Bitmap - 称作位图，一般位图的文件格式后缀为bmp，当然编码器也有很多如RGB565、RGB888。作为一种逐像素的显示对象执行效率高，但是缺点也很明显存储效率低。我们理解为一种存储对象比较好。
    Drawable - 作为Android平下通用的图形对象，它可以装载常用格式的图像，比如GIF、PNG、JPG，当然也支持BMP，当然还提供一些高级的可视化对象，比如渐变、图形等。

A bitmap is a Drawable. A Drawable is not necessarily a bitmap. Like all thumbs are fingers but not all fingers are thumbs.
Bitmap是Drawable . Drawable不一定是Bitmap .就像拇指是指头,但不是所有的指头都是拇指一样.

The API dictates: API规定:

Though usually not visible to the application, Drawables may take a variety of forms: 尽管通常情况下对于应用是不可见的,Drawables 可以采取很多形式:

Bitmap: the simplest Drawable, a PNG or JPEG image. Bitmap: 简单化的Drawable, PNG 或JPEG图像. 
Nine Patch: an extension to the PNG format allows it to specify information about how to stretch it and place things inside of it.
Shape: contains simple drawing commands instead of a raw bitmap, allowing it to resize better in some cases.
Layers: a compound drawable, which draws multiple underlying drawables on top of each other.
States: a compound drawable that selects one of a set of drawables based on its state.
Levels: a compound drawable that selects one of a set of drawables based on its level.
Scale: a compound drawable with a single child drawable, whose overall size is modified based on the current level.

小结：

对比项          显示清晰度      占用内存     支持缩放    支持色相色差调整      支持旋转     支持透明色       绘制速度           支持像素操作

Bitmap          相同                大                 是                是                            是                是                     慢                      是
Drawable       相同                小                 是               否                            是                 是                     快                      否

Drawable在内存占用和绘制速度这两个非常关键的点上胜过Bitmap

1)Drawable和Bitmap之间可以互相转换。Drawable占用内存远小于Bitmap。
(2)setImageDrawable使用资源文件；setImageBitmap使用bitmap图片，该图片可能是读取本地相册，或者从资源文件转换而来。
(3) public void setImageResource (int resId)占用UI thread;

ImageView iv; 
String fileName = "/data/data/com.test/aa.png; 
Bitmap bm = BitmapFactory.decodeFile(fileName); 
iv.setImageBitmap(bm); //占用内存，ie：


Bitmap image = BitmapFactory.decodeFile(imgFile.getAbsolutePath());
imageView.setImageBitmap(image);


Bitmap image = BitmapFactory.decodeFile(imgFile.getAbsolutePath());
BitmapDrawable bitmapDrawable = new BitmapDrawable(image);
imageView.setImageDrawable(bitmapDrawable);

Drawable和Bitmap区别