iOS压缩图片大小

//图片伸缩到指定大小  
- (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize forImage:(UIImage *)originImage  
{  
    UIImage *sourceImage = originImage;// 原图  
    UIImage *newImage = nil;// 新图  
    // 原图尺寸  
    CGSize imageSize = sourceImage.size;  
    CGFloat width = imageSize.width;  
    CGFloat height = imageSize.height;  
    CGFloat targetWidth = targetSize.width;// 目标宽度  
    CGFloat targetHeight = targetSize.height;// 目标高度  
    // 伸缩参数初始化  
    CGFloat scaleFactor = 0.0;  
    CGFloat scaledWidth = targetWidth;  
    CGFloat scaledHeight = targetHeight;  
    CGPoint thumbnailPoint = CGPointMake(0.0,0.0);  
      
    if (CGSizeEqualToSize(imageSize, targetSize) == NO)  
    {// 如果原尺寸与目标尺寸不同才执行  
        CGFloat widthFactor = targetWidth / width;  
        CGFloat heightFactor = targetHeight / height;  
          
        if (widthFactor > heightFactor)  
            scaleFactor = widthFactor; // 根据宽度伸缩  
        else  
            scaleFactor = heightFactor; // 根据高度伸缩  
        scaledWidth= width * scaleFactor;  
        scaledHeight = height * scaleFactor;  
          
        // 定位图片的中心点  
        if (widthFactor > heightFactor)  
        {  
            thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;  
        }  
        else if (widthFactor < heightFactor)  
        {  
            thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;  
        }  
    }  
      
    // 创建基于位图的上下文  
    UIGraphicsBeginImageContext(targetSize);  
      
    // 目标尺寸  
    CGRect thumbnailRect = CGRectZero;  
    thumbnailRect.origin = thumbnailPoint;  
    thumbnailRect.size.width= scaledWidth;  
    thumbnailRect.size.height = scaledHeight;  
      
    [sourceImage drawInRect:thumbnailRect];  
      
    // 新图片  
    newImage = UIGraphicsGetImageFromCurrentImageContext();  
    if(newImage == nil)  
        NSLog(@"could not scale image");  
      
    // 退出位图上下文  
    UIGraphicsEndImageContext();  
    return newImage;  
}