实用：裁剪和获取图片

- (UIImage *)reSizeImage:(UIImage *)image toSize:(CGSize)reSize
 
{
    UIGraphicsBeginImageContext(CGSizeMake(reSize.width, reSize.height));
    [image drawInRect:CGRectMake(0, 0, reSize.width, reSize.height)];
    UIImage *reSizeImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
     
    return reSizeImage;
     
}
 
- (UIImage *)squareImageWithImage:(UIImage *)image scaledToSize:(CGSize)newSize {
    double ratio;
    double delta;
    CGPoint offset;
     
    //make a new square size, that is the resized imaged width
    CGSize sz = CGSizeMake(newSize.width, newSize.width);
     
    //figure out if the picture is landscape or portrait, then
    //calculate scale factor and offset
    if (image.size.width > image.size.height) {
        ratio = newSize.width / image.size.width;
        delta = (ratio*image.size.width - ratio*image.size.height);
        offset = CGPointMake(delta/2, 0);
    } else {
        ratio = newSize.width / image.size.height;
        delta = (ratio*image.size.height - ratio*image.size.width);
        offset = CGPointMake(0, delta/2);
    }
     
    //make the final clipping rect based on the calculated values
    CGRect clipRect = CGRectMake(-offset.x, -offset.y,
                                 (ratio * image.size.width) + delta,
                                 (ratio * image.size.height) + delta);
     
     
    //start a new context, with scale factor 0.0 so retina displays get
    //high quality image
    if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {
        UIGraphicsBeginImageContextWithOptions(sz, YES, 0.0);
    } else {
        UIGraphicsBeginImageContext(sz);
    }
    UIRectClip(clipRect);
    [image drawInRect:clipRect];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
     
    return newImage;
}
- (CGImageRef)imageFromSampleBuffer:(CMSampleBufferRef)sampleBuffer
{
    // Get a CMSampleBuffer\'s Core Video image buffer for the media data
    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
     
    // Lock the base address of the pixel buffer
    CVPixelBufferLockBaseAddress(imageBuffer, 0);
     
    // Get the base address
    void *baseAddress = CVPixelBufferGetBaseAddressOfPlane(imageBuffer, 0);
     
    // Get the number of bytes per row for the pixel buffer
    size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
    // Get the pixel buffer width and height
    size_t width = CVPixelBufferGetWidth(imageBuffer);
    size_t height = CVPixelBufferGetHeight(imageBuffer);
     
    // Create a device-dependent RGB color space
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
     
    // Create a bitmap graphics context with the sample buffer data
    CGContextRef context = CGBitmapContextCreate(baseAddress, width, height, 8,
                                                 bytesPerRow, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst);
     
    // Create a Quartz image from the pixel data in the bitmap graphics context
    CGImageRef quartzImage = CGBitmapContextCreateImage(context);
     
    // Unlock the pixel buffer
    CVPixelBufferUnlockBaseAddress(imageBuffer,0);
     
    // Free up the context and color space
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
     
    // We should return CGImageRef rather than UIImage because QR Scan needs CGImageRef
    return quartzImage;
}