问题
My app supports various screen densities from ldpi to xhdpi. It contains some drawables (images) that need to be displayed properly on all densities. I'm wondering how many various resolutions of the drawables are actually necessary to provide, considering that Android runtime is scaling resources when it cannot find one for the appropriate density.
If you provide only mdpi drawables, they will be scaled up on hdpi/xhdpi (blurry) and scaled down on ldpi screen.
However, if you provide only xhdi resources, then they will be scaled down on all lower density screens. Scaling down doesn't make them blurry and they will look fine.
So why bother providing drawables for other densities than xhdpi?
回答1:
There are several reasons to used prescaled drawables:
Scaling down to lower resolutions takes resources (CPU time, memory and it drains your battery faster), especially if you have a lot of images this might be undesired.
Scaling down a large image might result in worse quality than using a 'pre-tuned' smaller image. Especially if your images are based on a vector original. Or they contain very fine lines or details, which will get lost when due to the scaling.
For example:
becomes
See here for the source of these images and more information on scaling artifacts.On lower resolutions the image pixels are larger (or the image gets smaller), so it might be required to remove certain aspects of the image to keep it clear/understandable.
becomes 
回答2:
Your assumption about scaling resources isn't correct. Some large images may still look nice when they are scaled down, but this isn't the general case. If the image has smooth transitions and no gradients, it will be nicely scaled down. Images that contain sharp transitions or contain text for example will look ugly in lower resolutions.
回答3:
If you make the scaling down yourself the result might be better compared to auto scaling plus your app doesnt consume resources for scaling if the drawables are prepared.
回答4:
Not necessary but for example, you can use different layouts with different drawables for different screen resolutions. You have the option to do it. If you and your users are satisfied with the result, then cool. If you need different behavior in different densities and resolutions, then this feature comes really useful.
来源：https://stackoverflow.com/questions/14214843/why-provide-multiple-drawables-for-various-densities