iphone - UIGraphicsBeginImageContextWithOptions 和 UIImageJPEGRepresentation 不能很好地协同工作

Question

所以我有这段代码来创建一个 UIImage:

UIGraphicsBeginImageContextWithOptions(border.frame.size, YES, 0);
[border.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *thumbnailImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

此时，图像上的尺寸是正确的，80x100。

然后它运行这段代码:

NSData *fullImageData = UIImageJPEGRepresentation(image, 1.0f);

并且图像的 NSData 返回尺寸为 160x200 的图像 - 是应有尺寸的两倍。

很明显，原因是这条线:

UIGraphicsBeginImageContextWithOptions(border.frame.size, YES, 0);

末尾的 0 是比例，因为它是 0，所以它是设备比例因子。我保持这种方式以保持清晰的图像。然而，当我将图像设置为 1 时，尽管图像保持其应有的大小，但它并没有以视网膜质量出现。我想要做的是保持视网膜质量，同时保持合适的大小。有办法做到这一点吗？

Answer 1

在调用 UIImageJPEGRepresentation 之前尝试调整 UIImage 的大小

- (UIImage *)resizeImage:(UIImage*)image newSize:(CGSize)newSize {
    CGRect newRect = CGRectIntegral(CGRectMake(0, 0, newSize.width, newSize.height));
    CGImageRef imageRef = image.CGImage;

    UIGraphicsBeginImageContextWithOptions(newSize, NO, 0);
    CGContextRef context = UIGraphicsGetCurrentContext();

    // Set the quality level to use when rescaling
    CGContextSetInterpolationQuality(context, kCGInterpolationHigh);
    CGAffineTransform flipVertical = CGAffineTransformMake(1, 0, 0, -1, 0, newSize.height);

    CGContextConcatCTM(context, flipVertical);  
    // Draw into the context; this scales the image
    CGContextDrawImage(context, newRect, imageRef);

    // Get the resized image from the context and a UIImage
    CGImageRef newImageRef = CGBitmapContextCreateImage(context);
    UIImage *newImage = [UIImage imageWithCGImage:newImageRef];

    CGImageRelease(newImageRef);
    UIGraphicsEndImageContext();    

    return newImage;
}

if([UIScreen mainScreen].scale > 1)
    {
        thumbnailImage = [self thumbnailImage newSize:CGSizeMake(thumbnailImage.size.width/[UIScreen       mainScreen].scale, thumbnailImage.size.height/[UIScreen mainScreen].scale)];
    }