iphone - 来自 UIImagePickerViewController 图像的方形缩略图

Question

在我的应用程序中，用户使用 UIImagePickerViewController 选择图像或拍照。选择图像后，我想在方形 UIImageView (90x90) 上显示其缩略图。

我正在使用 Apple's code创建缩略图。问题是缩略图不是正方形的，这个函数在设置 kCGImageSourceThumbnailMaxPixelSize 键为 90 后，似乎只是调整图像的高度，据我所知 kCGImageSourceThumbnailMaxPixelSize 键应该负责设置缩略图的高度和宽度。

这是我的代码的一瞥:

- (void)imagePickerController:(UIImagePickerController *)picker
    didFinishPickingMediaWithInfo:(NSDictionary *)info {

    UIImage *image = [info objectForKey:UIImagePickerControllerOriginalImage];

    NSData *imageData = UIImageJPEGRepresentation (image, 0.5);

    // My image view is 90x90
    UIImage *thumbImage = MyCreateThumbnailImageFromData(imageData, 90);

    [myImageView setImage:thumbImage];

    if (picker.sourceType == UIImagePickerControllerSourceTypeCamera) {

        UIImageWriteToSavedPhotosAlbum(image, self, @selector(image:didFinishSavingWithError:contextInfo:), nil);
    }

    [picker dismissViewControllerAnimated:YES completion:nil];
}

UIImage* MyCreateThumbnailImageFromData (NSData * data, int imageSize) {

    CGImageRef        myThumbnailImage = NULL;
    CGImageSourceRef  myImageSource;
    CFDictionaryRef   myOptions = NULL;
    CFStringRef       myKeys[3];
    CFTypeRef         myValues[3];
    CFNumberRef       thumbnailSize;

    // Create an image source from NSData; no options.
    myImageSource = CGImageSourceCreateWithData((__bridge CFDataRef)data,
                                                NULL);

    // Make sure the image source exists before continuing.
    if (myImageSource == NULL){
        fprintf(stderr, "Image source is NULL.");
        return  NULL;
    }

    // Package the integer as a  CFNumber object. Using CFTypes allows you
    // to more easily create the options dictionary later.
    thumbnailSize = CFNumberCreate(NULL, kCFNumberIntType, &imageSize);

    // Set up the thumbnail options.
    myKeys[0] = kCGImageSourceCreateThumbnailWithTransform;
    myValues[0] = (CFTypeRef)kCFBooleanTrue;
    myKeys[1] = kCGImageSourceCreateThumbnailFromImageIfAbsent;
    myValues[1] = (CFTypeRef)kCFBooleanTrue;
    myKeys[2] = kCGImageSourceThumbnailMaxPixelSize;
    myValues[2] = thumbnailSize;

    myOptions = CFDictionaryCreate(NULL, (const void **) myKeys,
                                   (const void **) myValues, 2,
                                   &kCFTypeDictionaryKeyCallBacks,
                                   & kCFTypeDictionaryValueCallBacks);

    // Create the thumbnail image using the specified options.
    myThumbnailImage = CGImageSourceCreateThumbnailAtIndex(myImageSource,
                                                           0,
                                                           myOptions);

    UIImage* scaled = [UIImage imageWithCGImage:myThumbnailImage];

    // Release the options dictionary and the image source
    // when you no longer need them.

    CFRelease(thumbnailSize);
    CFRelease(myOptions);
    CFRelease(myImageSource);

    // Make sure the thumbnail image exists before continuing.
    if (myThumbnailImage == NULL) {
        fprintf(stderr, "Thumbnail image not created from image source.");
        return NULL;
    }
    return scaled;
}

这就是我的 ImageView 的实例化方式:

myImageView = [[UIImageView alloc] init];
imageView.contentMode = UIViewContentModeScaleAspectFit;

CGRect rect = imageView.frame;
rect.size.height = 90;
rect.size.width = 90;

imageView.frame = rect;
[imageView setUserInteractionEnabled:YES];

如果我不设置 imageView.contentMode = UIViewContentModeScaleAspectFit; 缩略图会变形，因为它只是我原始图像的一个版本，高度为 90 像素。

那么，为什么我的缩略图不是正方形的？

Answer 1

最简单的做法是将 imageView 上的 contentMode 设置为 UIViewContentModeScaleAspectFill。然而，这可能并不理想，因为它会将整个图像保存在内存中。

这是我用来调整图像大小的一些代码。

+ (UIImage *)imageWithImage:(UIImage *)image scaledToSize:(CGSize)size
{
    UIGraphicsBeginImageContextWithOptions(size, NO, 0);
    [image drawInRect:CGRectMake(0, 0, size.width, size.height)];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();    
    UIGraphicsEndImageContext();
    return newImage;
}

如果尺寸与图像的纵横比不同，此版本将防止图像失真。

+ (UIImage *)imageWithImage:(UIImage *)image scaledToFillSize:(CGSize)size
{
    CGFloat scale = MAX(size.width/image.size.width, size.height/image.size.height);
    CGFloat width = image.size.width * scale;
    CGFloat height = image.size.height * scale;
    CGRect imageRect = CGRectMake((size.width - width)/2.0f,
                                  (size.height - height)/2.0f,
                                  width,
                                  height);

    UIGraphicsBeginImageContextWithOptions(size, NO, 0);
    [image drawInRect:imageRect];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();    
    UIGraphicsEndImageContext();
    return newImage;
}

通常您只需要图像的顶部正方形(而不是中心)。并且您希望最终图像具有特定大小，例如 128x128，如下例所示。

- (UIImage *)squareAndSmall // as a category (so, 'self' is the input image)
{
    // fromCleverError's original
    // http://stackoverflow.com/questions/17884555
    CGSize finalsize = CGSizeMake(128,128);

    CGFloat scale = MAX(
        finalsize.width/self.size.width,
        finalsize.height/self.size.height);
    CGFloat width = self.size.width * scale;
    CGFloat height = self.size.height * scale;

    CGRect rr = CGRectMake( 0, 0, width, height);

    UIGraphicsBeginImageContextWithOptions(finalsize, NO, 0);
    [self drawInRect:rr];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();    
    UIGraphicsEndImageContext();
    return newImage;
}