bitmap on ldpi mdpi hdpi nodpi

• ldpi : Low-density screens; approximately 120dpi.
• mdpi : Medium-density (on traditional HVGA) screens; approximately 160dpi.
• hdpi : High-density screens; approximately 240dpi.
• nodpi : This can be used for bitmap resources that you do not want to be scaled to match the device density.

There is thus a 4:3 scaling factor between each density, so a 9x9 bitmap in ldpi is 12x12 in mdpi and 16x16 in hdpi.