convert txt:

> This is the Unix philosophy: ... Write programs to handle text streams,
> because that is a universal interface.
>	Doug McIlroy

But images do not use text format, which happens to be my problem. I need to
find the top left red pixel and make a 210x320 image starting at that pixel.

So I can divide the task into two parts:
- Find the top left red pixel
- Crop the image

Someone else already did the cropping part, see
https://superuser.com/a/1163824, and that leaves me with finding the top left
red pixel. I could have used libpng or libjpeg, but that is format-dependent.
So the only choice left is to convert it to a text format that standard
command-line tools can process. After 5 minutes of web search on "imagemagick
convert image to text," I got:

	$ convert image.png txt:
	0,0: (36,147,252)  #FF0000  srgb(36,147,252)
	1,0: (36,147,252)  #FF0000  srgb(36,147,252)
	2,0: (36,147,252)  #FF0000  srgb(36,147,252)
	3,0: (36,147,252)  #FF0000  srgb(36,147,252)
	4,0: (36,147,252)  #FF0000  srgb(36,147,252)
	5,0: (36,147,252)  #FF0000  srgb(36,147,252)
	6,0: (36,147,252)  #FF0000  srgb(36,147,252)
	7,0: (36,147,252)  #FF0000  srgb(36,147,252)
	8,0: (36,147,252)  #FF0000  srgb(36,147,252)
	...

This sed command:

       $ sed -n "/#FF0000/{p;q}"

prints the first line that contains #FF0000 (red in RGB) and quit. To make it
print the x and y coordinate of the first red pixel, I use:

	$ sed -n '/#FF0000/{s/^\([0-9]\+\),\([0-9]\+\)\(.*\)/\1 \2/g;p;q}'
	0 0

Put it all together, I got:
	crop="210x320+`convert original.png | sed -n '/#FF0000/{s/^\([0-9]\+\),\([0-9]\+\)\(.*\)/\1+\2/g;p;q}'`"
	convert original.png -crop "$crop" cropped.png

Thanks for reading.