ios - Change NSData Size of a UIImage - Compress And Resize UIImage With a Maximum Megabyte Mb 限制

Question

我有一个用户上传的 UIImage。

如果图像数据超过 10Mb，我如何将它的大小调整到最大 10Mb？到目前为止，我发现的最接近数据大小调整的是:

NSData *imageData = UIImageJPEGRepresentation(theUploadedImage.image, 0.5f);

但我似乎无法控制 Mb 的文件大小...仅控制 JPG 表示中的第二个参数(图像质量 float )

Answer 1

必须创建我自己的函数来尽可能小地压缩图像，如果它仍然超过我的“最大大小”，那么它会调整大小、保留并再次开始压缩迭代。如果图像超过文件大小时，这可以很好地使图像尽可能接近目标“最大图像文件大小”。还包括 1024 次迭代后的故障保护。 (这应该永远不会超过一分钟(但这种情况永远不会发生......谁在 iPhone 上使用千兆字节的图像？哈哈))...

-(void)shrinkImage {


    //IMPORTANT!!! THIS CODE WAS CREATED WITH "ARC" IN MIND... DO NOT USE WITHOUT ARC UNLESS YOU ALTER THIS CODE TO MANAGE MEMORY


    float compressionVal = 1.0;
    float maxVal = 9.7;//MB

    UIImage *compressedImage = theUploadedImage.image; //get UIImage from imageView

    int iterations = 0;
    int totalIterations = 0;

    float initialCompressionVal = 0.00000000f;

    while (((((float)(UIImageJPEGRepresentation(compressedImage, compressionVal).length))/(1048576.000000000f)) > maxVal) && (totalIterations < 1024)) {

        NSLog(@"Image is %f MB", (float)(((float)(UIImageJPEGRepresentation(compressedImage, compressionVal)).length)/(1048576.000000f)));//converts bytes to MB

        compressionVal = (((compressionVal)+((compressionVal)*((float)(((float)maxVal)/((float)(((float)(UIImageJPEGRepresentation(compressedImage, compressionVal).length))/(1048576.000000000f)))))))/(2));
        compressionVal *= 0.97;//subtracts 3% of it's current value just incase above algorithm limits at just above MaxVal and while loop becomes infinite.

        if (initialCompressionVal == 0.00000000f) {
            initialCompressionVal = compressionVal;
        }

        iterations ++;

        if ((iterations >= 3) || (compressionVal < 0.1)) {
            iterations = 0;
            NSLog(@"%f", compressionVal);

            compressionVal = 1.0f;


            compressedImage = [UIImage imageWithData:UIImageJPEGRepresentation(compressedImage, compressionVal)];



            float resizeAmount = 1.0f;
            resizeAmount = (resizeAmount+initialCompressionVal)/(2);//percentage
            resizeAmount *= 0.97;//3% boost just incase image compression algorithm reaches a limit.
            resizeAmount = 1/(resizeAmount);//value
            initialCompressionVal = 0.00000000f;


            UIView *imageHolder = [[UIView alloc] initWithFrame:CGRectMake(0,0,(int)floorf((float)(compressedImage.size.width/(resizeAmount))), (int)floorf((float)(compressedImage.size.height/(resizeAmount))))];//round down to ensure frame isnt larger than image itself

            UIImageView *theResizedImage = [[UIImageView alloc] initWithFrame:CGRectMake(0,0,(int)ceilf((float)(compressedImage.size.width/(resizeAmount))), (int)ceilf((float)(compressedImage.size.height/(resizeAmount))))];//round up to ensure image fits
            theResizedImage.image = compressedImage;


            [imageHolder addSubview:theResizedImage];


            UIGraphicsBeginImageContextWithOptions(CGSizeMake(imageHolder.frame.size.width, imageHolder.frame.size.height), YES, 1.0f);
            CGContextRef resize_context = UIGraphicsGetCurrentContext();
            [imageHolder.layer renderInContext:resize_context];
            compressedImage = UIGraphicsGetImageFromCurrentImageContext();
            UIGraphicsEndImageContext();


            //after 3 compressions, if we still haven't shrunk down to maxVal size, apply the maximum compression we can, then resize the image (90%?), then re-start the process, this time compressing the compressed version of the image we were checking.

            NSLog(@"resize");
        }

        totalIterations ++;

    }

    if (totalIterations >= 1024) {
        NSLog(@"Image was too big, gave up on trying to re-size");//too many iterations failsafe. Gave up on trying to resize.
    } else {

        NSData *imageData = UIImageJPEGRepresentation(compressedImage, compressionVal);
        NSLog(@"FINAL Image is %f MB ... iterations: %i", (float)(((float)imageData.length)/(1048576.000000f)), totalIterations);//converts bytes to MB

        theUploadedImage.image = [UIImage imageWithData:imageData];//save new image to UIImageView.

    }
}