Guild of British Camera Technicians

The expansion in Digital Cinematography is one of the largest developments to hit the film' industry in recent years, with multiple cameras coming to mark et, with original ‘HD' format cameras such as Sony's f900/950, Panasonic's VariCam and Thomson's Viper initiating a surge of interest in Digital Cinematography.

Red One

None of the new generation Digital Cinematography Cameras has captured the imagination more than the Red One, from new manufacturer RED www.red.com

A Complete Package

And it's not just a camera body; but a full camera system, with lenses, viewfinder, rails, battery packs, on-camera disc storage, etc, etc, that Red are bringing to market.

It is virtually possible to order all you need for hand-held cinematography from Red and shoot your own film…

Image, Compression and Bayer

To help manage the massive amounts of data that a 4K image generates Red have developed a wavelet based compression codec that works on the raw Bayer data to capture 4K at some 27MB/sec, called RedCode.

To put that figure into perspective, an uncompressed 4K image stream would be 1.2GB/s (at 12bit Lin, non-Bayer, for the tech-heads amongst you).

That's a huge difference!

So what exactly does this mean?

Bayer filtering is the method employed by single CCD chip cameras to capture a colour image. By using a single chip the complexity of a beam splitter and 3 chip alignment is avoided, but there then needs to be an alternative way to capture RGB data.

This is done by making each pixel on the CCD responsive to either R, G or B only, but in a form that has twice as many G pixels as R or B, mainly as the human eye is more sensitive to green, so this helps maintain image contrast, which is seen as resolution…

But, this means there is less colour information than the number of pixels might suggest… And it means there needs to be some seriously clever processing done to the image to get back to a viewable image – and it is this processing that determines the final image quality. Fast and simple processing can be used to generate a viewable image but image quality is sacrificed in the quest for speed.

A high quality process will take time to perform, but will lessen, if not remove, the potential for artefacts. But to do this takes a powerful image processing system.

The image on the left shows a Bayer filter CCD.

As a result of this the data size per frame is reduced from a full RGB image, and with some clever image processing the resultant viewable image will look almost identical.

Rather than the 1.2GB/s data rate required for a full RGB image, the Bayer output drops to 400 GB/s.

An immediate saving: but is this now really a 4K image? An interesting question... and

it's very hard to answer, although there is a very good description of Bayer resolution at http://www.licha.de/astro_article_ccd_camera_bayer_matrix.php.

Basically, it is easy to de-Bayer using groups of 4 pixels to generate one final pixel, giving a high quality image with no Bayer artefacts. But this results in a final image a ¼ of the area resolution of the CCD. So this would mean a 2K image from Red's 4K capture.

It's a lot harder, and takes a lot more processing to generate a de-Bayered image that contains 4K worth of final image pixels – but not impossible if done right.

This Bayer filtered image at 400GB/s bate rate is further compressed (as Bayer is already a form of compression) using wavelet compression at approx 12:1, giving the final data rate of 27MB/s.

Wavelet compression is a more analogue form of compression than DCT, which has been used in most recent video compression systems and which causes ‘blocking' as the image is first divided into 8x8 pixel blocks prior to compression.

Wavelet compression generally causes less compression artefacts, but can make the image look soft if too much compression is used.

But starting with a high resolution image means more compression can be used without causing visible image degradation. 4K allows for a reasonable level of compression without too many problems.

It is then down to the de-compression and de-Bayering algorithms used to determine the final image quality – Professional 4K, or 4K that looks like it's been DIY'd.

It is here things are not as simple as they may seem.

4K Workflow

In an attempt to provide a solution for production through to post-production Red have developed their own workflow for dealing with captured images, working with some well known manufacturers – Apple – and some less well known – Assimilate – in building what at first appears to be a very cost effective methodology for working with the camera's 4K output.

And for those that don't want 4K images the camera can deliver 2K, and HD, or just about any format you desire, enabling you to adopt a traditional HD workflow if required.

So, all you need is a RED ONE and Apple FCP or Assimilate Scratch and you're ready to go!

Um, Well, I guess, but that means working within the compressed image space to keep data rates as low as possible and give FCP/Scratch a fighting chance of working interactively…

The issue with this is just how much work can you do staying compressed, and generate a final image of quality to do justice to the 4K capture medium?

The answer is not a lot!

Axis Films & Red

At Axis Films we saw the potential for the RED ONE early on, and have a number of cameras and camera kits on order, and will be one of the first to be up and running in the UK.

But a big part of Axis Film's commitment and support of RED ONE is in being able to offer a truly interactive workflow from scene-to-screen, and not just for customers using Axis Films' own RED cameras.

To gain the most from the awesome potential of the RED camera requires a bit more than a workflow remaining within the compressed and Bayer encoded world of Red's RedCode format, as exciting as this format is for image capture.

As with Peter Jackson's short test film (http://www.red.com/gallery-video.shtml) the best workflow to use to provide the best possible final image quality, without compromise, is a traditional offline/online one, using the compressed footage in its RedCode form to perform offline editorial via FCP (when FCP can work directly with the compressed footage, which it can't do as yet other than for simple cuts editing) Alternatively de-compress and de-Bayer to a lower resolution format for offline (1K or below) using a intermediate de-compression and de-Bayering quality for maximum speed of offline workflow.

The offline EDL or AAF can then be used to ingest the selected circle takes into an online DI system, such as Axis Post's Pablo, and de-compress and de-Bayer to full 4K dpx format, enabling full DI processing, including colour correction, vfx, graphics, text and titles, etc, to be added without fear of adding artefacts from poor de-compression and de-Bayering, and from concatenating compression if remaining within the compressed image world.

Being able to ensure that –de-compression and de-Bayering is performed at full online quality, rather than via a ‘quick and dirty' process for immediate playback on an offline system, is one of the most critical issues for final image quality.

FCP will (can?) be able to de-compress and de-Bayer the 4K footage on the fly, but only to a proxy resolution, and through the use of a less that ideal de-compression and de-Bayering process, meaning that the image displayed will be acceptable for offline, but not for online.

To use FCP to generate usable online images will require a lot of time consuming processing to generate truly high quality 4K uncompressed image, which the system will then not be able to replay in real-time.

Oh and FCP is 8 bit YUV (4:2:2) so not at all on line quality……………………………

It is here Axis has invested in the workflow to be able to work in real-time with full quality 4K images in a truly interactive environment, allowing the production to work directly with their images to generate the best end result possible.

For further information please contact:

Axis Films & Axis Post
47 Shepperton Studios
Middlesex
TW17 0QD
United Kingdom

Tel: +44 (0)1932 - 592 244
Fax: +44 (0) 1932 - 592 246
Mobile: +44 (0) 7971 - 494 901
Email: paul@axisfilms.co.uk
Web: www.axisfilms.co.uk

Contact: Paul Carter