Iconography
So, to create an icon for different densities, you should follow the 2:3:4:6:8 scaling ratio between the five primary densities (medium, high, x-high, xx-high, and xxx-high respectively).
Launcher
Sizes & scale
Launcher icons on a mobile device must be48x48dp.
Launcher icons for display on Google Play must be512x512 pixels.
Proportions
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Full asset,48x48 dp
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Style
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Use a distinct silhouette. Three-dimensional, front view, with a slight perspective as if viewed from above, so that users perceive some depth.
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Action Bar
Sizes & scale
Action bar icons for phones should be32x32dp.
Focal area & proportions
Full asset,32x32 dp.Optical square,24x24 dp
Style
Pictographic, flat, not too detailed, with smooth curves or sharp shapes. If the graphic is thin, rotate it 45° left or right to fill the focal space. The thickness of the strokes and negative spaces should be a minimum of 2 dp.
Colors
Colors: #333333 Enabled: 60% opacity Disabled: 30% opacity(Holo Light)
Colors: #FFFFFF Enabled: 80% opacity Disabled: 30% opacity(Holo Dark)
Small / Contextual Icons
Sizes & scale
Small icons should be16x16dp.
Focal area & proportions
Full asset,16x16 dp.Optical square,12x12 dp
Style
Neutral, flat, and simple. Filled shapes are easier to see than thin strokes. Use a single visual metaphor so that a user can easily recognize and understand its purpose.
Colors
Use non-neutral colors sparingly and with purpose. For example, Gmail uses yellow in the star icon to indicate a bookmarked message. If an icon is actionable, choose a color that contrasts well with the background.
Notification Icons
Sizes & scale
Notification icons must be24x24dp.
Focal area & proportions
Full asset,24x24 dp.Optical square,22x22 dp
Style
Keep the style flat and simple, using the same single, visual metaphor as your launcher icon.
Colors
Notification icons must be entirely white. Also, the system may scale down and/or darken the icons.
Design Tips
Use vector shapes where possible
Start with large artboards
When scaling, redraw bitmap layers as needed
Use common naming conventions for icon assets
Set up a working space that organizes files by density
Remove unnecessary metadata from final assets
Tools such asOptiPNG or Pngcrush can ensure that this metadata is removed and that your image asset file sizes are optimized.
http://developer.android.com/design/style/iconography.html#notification