iOS 圖片壓縮方法

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iOS 圖片壓縮方法

兩種圖片壓縮方法

兩種壓縮圖片的方法：壓縮圖片質(zhì)量(Quality)，壓縮圖片尺寸(Size)。

壓縮圖片質(zhì)量

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

通過 UIImage 和 NSData 的相互轉(zhuǎn)化，減小 JPEG 圖片的質(zhì)量來壓縮圖片。UIImageJPEGRepresentation:: 第二個參數(shù) compression 取值 0.0~1.0捅伤，值越小表示圖片質(zhì)量越低，圖片文件自然越小。

壓縮圖片尺寸

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

給定所需的圖片尺寸 size炉爆，resultImage 即為原圖 image 繪制為 size 大小的圖片。

壓縮圖片使圖片文件小于指定大小

如果對圖片清晰度要求不高卧晓，要求圖片的上傳芬首、下載速度快的話，上傳圖片前需要壓縮圖片逼裆。壓縮到什么程度要看具體情況郁稍，但一般會設(shè)定一個圖片文件最大值，例如 100 KB胜宇∫可以用上訴兩種方法來壓縮圖片。假設(shè)圖片轉(zhuǎn)化來的 NSData 對象為 data桐愉，通過data.length即可得到圖片的字節(jié)大小财破。

壓縮圖片質(zhì)量

比較容易想到的方法是，通過循環(huán)來逐漸減小圖片質(zhì)量从诲，直到圖片稍小于指定大小(maxLength)左痢。

+ (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;
}

這樣循環(huán)次數(shù)多，效率低，耗時長俊性。

可以通過二分法來優(yōu)化略步。

+ (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)!
}

當(dāng)圖片大小小于 maxLength，大于 maxLength * 0.9 時定页，不再繼續(xù)壓縮趟薄。最多壓縮 6 次，1/(2^6) = 0.015625 < 0.02拯勉，也能達(dá)到每次循環(huán) compression 減小 0.02 的效果竟趾。這樣的壓縮次數(shù)比循環(huán)減小 compression 少，耗時短宫峦。需要注意的是岔帽，當(dāng)圖片質(zhì)量低于一定程度時，繼續(xù)壓縮沒有效果导绷。也就是說犀勒，compression 繼續(xù)減小，data 也不再繼續(xù)減小妥曲。壓縮圖片質(zhì)量的優(yōu)點(diǎn)在于贾费，盡可能保留圖片清晰度，圖片不會明顯模糊檐盟；缺點(diǎn)在于褂萧，不能保證圖片壓縮后小于指定大小。

壓縮圖片尺寸

與之前類似葵萎，比較容易想到的方法是导犹，通過循環(huán)逐漸減小圖片尺寸，直到圖片稍小于指定大小(maxLength)羡忘。具體代碼省略谎痢。同樣的問題是循環(huán)次數(shù)多，效率低卷雕，耗時長节猿。可以用二分法來提高效率漫雕，具體代碼省略滨嘱。這里介紹另外一種方法，比二分法更好蝎亚，壓縮次數(shù)少九孩，而且可以使圖片壓縮后剛好小于指定大小(不只是 < maxLength， > maxLength * 0.9)发框。

+ (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)];是用新圖 resultImage 繪制躺彬，也可以用原圖 image 來繪制煤墙。用原圖繪制，壓縮后圖片更接近指定大小宪拥，但是壓縮次數(shù)較多仿野，耗時較長。一張大小為 6064 KB 的圖片她君，壓縮圖片尺寸脚作，原圖繪制與新圖繪制結(jié)果如下

指定大小(KB)	原圖繪制壓縮后大小(KB)	原圖繪制壓縮次數(shù)	新圖繪制壓縮后大小(KB)	新圖繪制壓縮次數(shù)
500	498	6	498	3
300	299	4	296	3
100	99	5	98	3
50	49	6	48	3

兩種繪制方法壓縮后大小很接近，與指定大小也很接近缔刹，但原圖繪制壓縮次數(shù)可達(dá)到新圖繪制壓縮次數(shù)的兩倍球涛。建議使用新圖繪制，減少壓縮次數(shù)校镐。壓縮后圖片明顯比壓縮質(zhì)量模糊亿扁。

需要注意的是繪制尺寸的代碼CGSize size = CGSizeMake((NSUInteger)(resultImage.size.width * sqrtf(ratio)), (NSUInteger)(resultImage.size.height * sqrtf(ratio)));，每次繪制的尺寸 size鸟廓，要把寬 width 和高 height 轉(zhuǎn)換為整數(shù)从祝，防止繪制出的圖片有白邊。

壓縮圖片尺寸可以使圖片小于指定大小引谜，但會使圖片明顯模糊(比壓縮圖片質(zhì)量模糊)牍陌。

兩種圖片壓縮方法結(jié)合

如果要保證圖片清晰度，建議選擇壓縮圖片質(zhì)量员咽。如果要使圖片一定小于指定大小毒涧，壓縮圖片尺寸可以滿足。對于后一種需求贝室，還可以先壓縮圖片質(zhì)量链嘀，如果已經(jīng)小于指定大小，就可得到清晰的圖片档玻，否則再壓縮圖片尺寸。

+ (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 代碼已上傳 GitHub：https://github.com/Silence-GitHub/CompressImageDemo

轉(zhuǎn)載請注明出處：http://www.cnblogs.com/silence-cnblogs/p/6346729.html