UIImage压缩和缩放
节省大量内存，重绘缩略图；

3种方法，分别是调整、居中、填充方式。

1、等比例缩放。需要根据图像的长宽比在其四周留出一些额外区域，用透明像素为图像消关。
+ (CGSize) fitSize: (CGSize)thisSize inSize: (CGSize) aSize
{
CGFloat scale;
CGSize newsize = thisSize;

if (newsize.height && (newsize.height > aSize.height))
{
scale = aSize.height / newsize.height;
newsize.width *= scale;
newsize.height *= scale;
}

if (newsize.width && (newsize.width >= aSize.width))
{
scale = aSize.width / newsize.width;
newsize.width *= scale;
newsize.height *= scale;
}

return newsize;
}
2、抽取部分图像来匹配可用空间。并裁剪掉落在像素区外的任何元素。
//返回调整的缩略图
+ (UIImage *) image: (UIImage *) image fitInSize: (CGSize) viewsize
{
// calculate the fitted size
CGSize size = [ImageHelper fitSize:image.size inSize:viewsize];

UIGraphicsBeginImageContext(viewsize);

float dwidth = (viewsize.width - size.width) / 2.0f;
float dheight = (viewsize.height - size.height) / 2.0f;

CGRect rect = CGRectMake(dwidth, dheight, size.width, size.height);
[image drawInRect:rect];

UIImage *newimg = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

return newimg;
}
3、通过匹配可用空间的长度和宽度来填充图像。每个像素都被使用，但是图像将水平或垂直裁剪。
//返回居中的缩略图
+ (UIImage *) image: (UIImage *) image centerInSize: (CGSize) viewsize
{
CGSize size = image.size;

UIGraphicsBeginImageContext(viewsize);
float dwidth = (viewsize.width - size.width) / 2.0f;
float dheight = (viewsize.height - size.height) / 2.0f;

CGRect rect = CGRectMake(dwidth, dheight, size.width, size.height);
[image drawInRect:rect];

UIImage *newimg = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

return newimg;
}

//返回填充的缩略图
+ (UIImage *) image: (UIImage *) image fillSize: (CGSize) viewsize

{
CGSize size = image.size;

CGFloat scalex = viewsize.width / size.width;
CGFloat scaley = viewsize.height / size.height;
CGFloat scale = MAX(scalex, scaley);

UIGraphicsBeginImageContext(viewsize);

CGFloat width = size.width * scale;
CGFloat height = size.height * scale;

float dwidth = ((viewsize.width - width) / 2.0f);
float dheight = ((viewsize.height - height) / 2.0f);

CGRect rect = CGRectMake(dwidth, dheight, size.width * scale, size.height * scale);
[image drawInRect:rect];

UIImage *newimg = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

return newimg;
}

//相对比较简单的压缩
- (UIImage *)image:(UIImage *)image centerInSize:(CGSize)viewsize
{
UIGraphicsBeginImageContext(CGSizeMake(viewsize.width, viewsize.height));
[image drawInRect:CGRectMake(0, 0, viewsize.width, viewsize.height)];
UIImage *reSizeImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

return reSizeImage;
}

测试

NSDictionary *cate = [self.cates objectAtIndex:indexPath.row];    UIImage *img1 = [UIImage imageNamed:[[cate objectForKey:@"imageName"] stringByAppendingString:@".jpg"]];    NSData *data1 = UIImageJPEGRepresentation(img1, 1.0);    NSLog(@"data1 = %d",data1.length);    UIImage *img2 = [self image:[UIImage imageNamed:[[cate objectForKey:@"imageName"] stringByAppendingString:@".png"]] centerInSize:CGSizeMake(140, 140)];    NSData *data2 = UIImageJPEGRepresentation(img2, 1.0);    NSLog(@"data2 = %d",data2.length);

打印：
2013-07-01 14:04:03.194 Abviewer_iPhone[8829:907] data1 = 288658
2013-07-01 14:04:03.198 Abviewer_iPhone[8829:907] data2 = 859