A36851 HaberAske Scope TEXT AW:A31822 HaberAske Skylark
12/3/10
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Scope 2009/10 Physical Sciences
Visual Effects Engineering Sahil Patel From adverts in our televisions at home to cinemas, nowadays we are surrounded by some form of visual effects wherever we turn. Visual effects have become a fundamental element in almost all of the films made but there is a lack of understanding of how photo realistic images are made. Visual effects are a broad, open aspect of films and there are several definitions that have been adopted over the years so the best definition that encapsulates this topic is: “Practices, methods and technologies relating to the creation and manipulation of elements within moving images that enable storytellers to guide an audience’s conception of time, space and/or reality, thereby eliciting a desired emotional response and/or conveying critical story information.” The Basics of VFX Motion Capture (Mocap) involves translating real human movements onto a 3-D model that mimics those actions and ultimately behaves like a human does. The process is relatively simple; an actor wears a suit dotted with markers and the actors’ performance is captures and stored as animation data. When the data is seen on a computer, the dots that represent the outline of the actor can be mapped onto a 3-D model. The model can be designed as required but still retains the human actions from the motion capture session. One huge advantage is the freedom to place and move the camera wherever desired so the perspective of the shot is flexible at all times. However, the technique is limited in the sense that creating characters that do not follow the laws of physics cannot be captured. Animation and ‘CGI’ There are several techniques which all classify as animation so there is no single definition of animation. However, the three types in chronological order are traditional, stop-motion and computer animation. 90% of all animated productions are computer generated and traditional has almost completely been phased out of use since children’s cartoons can now be computer generated. CGI stands for Computer Generated Imagery and is a term used often and sometimes haphazardly. Computer animation is by definition, the same as CGI but nowadays films like Wall-E are categorized as animated films so CGI would therefore generally refer to the creation of images intended to appear realistic and blend into the live action of the frame. Most films are shot in a 2:35:1 aspect ratio at 24Hz (frames
per second). A single frame of a complex animated film such as Wall-E would range from 2-15 hours to render completely. Average rendering times for one frame of CGI have increased in the last 10 years which doesn’t seem logical when GPU/CPU speeds and memory space have been doubling every 18 months. The reason for longer rendering times, even with faster computers, is that big-budget films are demanding more complex and realistic shots and with advances in rendering and ray tracing capacity, films can afford to be more ambitious. The perfect and well-known example of intricate CGI is the Transformers franchise where any frame which has 3 or more transformers moving in it will typically take 38 hours to render. The key to making an image realistic is ray tracing (see definitions) which makes a CGI model blend and react realistically to light sources in the live-action part of the image. The realism of a visual effect is often judged by how detailed or sharp it is, however, its reaction to light and other objects surrounding it are the determining factors to impressive CGI. So an image with a resolution of 4000 pixels by 3000 pixels but rasterised will look far worse in terms of realism than the image above that has been ray traced at 1920x1080 pixels. In conclusion, we come back to our definition of visual effects to guide an audience’s perception of reality by obtaining as realistic an image as possible. Extending rotoscoping: Blue/Green screen I have explained (see definitions) rotoscoping as the way of creating composite images by pasting or drawing over filmed backgrounds use of traditional rotoscoping was in the original three Star Wars films, where it was used to create the glowing light saber effect, by creating a matte based on sticks held by the actors. To achieve this, editors traced a line over each frame with the prop, then enlarged each line and digitally added the glow. In the latter three Star Wars films, green screen was used to create the light saber effect. The actors still use sticks which make it seem like old fashioned rotoscoping is still being used but sticks are used to make saber sequences easier to film practically. The light saber is added through CGI in post-production. Green screen is effectively the modern alternative to rotoscoping as it is the same idea of modifying or adding a CGI element to a filmed scene. The process is simple; the green screen is green because it the shade which is furthest away from skin colour so a good separation from the foreground (actor) to the background can be achieved. The desired background can be added in post-production
(keyed out). A blue screen serves the same purpose as a green one but is less sensitive to cameras so more light is needed to film against a blue screen. The reason for green being more sensitive is that more pixels are allocated to the green part of the spectrum than red or blue.
Definitions Traditional animation- By using drawings for each frame (created by an artist), the drawings are photographed onto motion picture film. The final product will run at about 8 frames (drawings) per second to create a fluid animation. Rotoscoping- Invented by Max Fleischer, rotoscoping is a way tracing an object, a silhouette (called a matte) is created that can be used to extract that object from a scene for use on a different background. Stop-motion animation- Using physical objects and/or models and manipulating them and capturing the modified models onto film one by one. The most famous modern example of stop-motion animation is Wallace and Grommit: The Curse of the Were-Rabbit which used clay models to capture each frame. Computer animation- This is essentially the successor to stopmotion animation in the sense that the computer creates each frame, but each new image is advanced in the time frame. 3-D models can be built using simple animation software and by rigging the model with a virtual skeleton, facial detail can be added before rendering the final frame. Bayer Filter- filter pattern used for sensor chips in a digital still camera. More pixels are dedicated to green than to red and blue, because the human eye is more sensitive to green, producing a better colour image.
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