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Add images to a set that are customized for display in different size classes supported by your app.
In the set list, select an image set.
在集合列表(set list)中，选择一个图片集。
Open the utilities area for the workspace window by clicking the Show Utilities button (
).
点击()显示工作区窗口的工具区(utilities area).
In the inspector bar, click the Attributes Inspector button (
).
在inspector 栏，点击属性检查按钮 ()。
Choose Universal from the Device pop-up menu.
在设备弹出菜单那选择Universal.
Choose a specific size class from the Width or Height size pop-up menu or from both.
在width 或者Height弹出菜单中选择一个指定的尺寸类(size class)。
After you choose a size class, Xcode adds placeholders for image files in the set viewer area.
选择完成后,Xcode 在图片集查看区(set viewer)为图片文件添加占位符。
The size class settings for a placeholder image are shown in coded format following the size of the image. The format is [horizontal size class, vertical size class]. Each size class can be one of three values:
size class(尺寸类) 为一个占位图片的格式是把图片的尺寸以编码格式显示。 格式为[横向尺寸类 纵向尺寸类]. 每个尺寸类 可以是以下三个值中的一个：
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Any size class 任何尺寸类 |
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Regular size class 常规尺寸类 |
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Compact size class 紧凑尺寸类 |
For example, 
is a 2x resolution image used with a compact horizontal size class and any vertical size class.
比如， 2x[- *] 是一个2倍分辨率图片，它的尺寸类是紧凑横向尺寸类和纵向任何尺寸类。
Asset Catalog Help (七)---Customizing Image Sets for Size Classes
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原文地址：http://www.cnblogs.com/patientAndPersist/p/4428689.html
