Slicing and dicing images for Google Play
Use target- and pattern-specific variables:

$(ldpi_dir)/btn_%.png : GEOMETRY := 24x
$(mdpi_dir)/btn_%.png : GEOMETRY := 36x
$(hdpi_dir)/btn_%.png : GEOMETRY := 72x


and similar definitions for launcher_%.png

Then you can combine all the individual recopies into a single one:

$(ldpi_dir)/%.png $(mdpi_dir)/%.png $(hdpi_dir)/%.png) :
    convert -geometry $(GEOMETRY) $&lt; $@
    identify $@