mipmap目录和drawable目录的区别

官方文档：

Mipmapping for drawables

Using a mipmap as the source for your bitmap or drawable is a simple way to provide a quality image and various image scales, which can be particularly useful if you expect your image to be scaled during an animation.

Android 4.2 (API level 17) added support for mipmaps in the Bitmap class—Android swaps the mip images in your Bitmap when you've supplied a mipmap source and have enabled setHasMipMap(). Now in Android 4.3, you can enable mipmaps for a BitmapDrawable object as well, by providing a mipmap asset and setting the android:mipMap attribute in a bitmap resource file or by calling hasMipMap().

应用场景：

If you know that you are going to draw this bitmap at less than 50% of its original size, you may be able to obtain a higher quality by turning this property on. Note that if the renderer respects this hint it might have to allocate extra memory to hold the mipmap levels for this bitmap.

说明：

1. Mipmaps在android2.2+就存在了，直到android4.3 google才推荐使用。

2. 把图片放到mipmap目录可以提高系统渲染图片的速度和质量，减少GPU压力。

3. 把应用的launcher图标放在 mipmap 目录下，而其它图片资源仍然放在 drawable目录下。