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];
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;}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;}
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;}