cd 4x6 ; pnmsplit )
mkdir 5x7
cd 4x6
for i in *
do
  pnmcat -tb -black ../edge.pgm $i ../edge.pgm |
  pgmtoppm white-black | ppmtopgm | pnmgamma 0.25 |
  pnmscale -xysize 1500 2100 | pnmcat -lr ../back.pgm - |
  pnmtopng -force -interlace -phys 11811 11811 1 > ../5x7/$i.png
done
cd ../5x7
geeqie
# delete any you find ugly, often the last one is incomplete

# finally, print them - load up your printer with card blanks first
lp -d R220 -o landscape *.png