There are two cases you deal with when working with images in Android: You want to load an image for your device density and you are going to use it “as is”, without changing its actual size. In this case you should work with drawables and Android will give you the best fitting image. You want to load an image for your device density, but this image is going to be scaled up or down. For instance this is needed when you want to show a bigger launcher icon, or you have an animation, which increases image’s size. In such cases, to ensure best image quality, you should put your image into mipmap folder. What Android will do is, it will try to pick up the image from a higher density bucket instead of scaling it up. SO Thus, the rule of thumb to decide where to put your image into would be: Launcher icons always go into mipmap folder. Images, which are often scaled up (or extremely scaled down) and whose quality is critical for the app, go into mipmap folder as well. All other images are usual drawables.

本文引证。

Android 4.3中mipmaps的实现正是1983年维基百科文章中解释的技术:)

mipmap集的每个位图图像都是 主要纹理，但在一定程度上减少了细节。虽然 当视图足够渲染时，主纹理仍将被使用 它的细节充分，渲染器将切换到合适的mipmap图像 (…)当从远处或小尺寸观察纹理时。

虽然这被描述为3D图形的技术(因为它提到了“从远处观看”)，但它同样适用于2D图形(翻译为“绘制的是一个更小的空间”，即。“缩减规模”)。

对于一个具体的Android例子，假设你有一个带有特定背景可绘制对象的视图(特别是BitmapDrawable)。现在使用动画将其缩放到原始大小的0.15。通常，这需要为每一帧降低背景位图的比例。然而，这种“极端”的缩小尺度可能会产生视觉上的伪影。

但是，您可以提供一个mipmap，这意味着图像已经预先渲染了一些特定的比例(比如1.0、0.5和0.25)。每当动画“越过”0.5的阈值时，它不会继续缩小原始的1.0大小的图像，而是切换到0.5的图像并缩小它，这应该会提供更好的结果。随着动画的继续。

这有点理论化，因为它实际上是由渲染器完成的。根据Bitmap类的源代码，它只是一个提示，渲染器可能尊重它，也可能不尊重。

/**
 * Set a hint for the renderer responsible for drawing this bitmap
 * indicating that it should attempt to use mipmaps when this bitmap
 * is drawn scaled down.
 *
 * 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.
 *
 * This property is only a suggestion that can be ignored by the
 * renderer. It is not guaranteed to have any effect.
 *
 * @param hasMipMap indicates whether the renderer should attempt
 *                  to use mipmaps
 *
 * @see #hasMipMap()
 */
public final void setHasMipMap(boolean hasMipMap) {
    nativeSetHasMipMap(mNativeBitmap, hasMipMap);
}

不过，我不太确定为什么这特别适合应用程序图标。尽管平板电脑上的Android以及一些启动器(如GEL)要求图标“更高密度”以显示它更大，但这应该使用常规机制(即draw -xxxhdpi， &c)来完成。

mipmap有两种不同的用法:

For launcher icons when building density specific APKs. Some developers build separate APKs for every density, to keep the APK size down. However some launchers (shipped with some devices, or available on the Play Store) use larger icon sizes than the standard 48dp. Launchers use getDrawableForDensity and scale down if needed, rather than up, so the icons are high quality. For example on an hdpi tablet the launcher might load the xhdpi icon. By placing your launcher icon in the mipmap-xhdpi directory, it will not be stripped the way a drawable-xhdpi directory is when building an APK for hdpi devices. If you're building a single APK for all devices, then this doesn't really matter as the launcher can access the drawable resources for the desired density. The actual mipmap API from 4.3. I haven't used this and am not familiar with it. It's not used by the Android Open Source Project launchers and I'm not aware of any other launcher using.

如果你为HDPI这样的目标屏幕分辨率构建APK, Android资产打包工具AAPT就可以为你不需要的其他分辨率剔除绘图。但是如果它在mipmap文件夹中，那么不管目标分辨率如何，这些资产都将留在APK中。

我在另一个线程中提到的一件事值得指出——如果你为不同的密度构建不同版本的应用，你应该了解“mipmap”资源目录。这与“可绘制”资源完全相同，只是在创建不同的apk目标时它不参与密度剥离。

https://plus.google.com/105051985738280261832/posts/QTA9McYan1L

这些mipmap图像与其他熟悉的可绘制图像有何不同?

以下是我的观点，试图解释其中的区别。在Android中处理图像时有两种情况:

You want to load an image for your device density and you are going to use it "as is", without changing its actual size. In this case you should work with drawables and Android will give you the best fitting image. You want to load an image for your device density, but this image is going to be scaled up or down. For instance this is needed when you want to show a bigger launcher icon, or you have an animation, which increases image's size. In such cases, to ensure best image quality, you should put your image into mipmap folder. What Android will do is, it will try to pick up the image from a higher density bucket instead of scaling it up. This will increase sharpness (quality) of the image.

因此，决定把你的图像放在哪里的经验法则是:

启动器图标总是进入mipmap文件夹。 图片，通常是按比例放大(或极度缩小)，其质量是至关重要的应用程序，也进入mipmap文件夹。 所有其他图像通常都是可绘制的。