今天翻了下墙，解决了一直以来的疑惑问题：
为什么Android5.0以及6.0的recovery版本，机器人动画怎么就只有一张图片？
这个问题，我百思不得其解，看了很多网文，也只是有了个概念。
请参考以下文档，这是我从谷歌Android开源网拉下来的原文:
https://source.android.com/devices/tech/ota/device_code
Recovery UI images
Android 5.x
The recovery user interface consists images. Ideally, users never interact with the UI: During a normal update, the phone boots into recovery, fills the installation progress bar, and boots back into the new system without input from the user. In the event of a system update problem, the only user action that can be taken is to call customer care.
An image-only interface obviates the need for localization. However, as of Android 5.x the update can display a string of text (e.g. "Installing system update...") along with the image. For details, see Localized recovery text.
bootable/recovery/interlace-frames.py takes a set of input frames and combines them into the necessary composite image used by recovery.bootable/recovery/res$DENSITY/images (e.g., bootable/recovery/res-hdpi/images). To use a static image during installation, you need only provide the icon_installing.png image and set the number of frames in the animation to 0 (the error icon is not animated; it is always a static image).The Android 4.x and earlier recovery UI uses the error image (shown above) and the installing animation plus several overlay images:
During installation, the on-screen display is constructed by drawing the icon_installing.png image, then drawing one of the overlay frames on top of it at the proper offset. Here, a red box is superimposed to highlight where the overlay is placed on top of the base image:
Subsequent frames are displayed by drawing only the next overlay image atop what's already there; the base image is not redrawn.
The number of frames in the animation, desired speed, and x- and y-offsets of the overlay relative to the base are set by member variables of the ScreenRecoveryUI class. When using custom images instead of default images, override the Init() method in your subclass to change these values for your custom images (for details, see ScreenRecoveryUI). The script bootable/recovery/make-overlay.py can assist in converting a set of image frames to the "base image + overlay images" form needed by recovery, including computing of the necessary offsets.
Default images are located in bootable/recovery/res/images. To use a static image during installation, you need only provide the icon_installing.png image and set the number of frames in the animation to 0 (the error icon is not animated; it is always a static image).
上面说了这么多，其实也是把这上面的图片通过make-overlay.py这个python脚本来对4.x以及早期版本的android recovery的图片进行合成，中间让我们看到的转转转那个overlay的效果就是上面这些组图合成的，合成最终的图片存放在bootable/recovery/res/images这个路径下。