Real-Time Dynamic Shadow Generation

Files

• Program: realtimeshadows.zip (source not included)
• Power Point Presentation: realtimeshadows.ppt

Introduction

This project was created for CSE528(Computer Graphics) at SUNY Stony Brook. It demonstrates the use shadow algorithms based in a small virtual environment. The idea of my project was to implement shadow algorithms to generate shadows that look good and have a flexible use with a low cost frame rate hit.

Importance of Shadows in Virtual Environments

Shadows are a very important piece of the puzzle in a virtual environment. They play a major role in the determination of special relationships between objects in a virtual scene. Without shadows the realism of a scene is lost and the spatial location of models can be ambiguous. The picture on the right illustrates the importance of shadows in the determination of spatial relationships. The scene on the left consists of a thin blue rectangle, a purple cube and one light source off in the distance a bit. Where is the light coming from? Where are the objects in relation to each other? Is the blue rectangle resting on the floor? Can shadows answer these questions? In the scene on the right, shadows are being cast and the answers to these questions are easily found.

Real-Time Dynamic Shadow Algorithms

• Planar Projected Shadows

  Very Fast
  Squishes the Model onto a plane in just one matrix multiplication
  Easy To Implement and with good results when casting shadows onto one plane
  Can extend to cast onto multiple planes, but each plane will require another rendering of the model.
  
  Plane Eq: Ax + By + Cz + D = 0
  [x, y, z] is the difference between the center of the model and the light source. Ideally you want to use the distance from each vertex. However it is not as fast since the matrix will have to be computed on a per vertex basis instead of a per model basis.
  The vertices of the model are multiplied by the Planar Projection matrix.
  You can find this matrix in the openGL Red Book.

• Shadow Mapping

  The algorithm is as follows:
  Render scene from the point of view of the light with depth test enabled.
  Result is a shadow map from the lights perspective stored in the depth buffer.
  Next, render scene from cameras point of view and determine the pixels (X,Y,Z) coordinates with respect to the Lights coordinate system.
  Compare the Z value of the pixel with the value stored at position (X,Y) in the depth map.
  If the values are equal then the pixel is lit, if it is greater then the pixel is shadowed.
  
  The picture on the left demonstrates this algorithm when the pixel fragment is in shadow, the picture on the right shows what happens when a pixel is lit.

• Shadow Volumes

  

  A shadow volume is a volume in space for which objects that reside within that volume are shadowed. A shadow volume extends from the occluder at one end towards infinity at the other. The boundary of a shadow volume defines the point in space where an object goes from being un-shadowed to shadowed. The picture on the right demonstrates what the shadow volumes are.

  How it works:
  
  Makes use of the stencil buffer to block out areas in which a shadow is cast.
  First render scene with only ambient light.
  Compute shadow volume.
  Render the front facing polygons of the shadow volume and increment the stencil buffer for each successful stencil test
  Render the back facing polygons and decrement the stencil buffer for successful stencil tests.
  Finally render the scene with diffuse and specular lights in all places that the stencil buffer is equal to 0.

The Future of Shadow Algorithms in Real-Time

With the ever-increasing need for more realism and at a higher frame rate it is no surprise that the future of shadow rendering is starting to take a more hardware specific approach. The same algorithms are being used but fine tuned and accelerated with hardware specific techniques for the shadow rendering algorithms. NVIDIA Corporations? new graphics card the GeForce3 is the first graphics card to incorporate in it a shadow buffer ? a segment of memory that is dedicated to do one specific thing only ? help in rendering shadows in real-time.

The algorithm that the new NVIDIA card will use is based on a shadow mapping technique. Instead of mapping shadows though it maps un-shadowed areas. This simplifies the hardware and is the approach that shadow algorithms are starting to implement.

NVIDIA Corporations? new GeForce3 will revolutionize the way in which shadows are created in virtual environments. Right from the mouth of NVIDIA: ?NVIDIA shadow buffers represent a giant leap forward for the PC graphics industry. The unique hardware features of the GeForce3 Ti GPUs make realistic, real-time shadows available for the first time ever on consumer PCs and bridge the gap between cinema style shadow effects and the state-of-the-art for PC graphics.?

A Few Nice Pictures