适用于iOS的最快2D图像旋转

Question

I would like to rotate a 2D image (in the x/y plane) in realtime or near realtime (10 frames per second would be adequate). The question at hand is how large an image I can rotate at such speeds. The reason that I can't just run the experiment myself is that I can't be sure I'm doing it right. I am not sure if I get full speed by using CATransform3DRotate, CGAffineTransformMakeRotation or if I have to go to OpenGL.
I am betting that the hardware is intended to rotate screen size images (or smaller) and not full resolution (3226 by 2448) images. Unfortunately we have to work with images larger than the screen and sometimes as large as the whole sensor. I have implemented rotation with CGAffineTransformMakeRotation and if this is all there is to it then I can start small and build up – is it?

UIImage * ImageRotatedByRadians(UIImage *oldImage, CGFloat radians)
{
NSLog( @"\n\nRotateImage->imageRotatedByRadians: (%f)\n\n", radians );

// Calculate the size of the rotated view's containing box for our drawing space.
UIView *rotatedViewBox = [[UIView alloc] initWithFrame:CGRectMake( 0, 0, oldImage.size.width, oldImage.size.height )];
CGAffineTransform t = CGAffineTransformMakeRotation( radians );
rotatedViewBox.transform = t;
CGSize rotatedSize = rotatedViewBox.frame.size;

// Create the bitmap context.
UIGraphicsBeginImageContext( rotatedSize );
CGContextRef bitmap = UIGraphicsGetCurrentContext();

// Move the origin to the middle of the image so we will rotate and scale around the center.
CGContextTranslateCTM( bitmap, rotatedSize.width / 2, rotatedSize.height / 2 );

// Rotate the image context.
CGContextRotateCTM( bitmap, radians );

// Now, draw the rotated/scaled image into the context.
CGContextScaleCTM( bitmap, 1.0, -1.0 );
CGContextDrawImage( bitmap, CGRectMake( -oldImage.size.width / 2, -oldImage.size.height / 2, oldImage.size.width, oldImage.size.height), [oldImage CGImage] );

UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
//UIImageWriteToSavedPhotosAlbum(newImage, nil, nil, nil);

UIGraphicsEndImageContext();

return newImage;
}

Questions: What is the fastest 2D rotation mechanism on iPhones 5 and later?

What is the largest image area that can be rotated at at least 10 frames per second?

We are working with 256 grayscale images (which should speed things up, I would think).

If the answer is OpenGL I would appreciate either code or a pointer to a place to start.

Answer 1

so the fast part is rotating the image using the UIViews transformation, but the slow part is when you are actually writing the rotated pixel data out to another UIImage

to run a method continuously linked to the refresh of the screen you can use the CADisplayLink like so

//inside viewDidLoad or something
displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(render:)]; //implement a method called render or some other name
displayLink.frameInterval = 2; // 2 = 30fps, 1 = 60fps 3 = 20fps etc basically the frameInterval = 60/frameInterval
[displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];

inside the render: function you could do something like

- (void)render:(CADisplayLink*)displayLink {
    rotatedBoxView.transform = CGAffineTransformMakeRotation( radians );
    //do other image stuff
}

if you want to speed up the UIImage creation i think you will need to find out how to do it on another thread, could be as easy as putting it inside a dispatch_async block but i have a feeling that UIGraphicsGetCurrentContext(); stuff doesnt play nicely with threads

but i think no matter what method you use, OpenGL or otherwise, your bottle neck is just the creating the new UIImage so i wouldnt fuss too much about speeding up the rotation

havent tested any of the code but could be a start in the right direction

not a complete answer but hope it helps a bit