How to turn a CVPixelBuffer into a UIImage?

Unless your image data is in some different format that requires swizzle or conversion - i would recommend no incrementing of anything... just smack the data into your context memory area with memcpy as in:

//not here... unsigned char *buffer = CVPixelBufferGetBaseAddress(pixelBuffer);

UIGraphicsBeginImageContext(CGSizeMake(w, h));

CGContextRef c = UIGraphicsGetCurrentContext();

void *ctxData = CGBitmapContextGetData(c);

// MUST READ-WRITE LOCK THE PIXEL BUFFER!!!!
CVPixelBufferLockBaseAddress(pixelBuffer, 0);
void *pxData = CVPixelBufferGetBaseAddress(pixelBuffer);
memcpy(ctxData, pxData, 4 * w * h);
CVPixelBufferUnlockBaseAddress(pixelBuffer, 0);

... and so on...

Try this one in Swift.

Swift 4.2:

import VideoToolbox

extension UIImage {
    public convenience init?(pixelBuffer: CVPixelBuffer) {
        var cgImage: CGImage?
        VTCreateCGImageFromCVPixelBuffer(pixelBuffer, nil, &cgImage)

        guard let cgImage = cgImage else {
            return nil
        }

        self.init(cgImage: cgImage)
    }
}

Swift 5:

import VideoToolbox

extension UIImage {
    public convenience init?(pixelBuffer: CVPixelBuffer) {
        var cgImage: CGImage?
        VTCreateCGImageFromCVPixelBuffer(pixelBuffer, options: nil, imageOut: &cgImage)

        guard let cgImage = cgImage else {
            return nil
        } 

        self.init(cgImage: cgImage)
    }
}

Note: This only works for RGB pixel buffers, not for grayscale.

Another way to get an UIImage. Performs ~10 times faster, at least in my case:

int w = CVPixelBufferGetWidth(pixelBuffer);
int h = CVPixelBufferGetHeight(pixelBuffer);
int r = CVPixelBufferGetBytesPerRow(pixelBuffer);
int bytesPerPixel = r/w;

unsigned char *buffer = CVPixelBufferGetBaseAddress(pixelBuffer);

UIGraphicsBeginImageContext(CGSizeMake(w, h));

CGContextRef c = UIGraphicsGetCurrentContext();

unsigned char* data = CGBitmapContextGetData(c);
if (data != NULL) {
   int maxY = h;
   for(int y = 0; y<maxY; y++) {
      for(int x = 0; x<w; x++) {
         int offset = bytesPerPixel*((w*y)+x);
         data[offset] = buffer[offset];     // R
         data[offset+1] = buffer[offset+1]; // G
         data[offset+2] = buffer[offset+2]; // B
         data[offset+3] = buffer[offset+3]; // A
      }
   }
} 
UIImage *img = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

First of all the obvious stuff that doesn't relate directly to your question: AVCaptureVideoPreviewLayer is the cheapest way to pipe video from either of the cameras into an independent view if that's where the data is coming from and you've no immediate plans to modify it. You don't have to do any pushing yourself, the preview layer is directly connected to the AVCaptureSession and updates itself.

I have to admit to lacking confidence about the central question. There's a semantic difference between a CIImage and the other two types of image — a CIImage is a recipe for an image and is not necessarily backed by pixels. It can be something like "take the pixels from here, transform like this, apply this filter, transform like this, merge with this other image, apply this filter". The system doesn't know what a CIImage looks like until you chose to render it. It also doesn't inherently know the appropriate bounds in which to rasterise it.

UIImage purports merely to wrap a CIImage. It doesn't convert it to pixels. Presumably UIImageView should achieve that, but if so then I can't seem to find where you'd supply the appropriate output rectangle.

I've had success just dodging around the issue with:

CIImage *ciImage = [CIImage imageWithCVPixelBuffer:pixelBuffer];

CIContext *temporaryContext = [CIContext contextWithOptions:nil];
CGImageRef videoImage = [temporaryContext
                   createCGImage:ciImage
                   fromRect:CGRectMake(0, 0, 
                          CVPixelBufferGetWidth(pixelBuffer),
                          CVPixelBufferGetHeight(pixelBuffer))];

UIImage *uiImage = [UIImage imageWithCGImage:videoImage];
CGImageRelease(videoImage);

With gives an obvious opportunity to specify the output rectangle. I'm sure there's a route through without using a CGImage as an intermediary so please don't assume this solution is best practice.