MTRC Bath, UK

Digital Image Compositing

Digital image compositing is heavily used in film and television. Typically it brings together image components which were filmed or computer generated independently. The aim is to produce an image which integrates the components visually.

The 1984 Porter and Duff paper is usually cited as the key publication in this area because it brought together various important ideas in a consistent way. In 1991 we extended their approach to permit colour filter effects and back-lights. Our work was aimed at cel animation and greatly enhanced the effects possible. For the first time, lighting was placed in the hands of the animator.

Our 2006 paper solved a 20 year old problem, by showing that Porter and Duff's alpha colours formed a projective space. This extended further the range of operations possible during image composition but also showed that alpha colour calculations could be used in any colour-related operation. Furthermore, they could be applied to spectral rendering and even to subtractive colours. A patent was filed earlier in the year and this extends the description in the paper. The PowerPoint slides used at Eurographics '06 are available for download. The paper reference is given below.

0TRC's ARTcam work shows how to build a software camera which includes all the attributes of previous camera models. Moreover it offers the kind of flexibility that gose way beyond any physical camera.

Our work on vectorised images provides a practical solution to pixel-free image representation. Our 2009 publication (below) shows how to render a continuous image to a specific pixel resolution and demonstrates how easy operations such as rotation are when using vector format. We have working software which performs the conversion to and from this format and the results in the paper demonstrate how image quality is fully maintained.

Researchers

Phil Willis

Research Results

Examples

Centre right is an alpha composite, basically a weighted sum of the two input images on the top row. Centre left is our result, in which the colour from the front image illuminates or filters the rear image, which has a transparent background. The third row (left) shows the rear-heading light which, composited with the filtered face, produces a final result bottom right. Compare this with the conventional image, directly above it.

The papers are available from our Departmental publications database.

"Reconstructing vectorised photographic images", J.W. Patterson, C.D. Taylor, P.J. Willis, Proc.Conference for Visual Media Production CVMP, pp. 15-24, 2009, IEEE.
“ARTcams: Attributed Rational Tensor Cameras”, Chuan Li, Peter Hall and Philip Willis Proc. Computational Aesthetics in Graphics, Visualization, and Imaging, pp. 73-81, 2009. O. Deussen and P. Hall (Editors).
"Projective Alpha Colour" P J Willis Computer Graphics Forum. Vol. 25, no. 3, pp. 557-566. (ISSN 0167-7055) Eurographics Conference issue, Sept 2006.
UK Patent: P40290GB "Method for effective, easy, and low cost manipulation of light in digital imaging". Sept 2006.
"Generalised Compositing" P J Willis ACM Graphite 2007, pp. 129-134 and 312 (colour plate).
"A physically-based colour model" R J Oddy and P J Willis, Computer Graphics Forum 10 (2) 1991, pp 121-127. Also: Eurographics UK Conference, April 10-12th 1991, pp. 87-103. Winner of the Ken Brodlie prize for best paper.

Funding Agency

No external funding.