%g.jpg: %.jpg
	convert $< -set colorspace Gray -separate -average $@
%g.png: %.png
	convert $< -set colorspace Gray -separate -average $@
%g.pdf: %.pdf
	gs \
		-sDEVICE=pdfwrite \
		-sProcessColorModel=DeviceGray \
		-sColorConversionStrategy=Gray \
		-dOverrideICC \
		-dCompatibilityLevel=1.5 \
		-o $@ \
		-f $<