在内存限制下调整 UIImage 的大小

Question

我从网络资源 (>300MB) 下载了一个巨大的 UIImage，当我尝试渲染时由于内存问题导致应用程序崩溃。我正在尝试使用以下代码调整图像大小：

+ (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;
}

正如您可能已经猜到的那样，问题是这需要实际渲染图像，因此会导致同样的崩溃。有没有一种方法可以在不对系统造成如此巨大的内存压力的情况下调整大图像的大小？

Answer 1

你可以试试这个 + (UIImage *)decodedImageWithImage:(UIImage *)image maxSize:(NSInteger) maxSize. 代码来自 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;
}

Answer 2

问题是您将图像保存在内存中。将其下载到磁盘并使用 ImageIO 框架直接从磁盘读取 "thumbnail"（较小尺寸），而无需将全尺寸图像保存在内存中。

在内存限制下调整 UIImage 的大小

Resize UIImage under memory constraints

core-graphics

objective-c

uiimage

image-resizing

ios