iOS 图片压缩方法

时间  2019-11-10
标签 ios 图片 压缩 方法 栏目 iOS 繁體版
原文   原文链接

更多图片处理方法见图片组件 BBWebImagehtml

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)。ios

压缩图片质量

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

经过 UIImage 和 NSData 的相互转化，减少 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，图片文件天然越小。git

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。github

压缩图片使图片文件小于指定大小

若是对图片清晰度要求不高，要求图片的上传、下载速度快的话，上传图片前须要压缩图片。压缩到什么程度要看具体状况，但通常会设定一个图片文件最大值，例如 100 KB。能够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，经过data.length便可获得图片的字节大小。swift

压缩图片质量

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

+ (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。优化

能够经过二分法来优化。this

+ (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，再也不继续压缩。最多压缩 6 次，1/(2^6) = 0.015625 < 0.02，也能达到每次循环 compression 减少 0.02 的效果。这样的压缩次数比循环减少 compression 少，耗时短。须要注意的是，当图片质量低于必定程度时，继续压缩没有效果。也就是说，compression 继续减少，data 也再也不继续减少。压缩图片质量的优势在于，尽量保留图片清晰度，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。code

压缩图片尺寸

与以前相似，比较容易想到的方法是，经过循环逐渐减少图片尺寸，直到图片稍小于指定大小(maxLength)。具体代码省略。一样的问题是循环次数多，效率低，耗时长。能够用二分法来提升效率，具体代码省略。这里介绍另一种方法，比二分法更好，压缩次数少，并且可使图片压缩后恰好小于指定大小(不仅是 < maxLength， > maxLength * 0.9)。htm

+ (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，也能够用原图 image 来绘制。用原图绘制，压缩后图片更接近指定大小，可是压缩次数较多，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以下

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，减小压缩次数。压缩后图片明显比压缩质量模糊。

须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

若是要保证图片清晰度，建议选择压缩图片质量。若是要使图片必定小于指定大小，压缩图片尺寸能够知足。对于后一种需求，还能够先压缩图片质量，若是已经小于指定大小，就可获得清晰的图片，不然再压缩图片尺寸。

+ (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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