• A Drawable resource is a general concept for a drawing-able picture. A graphics file (bitmap) is the most basic instance, which is represented in Android by the BitmapDrawable class.
• In one of the res/drawable folders, each Drawable is saved as a separate file. Bitmaps for various resolutions are often stored in the res/drawable subfolders -mdpi, -hdpi, -xhdpi, -xxhdpi. By default, the ADT project creation wizard creates these directories. If these bitmaps are provided in separate folders, the Android system automatically selects the relevant one based on the device setup.
• The Android system scales the closest fit up or down if you don't offer bitmaps for all supported resolutions. This is usually undesirable since the bitmap may get blurry.

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