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iOS 图片压缩方法

iOS 图片压缩方法

作者: 叩首问路梦码为生 | 来源:发表于2020-09-14 17:22 被阅读0次

    iOS 图片压缩方法

    更多图片处理方法见图片组件 BBWebImage

    iOS 图片压缩方法

    两种图片压缩方法

    两种压缩图片的方法:压缩图片质量(Quality),压缩图片尺寸(Size)。

    压缩图片质量

    NSData *data = UIImageJPEGRepresentation(image, compression);
    UIImage *resultImage = [UIImage imageWithData:data];
    
    

    通过 UIImage 和 NSData 的相互转化,减小 JPEG 图片的质量来压缩图片。UIImageJPEGRepresentation:: 第二个参数 compression 取值 0.0~1.0,值越小表示图片质量越低,图片文件自然越小。

    压缩图片尺寸

    UIGraphicsBeginImageContext(size);
    [image drawInRect:CGRectMake(0, 0, size.width, size.height)];
    resultImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    

    给定所需的图片尺寸 size,resultImage 即为原图 image 绘制为 size 大小的图片。

    压缩图片使图片文件小于指定大小

    如果对图片清晰度要求不高,要求图片的上传、下载速度快的话,上传图片前需要压缩图片。压缩到什么程度要看具体情况,但一般会设定一个图片文件最大值,例如 100 KB。可以用上诉两种方法来压缩图片。假设图片转化来的 NSData 对象为 data,通过data.length即可得到图片的字节大小。

    压缩图片质量

    比较容易想到的方法是,通过循环来逐渐减小图片质量,直到图片稍小于指定大小(maxLength)。

    + (UIImage *)compressImageQuality:(UIImage *)image toByte:(NSInteger)maxLength {
        CGFloat compression = 1;
        NSData *data = UIImageJPEGRepresentation(image, compression);
        while (data.length > maxLength && compression > 0) {
            compression -= 0.02;
            data = UIImageJPEGRepresentation(image, compression); // When compression less than a value, this code dose not work
        }
    
        UIImage *resultImage = [UIImage imageWithData:data];
        return resultImage;
    }
    
    

    这样循环次数多,效率低,耗时长。

    可以通过二分法来优化。

    + (UIImage *)compressImageQuality:(UIImage *)image toByte:(NSInteger)maxLength {
        CGFloat compression = 1;
        NSData *data = UIImageJPEGRepresentation(image, compression);
        if (data.length < maxLength) return image;
        CGFloat max = 1;
        CGFloat min = 0;
        for (int i = 0; i < 6; ++i) {
            compression = (max + min) / 2;
            data = UIImageJPEGRepresentation(image, compression);
            if (data.length < maxLength * 0.9) {
                min = compression;
            } else if (data.length > maxLength) {
                max = compression;
            } else {
                break;
            }
        }
        UIImage *resultImage = [UIImage imageWithData:data];
        return resultImage;
    }
    
    
    static func compressImageQuality(_ image: UIImage, toByte maxLength: Int) -> UIImage {
        var compression: CGFloat = 1
        guard var data = UIImageJPEGRepresentation(image, compression),
            data.count > maxLength else { return image }
    
        var max: CGFloat = 1
        var min: CGFloat = 0
        for _ in 0..<6 {
            compression = (max + min) / 2
            data = UIImageJPEGRepresentation(image, compression)!
            if CGFloat(data.count) < CGFloat(maxLength) * 0.9 {
                min = compression
            } else if data.count > maxLength {
                max = compression
            } else {
                break
            }
        }
        return UIImage(data: data)!
    }
    
    

    当图片大小小于 maxLength,大于 maxLength * 0.9 时,不再继续压缩。最多压缩 6 次,1/(2^6) = 0.015625 < 0.02,也能达到每次循环 compression 减小 0.02 的效果。这样的压缩次数比循环减小 compression 少,耗时短。需要注意的是,当图片质量低于一定程度时,继续压缩没有效果。也就是说,compression 继续减小,data 也不再继续减小。压缩图片质量的优点在于,尽可能保留图片清晰度,图片不会明显模糊;缺点在于,不能保证图片压缩后小于指定大小。

    压缩图片尺寸

    与之前类似,比较容易想到的方法是,通过循环逐渐减小图片尺寸,直到图片稍小于指定大小(maxLength)。具体代码省略。同样的问题是循环次数多,效率低,耗时长。可以用二分法来提高效率,具体代码省略。这里介绍另外一种方法,比二分法更好,压缩次数少,而且可以使图片压缩后刚好小于指定大小(不只是 < maxLength, > maxLength * 0.9)。

    + (UIImage *)compressImageSize:(UIImage *)image toByte:(NSUInteger)maxLength {
        UIImage *resultImage = image;
        NSData *data = UIImageJPEGRepresentation(resultImage, 1);
        NSUInteger lastDataLength = 0;
        while (data.length > maxLength && data.length != lastDataLength) {
            lastDataLength = data.length;
            CGFloat ratio = (CGFloat)maxLength / data.length;
            CGSize size = CGSizeMake((NSUInteger)(resultImage.size.width * sqrtf(ratio)),
                                     (NSUInteger)(resultImage.size.height * sqrtf(ratio))); // Use NSUInteger to prevent white blank
            UIGraphicsBeginImageContext(size);
            // Use image to draw (drawInRect:), image is larger but more compression time
            // Use result image to draw, image is smaller but less compression time
            [resultImage drawInRect:CGRectMake(0, 0, size.width, size.height)];
            resultImage = UIGraphicsGetImageFromCurrentImageContext();
            UIGraphicsEndImageContext();
            data = UIImageJPEGRepresentation(resultImage, 1);
        }
        return resultImage;
    }
    
    
    static func compressImageSize(_ image: UIImage, toByte maxLength: Int) -> UIImage {
        guard var data = UIImageJPEGRepresentation(image, 1) else { return image }
    
        var resultImage: UIImage = image
        var lastDataLength: Int = 0
        while data.count > maxLength, data.count != lastDataLength {
            lastDataLength = data.count
            let ratio: CGFloat = CGFloat(maxLength) / CGFloat(data.count)
            let size: CGSize = CGSize(width: Int(resultImage.size.width * sqrt(ratio)),
                                    height: Int(resultImage.size.height * sqrt(ratio)))
            UIGraphicsBeginImageContext(size)
            resultImage.draw(in: CGRect(x: 0, y: 0, width: size.width, height: size.height))
            resultImage = UIGraphicsGetImageFromCurrentImageContext()!
            UIGraphicsEndImageContext()
            data = UIImageJPEGRepresentation(resultImage, 1)!
        }
        return resultImage
    }
    
    

    [resultImage drawInRect:CGRectMake(0, 0, size.width, size.height)];是用新图 resultImage 绘制,也可以用原图 image 来绘制。用原图绘制,压缩后图片更接近指定大小,但是压缩次数较多,耗时较长。一张大小为 6064 KB 的图片,压缩图片尺寸,原图绘制与新图绘制结果如下

    指定大小(KB) 原图绘制压缩后大小(KB) 原图绘制压缩次数 新图绘制压缩后大小(KB) 新图绘制压缩次数
    500 498 6 498 3
    300 299 4 296 3
    100 99 5 98 3
    50 49 6 48 3

    两种绘制方法压缩后大小很接近,与指定大小也很接近,但原图绘制压缩次数可达到新图绘制压缩次数的两倍。建议使用新图绘制,减少压缩次数。压缩后图片明显比压缩质量模糊。

    需要注意的是绘制尺寸的代码CGSize size = CGSizeMake((NSUInteger)(resultImage.size.width * sqrtf(ratio)), (NSUInteger)(resultImage.size.height * sqrtf(ratio)));每次绘制的尺寸 size,要把宽 width 和 高 height 转换为整数,防止绘制出的图片有白边

    压缩图片尺寸可以使图片小于指定大小,但会使图片明显模糊(比压缩图片质量模糊)。

    两种图片压缩方法结合

    如果要保证图片清晰度,建议选择压缩图片质量。如果要使图片一定小于指定大小,压缩图片尺寸可以满足。对于后一种需求,还可以先压缩图片质量,如果已经小于指定大小,就可得到清晰的图片,否则再压缩图片尺寸。

    + (UIImage *)compressImage:(UIImage *)image toByte:(NSUInteger)maxLength {
        // Compress by quality
        CGFloat compression = 1;
        NSData *data = UIImageJPEGRepresentation(image, compression);
        if (data.length < maxLength) return image;
    
        CGFloat max = 1;
        CGFloat min = 0;
        for (int i = 0; i < 6; ++i) {
            compression = (max + min) / 2;
            data = UIImageJPEGRepresentation(image, compression);
            if (data.length < maxLength * 0.9) {
                min = compression;
            } else if (data.length > maxLength) {
                max = compression;
            } else {
                break;
            }
        }
        UIImage *resultImage = [UIImage imageWithData:data];
        if (data.length < maxLength) return resultImage;
    
        // Compress by size
        NSUInteger lastDataLength = 0;
        while (data.length > maxLength && data.length != lastDataLength) {
            lastDataLength = data.length;
            CGFloat ratio = (CGFloat)maxLength / data.length;
            CGSize size = CGSizeMake((NSUInteger)(resultImage.size.width * sqrtf(ratio)),
                                     (NSUInteger)(resultImage.size.height * sqrtf(ratio))); // Use NSUInteger to prevent white blank
            UIGraphicsBeginImageContext(size);
            [resultImage drawInRect:CGRectMake(0, 0, size.width, size.height)];
            resultImage = UIGraphicsGetImageFromCurrentImageContext();
            UIGraphicsEndImageContext();
            data = UIImageJPEGRepresentation(resultImage, compression);
        }
    
        return resultImage;
    }
    
    
    static func compressImage(_ image: UIImage, toByte maxLength: Int) -> UIImage {
        var compression: CGFloat = 1
        guard var data = UIImageJPEGRepresentation(image, compression),
            data.count > maxLength else { return image }
    
        // Compress by size
        var max: CGFloat = 1
        var min: CGFloat = 0
        for _ in 0..<6 {
            compression = (max + min) / 2
            data = UIImageJPEGRepresentation(image, compression)!
            if CGFloat(data.count) < CGFloat(maxLength) * 0.9 {
                min = compression
            } else if data.count > maxLength {
                max = compression
            } else {
                break
            }
        }
        var resultImage: UIImage = UIImage(data: data)!
        if data.count < maxLength { return resultImage }
    
        // Compress by size
        var lastDataLength: Int = 0
        while data.count > maxLength, data.count != lastDataLength {
            lastDataLength = data.count
            let ratio: CGFloat = CGFloat(maxLength) / CGFloat(data.count)
            let size: CGSize = CGSize(width: Int(resultImage.size.width * sqrt(ratio)),
                                    height: Int(resultImage.size.height * sqrt(ratio)))
            UIGraphicsBeginImageContext(size)
            resultImage.draw(in: CGRect(x: 0, y: 0, width: size.width, height: size.height))
            resultImage = UIGraphicsGetImageFromCurrentImageContext()!
            UIGraphicsEndImageContext()
            data = UIImageJPEGRepresentation(resultImage, compression)!
        }
        return resultImage
    }
    
    

    Swift 代码已上传 GitHub:https://github.com/Silence-GitHub/CompressImageDemo

    转载请注明出处:http://www.cnblogs.com/silence-cnblogs/p/6346729.html

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