Resize UIImage under memory constraints

问题:

I have a massive UIImage downloaded from a web resource (>300MBs) which when I attempt to render is causing an app crash due to memory. I am trying to resize the image using the following code:

+ (UIImage *)imageWithImage:(UIImage *)image scaled:(float) scale {
    //UIGraphicsBeginImageContext(newSize);
    CGSize size = (CGSize){scale * image.size.width, scale * image.size.height};
    UIGraphicsBeginImageContextWithOptions(size, NO, 0.0);
    [image drawInRect:CGRectMake(0, 0, size.width, size.height)];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}

The problem is, as you may have guessed, that this requires actually rendering the image and thus causes the same crash. Is there a way to resize an large image without placing such an enormous memory strain on the system?

回答1:

The problem is that you are holding the image in memory. Download it to disk and use the ImageIO framework to read a "thumbnail" (smaller size) directly from disk without ever having to hold the full-size image in memory.

回答2:

You can try this one + (UIImage *)decodedImageWithImage:(UIImage *)image maxSize:(NSInteger) maxSize. Code from SDWebImage

#define MAX_IMAGE_SIZE 1000
@implementation UIImage (ForceDecode)
+ (CGSize) originalSize:(UIImage*) image
{
    return  CGSizeMake(CGImageGetWidth(image.CGImage), CGImageGetHeight(image.CGImage));
}
+ (UIImage *)decodedImageWithImage:(UIImage *)image {
    return [self decodedImageWithImage:image maxSize:MAX_IMAGE_SIZE];
}
+ (UIImage *)decodedImageWithImage:(UIImage *)image maxSize:(NSInteger) maxSize{
    if (image.images) {
        // Do not decode animated images
        return image;
    }

    CGImageRef imageRef = image.CGImage;
    CGSize imageSize = CGSizeMake(CGImageGetWidth(imageRef), CGImageGetHeight(imageRef));
    imageSize = [self checkMaxImageSize:imageSize maxSize:maxSize];
    CGRect imageRect = (CGRect){.origin = CGPointZero, .size = imageSize};

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo = CGImageGetBitmapInfo(imageRef);

    int infoMask = (bitmapInfo & kCGBitmapAlphaInfoMask);
    BOOL anyNonAlpha = (infoMask == kCGImageAlphaNone ||
                        infoMask == kCGImageAlphaNoneSkipFirst ||
                        infoMask == kCGImageAlphaNoneSkipLast);

    // CGBitmapContextCreate doesn't support kCGImageAlphaNone with RGB.
    // https://developer.apple.com/library/mac/#qa/qa1037/_index.html
    if (infoMask == kCGImageAlphaNone && CGColorSpaceGetNumberOfComponents(colorSpace) > 1) {
        // Unset the old alpha info.
        bitmapInfo &= ~kCGBitmapAlphaInfoMask;

        // Set noneSkipFirst.
        bitmapInfo |= kCGImageAlphaNoneSkipFirst;
    }
    // Some PNGs tell us they have alpha but only 3 components. Odd.
    else if (!anyNonAlpha && CGColorSpaceGetNumberOfComponents(colorSpace) == 3) {
        // Unset the old alpha info.
        bitmapInfo &= ~kCGBitmapAlphaInfoMask;
        bitmapInfo |= kCGImageAlphaPremultipliedFirst;
    }

    // It calculates the bytes-per-row based on the bitsPerComponent and width arguments.
    CGContextRef context = CGBitmapContextCreate(NULL,
                                                 imageSize.width,
                                                 imageSize.height,
                                                 CGImageGetBitsPerComponent(imageRef),
                                                 0,
                                                 colorSpace,
                                                 bitmapInfo);
    CGColorSpaceRelease(colorSpace);

    // If failed, return undecompressed image
    if (!context) return image;

    CGContextDrawImage(context, imageRect, imageRef);
    CGImageRef decompressedImageRef = CGBitmapContextCreateImage(context);

    CGContextRelease(context);

    UIImage *decompressedImage = [UIImage imageWithCGImage:decompressedImageRef scale:image.scale orientation:image.imageOrientation];
    CGImageRelease(decompressedImageRef);
    return decompressedImage;
}

+ (CGSize) checkMaxImageSize:(CGSize) size maxSize:(NSInteger) max
{
    CGFloat whRatio = size.width / size.height;
    if (size.height > max || size.width > max) {
        if (size.height > size.width) {
            size.width = max * whRatio;
            size.height = max;
        }else{
            size.width = max ;
            size.height = max / whRatio;
        }
    }
    return size;
}