andrioid图片适配 和 导演交流过之后顿悟了。

http://mobile.smashingmagazine.com/

在developer.android.com 上 google 告诉我们 alternative  layouts and bitmaps.   告诉了我们换算比例。但是没有告诉我们 基准。 

通过和“导演” 交流，通过一张图明白了 基准。。  640*900  《---》  XHDPI

Create Different Bitmaps

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You should always provide bitmap resources that are properly scaled to each of the generalized density buckets: low, medium, high and extra-high density. This helps you achieve good graphical quality and performance on all screen densities.

To generate these images, you should start with your raw resource in vector format and generate the images for each density using the following size scale:

• xhdpi: 2.0
• hdpi: 1.5
• mdpi: 1.0 (baseline)
• ldpi: 0.75

This means that if you generate a 200x200 image for xhdpi devices, you should generate the same resource in 150x150 for hdpi, 100x100 for mdpi, and 75x75 for ldpi devices.

Then, place the files in the appropriate drawable resource directory:

MyProject/    res/        drawable-xhdpi/            awesomeimage.png        drawable-hdpi/            awesomeimage.png        drawable-mdpi/            awesomeimage.png        drawable-ldpi/            awesomeimage.png

Any time you reference @drawable/awesomeimage, the system selects the appropriate bitmap based on the screen's density.