iOS开发之UIImage等比缩放

前面讲了截取UIImage指定大小区域，很方便的截取UIImage。今天要和大家分享的是UIImage的缩放。

两种缩放:

1. 缩放到指定大小，也就是指定的size.
2. 等比缩放。

缩放到指定大小

- (UIImage*)imageCompressWithSimple:(UIImage*)image scaledToSize:(CGSize)size
{
    UIGraphicsBeginImageContext(size);
    [image drawInRect:CGRectMake(0,0,size.width,size.height)];
    UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}

等比缩放

通过缩放系数

- (UIImage*)imageCompressWithSimple:(UIImage*)image scale:(float)scale
{
    CGSize size = image.size;
    CGFloat width = size.width;
    CGFloat height = size.height;
    CGFloat scaledWidth = width * scale;
    CGFloat scaledHeight = height * scale;
    UIGraphicsBeginImageContext(size); // this will crop
    [image drawInRect:CGRectMake(0,0,scaledWidth,scaledHeight)];
    UIImage* newImage= UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}

scale是缩放系数 。

通过计算得到缩放系数

- (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize
{

    UIImage *sourceImage = [UIImage imageNamed:@"test.jpg"];
    UIImage *newImage = nil;
    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;
    CGFloat targetWidth = targetSize.width;
    CGFloat targetHeight = targetSize.height;
    CGFloat scaleFactor = 0.0;
    CGFloat scaledWidth = targetWidth;
    CGFloat scaledHeight = targetHeight;
    CGPoint thumbnailPoint = CGPointMake(0.0,0.0);

    if (CGSizeEqualToSize(imageSize, targetSize) == NO)
    {
        CGFloat widthFactor = targetWidth / width;
        CGFloat heightFactor = targetHeight / height;
        if (widthFactor > heightFactor)
            scaleFactor = widthFactor; // scale to fit height
        else
            scaleFactor = heightFactor; // scale to fit width

        scaledWidth= width * scaleFactor;
        scaledHeight = height * scaleFactor;
        // center the image
        if (widthFactor > heightFactor)
        {
            thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
        }
        else if (widthFactor < heightFactor)
        {
            thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
        }
    }

    UIGraphicsBeginImageContext(targetSize); // this will crop
    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = thumbnailPoint;
    thumbnailRect.size.width= scaledWidth;
    thumbnailRect.size.height = scaledHeight;
    [sourceImage drawInRect:thumbnailRect];
    newImage = UIGraphicsGetImageFromCurrentImageContext();

    if(newImage == nil)
        NSLog(@"could not scale image");
    //pop the context to get back to the default
    UIGraphicsEndImageContext();

    return newImage;

}

很久之前写了一篇文章解决MWPhotoBrowser中的SDWebImage加载大图导致的内存警告问题。这个我记得当时从服务器拿到的照片大概有10几M的样子，加载出来会导致内存警告。所以我当时通过修改SDWebImage源码，就是把下载下来的照片进行缩放，内存就降下来了。一般情况下应该不会加载这么大的照片的，用户要是知道，早把你的APP给删掉了。

作者李刚是刚刚在线（ www. ）站长，百度百家专栏作者

iOS工程师非著名自媒体，微信公众号iOS开发：iOSDevTip运营者