iPhone快速参考：从一个图片截取部分区域生成新的小图
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0.0,?0.0, image.size.width, image.size.height);
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?? ?CGSize itemSize = CGSizeMake(w,h);
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?? ?UIGraphicsBeginImageContext(itemSize);
?? ?[image drawInRect:imageRect];
?? ?UIImage *smallimage?= UIGraphicsGetImageFromCurrentImageContext();
?? ?UIGraphicsEndImageContext();
