3D Compositing

1. 3D Compositing This Session: • Definitions – compositing, green screen, alpha mattes, Motion tracking, light matching. • Examples

2. Definitions - Compositing • The combining of visual elements from separate sources into single images, often to create the illusion that all those elements are parts of the same scene. • Live-action shooting for compositing is variously called “blue screen,” “green screen,” “chroma key,” and other names. • Today, most though not all compositing is achieved through digital image manipulation. • Pre-digital compositing techniques, however, go back as far as the trick films of Georges Méliès in the late 19th century; and some are still in use.

7. Problems with Chromakey / Green screen • Edge bleeding / spill • Limits colour options in foreground • Can’t deal with transparency • Cant deal with (or create shadows) Lets try mattes…..

8. Definitions – Alpha Compositing • alpha compositing is the process of combining an image with a background to create the appearance of partial transparency. It is often useful to render image elements in separate passes, and then combine the resulting multiple 2D images into a single, final image. • In order to combine these image elements correctly, it is necessary to keep an associated matte for each element. • This matte contains the coverage information — the shape of the geometry being drawn — making it possible to distinguish between parts of the image where the geometry was actually drawn and other parts of the image which are empty.

9. http://www.vimeo.com/1262558

12. <Rendered file Alpha matte file>

14. Advantages – Alpha Compositing • Buffers: 3D rendering programmes can not only output colour files and their associated alpha matte. Virtually any data stored as information in the render can be output: z y x

23. Putting it all together: • http://www.vimeo.com/2224272 • Alpha mattes: What comes next • http://www.vimeo.com/1513186

24. Motion Tracking / Match moving • Match moving is a visual effects technology to allow the insertion of virtual objects into real footage with the correct position, scale, orientation and motion in relation to the photographed objects in the scene. • The term is used loosely to refer to several different ways of extracting motion information from a motion picture, particularly camera movement. Match moving is related to rotoscoping. It is sometimes referred to as motion tracking.

25. Motion Tracking / Match moving • Motion tracking programmes – After Effects, Boujou, Syntheyes http://uk.youtube.com/watch?v=UHGv4LIE338 • http://www.2d3.com/ • http://www.computerarts.co.uk/tutorials/3d__and__animation/3d_motion_tracking • http://uk.youtube.com/watch?v=f3ruaZ5QDXI • http://uk.youtube.com/watch?v=RvteccETd6k&feature=related • http://uk.youtube.com/watch?v=yuHX5iGBzvw&feature=related

26. Motion Tracking / Match moving • Without sophisticated tracking software you will have to match angles, distances and movement by careful measurement and planning. Tips: • Carry a tape measure on the shoot (I have a laser ruler that you can borrow) • Shoot the scene twice – once with an object roughly the size of the 3D character that you will be building – and once without. You can use the ‘with’ footage as reference in the 3D programme. • Ensure that you set up your units in blender / Cinema 4D to match the units in the real world. • Keep the camera on a tripod (of known height)

27. What comes next: Light matching

30. Playing around with HDR Shop • http://gl.ict.usc.edu/HDRShop/ • Read: http://www.hdrsoft.com/resources/dri.html • JahShaka compositing (open source): http://jahshaka.org/