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 = 1guard var data = UIImageJPEGRepresentation(image, compression),data.count > maxLength else { return image }var max: CGFloat = 1var min: CGFloat = 0for _ in 0..<6 {compression = (max + min) / 2data = 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 blankUIGraphicsBeginImageContext(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 = imagevar lastDataLength: Int = 0while data.count > maxLength, data.count != lastDataLength {lastDataLength = data.countlet 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

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 qualityCGFloat 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 sizeNSUInteger 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 blankUIGraphicsBeginImageContext(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 = 1guard var data = UIImageJPEGRepresentation(image, compression),data.count > maxLength else { return image }// Compress by sizevar max: CGFloat = 1var min: CGFloat = 0for _ in 0..<6 {compression = (max + min) / 2data = 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 sizevar lastDataLength: Int = 0while data.count > maxLength, data.count != lastDataLength {lastDataLength = data.countlet 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
}