Visually Lossless and how to back it up.

CineForm is the company most guilty of using the term Visually Lossless (VL)-- we may even have coined it. Red has benefited from CineForm's success with wavelets, and they also uses the term, as they should. But it is a marketing term, so we all should do our best to back it up. While visual inspection of 1:1 pixel data (or magnified) at 12" viewing distances is a good way to test for VL, there is a different way to measure the amount of compression that is significant, without involving human inspection. There is a point in most high bit-rate image compression technologies when the amount of loss is insignificant; this occurs when the compression error falls well below the inherent noise floor of the imaging device.

Back in March '07, Scott Billups, George Palmer and Mark Chiolis helped us out with a Viper test shoot at Plus-8 to compare HDCAM-SR to CineForm 444 -- thanks again guys. Follow following this blog when remember many posts on the subject. Using a Billups Chart in front of the Viper in Filmstream mode, the stationary image noise power was measured in PSNR at 38.97dB -- this is the amount of difference between adjacent frames due to sensor noise or other environmentals, from an otherwise completely static image. Yet the difference between any uncompressed frame and the HDCAM-SR equivalent was 49.8dB, more than 10dB less distortion (massive) than the inherent noise level. CineForm 444 was 51.03db, better still (the higher number means less distortion.) While adjacent frame noise is not normally measured as a distortion, it is a very good indicator in the amount change that you must be less than for the image to look the same after compression -- assuming you are also trying to store the noise which I feel is an important part of being visually lossless. In film scans, the film grain is the noise floor you need to be less than, and the compression should preserve the shape of the grain to reach VL.

Taking this test further, and I get little wacky, but please bear with me. I average 72 frames of the uncompressed stationary image to extract out the sensor noise -- the 72 gives and effective 37dB of noise suppression. This new average frame is the ideal frame; it is what the camera was trying to see if noise could be eliminated in the camera. When I compare each unique uncompressed frame to the ideal, we can now measure the uncompressed acquisition distortion, just like as it was a lossy format. The PSNR error between uncompressed signal and the ideal frame was 41.86dB. When you compare the same ideal frame to the compressed results, you get the values of 41.38dB and 41.63dB for HDCAM-SR and CineForm respectively (CineForm 444 was compressing to around 5:1 using Filmscan1 mode and HDCAM SR compressed to around 4:1.) The less than 0.5dB difference between the lightly compressed solutions and the uncompressed capture, is why CineForm and Sony SR can safely claim to be visual lossless.

Ideal image:

Uncompressed image:

HDCAM-SR image:

CineForm image:

Download the full screen DPX versions here.

Notes
1: See in the black background to see how much noise has been removed through averaging sequential frames.

2: The reduction in noise in the ideal image can make it look softer, as noise is often seen as a visual cue for sharpness--add some noise to it and will seem sharper again.

3: There is some edge enhancement that must have occurred in camera, as all the images show it, so it is not a compression artifact.

Update: 10/1/07, Here is a good write up on the Viper's edge enhancement that I commented on in note 3.