Core Graphics使用

- (void)generateThumbnailFromImage:(UIImage *)image
{
    CGSize origImageSize = image.size;

    // The rectangle of the thumbnail
    CGRect newRect = CGRectMake(0, 0, 40, 40);

    // Figure out a scaling ratio to make sure we maintain the same aspect ratio
    float ratio = MAX(newRect.size.width / origImageSize.width,
                      newRect.size.height / origImageSize.height);

    // Create a transparent bitmap context with a scaling factor
    // equal to that of the screen
    UIGraphicsBeginImageContextWithOptions(newRect.size, NO, 0.0);

    // Create a path that is a rounded rectangle
    UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:newRect
                                                    cornerRadius:5.0];

    // Make all subsequent drawing clip to this rounded rectangle
    [path addClip];

    // Center the image in the thumbnail rectangle
    CGRect projectRect;
    projectRect.size.width = ratio * origImageSize.width;
    projectRect.size.height = ratio * origImageSize.height;
    projectRect.origin.x = (newRect.size.width - projectRect.size.width) / 2.0;
    projectRect.origin.y = (newRect.size.height - projectRect.size.height) / 2.0;

    // Draw the image on it
    [image drawInRect:projectRect];

    // Get the image from the image context; keep it as our thumbnail
    UIImage *smallImage = UIGraphicsGetImageFromCurrentImageContext();
    self.thumbnail = smallImage;

    // Cleanup image context resources; we're done
    UIGraphicsEndImageContext();
}