DirectX® 9.0 and SmartShader™ 2.0 White Paper

Animusic Pipe Dream

"Animusic" is a technique for generating a virtual orchestra from a piece of digitized music. One of the most talked about examples of this concept, a short movie called Pipe Dream, was first shown in 2001 at the Electronic Theatre of SIGGRAPH, the world's largest computer graphics conference. Like the Rendering with Natural Light demo, the original version of Pipe Dream was rendered offline at a rate of several minutes per frame. Just one year later, the Radeon 9700 PRO makes it possible to render the same sequence interactively, at over 30 frames per second.

The technical highlights of this demo are the high polygon count, dynamic shadows, and motion blurred objects. The full scene consists of approximately 550,000 polygons, approximately ten times more than the most advanced 3D games on the market today. Many additional polygons are required to recreate the dynamic shadows, but in spite of this the powerful vertex processing engines of the Radeon 9700 PRO don't miss a beat.

In order to maintain smooth frame rates, the shadows in the scene are divided into two types, static and dynamic. Static shadows are cast by stationary objects and stationary light sources, although the color and brightness of the light sources can vary. For these shadows, an algorithm is run once over the entire scene to determine the shape and position of the shadows from each light source. The static shadows are then inserted or "cut" into the existing polygons, making them a permanent part of the scene. Each of these polygons is tagged to reference the light source that created it, so they can be turned on and off as desired.

The moving objects in the scene cast dynamic shadows, which must be recalculated for every frame. This is done using a stencil shadow volume technique. For each light source, a vertex shader is used to determine which vertices on the moving shadow casting object are facing away from it. These vertices are then projected back away from the light source, creating a shadow volume behind the object. The shapes and positions of these shadow volumes are stored in a stencil buffer. When the final scene is rendered, every pixel is checked against the values in the stencil buffer. Pixels falling within the shadow volumes are darkened.

To increase the realism of the shiny, fast-moving balls, a sophisticated motion blurring technique is used. In each frame, the position of the ball is compared to its position in the previous frame to determine its instantaneous velocity. The ball itself is then split into two halves, one facing in the direction of forward motion and the other facing away. The back facing half is drawn where it was located in the previous frame, while the front facing half is drawn in its location for the current frame. The two halves are then joined together, producing a stretched version of the ball.

Figure 11: Motion Blur

Each ball is highly reflective; in fact if you look closely at a slow moving ball you can see the entire scene reflected in it. To enhance the motion blur effect, the reflectivity and opacity of each ball is scaled according to its velocity. In other words, faster moving balls become less shiny and more transparent. The levels of detail for the reflections are scaled as well, so that they become blurrier as the velocity increases.

A slightly different motion blur effect is applied to the vibrating strings, which are oscillating between two different positions. In each frame, two versions of the string are drawn: one in the instantaneous position for the current frame, and one in which it is stretched across its entire range of motion. The two versions of the string are then blended together to produce the effect.