GPU Raycasting Tutorial

The famous Stanford dragon rendered using GPU raycasting.

This post will try to explain how to implement a GPU based raycasting render, using open GL and Nvidia’s CG. This tutorial assumes some experiance with OpenGl and vertex-fragment shaders.

First of all why do we need this algortihm? Because it is a smart way to achieve high quality volume rendering and the raycasting algorithm is well suited for the modern GPU’s. Esspecially the new 8800 series because of the unified shading system.

The reason behind this tutorial is to help people getting startet with GPU raycasting because there is some technical difficulties that has to be adressed in order to render volumetric data like in the picture above.

The main core of the algorithm is to send one ray per screen pixel and trace this ray through the volume. This is possible to implement in a fragment program and the rendering can be done in realtime. The techinque is pretty flexible for instance effect like shadows can be implemented with a few lines of code.

Here is a conceptual image of the raycasting algorithm where one ray pr pixel is spawned and traced through the volume.

In order to generate the nessesary rays we use a clever trick by using OpenGl abillity to render geometry. How can this help us you might say? Well listen up my young apprentice. First we define a ray:

• A ray is just a origin point o and a direction vector dir.
• A ray descripes a line in 3d space by using the formula P(t) = o + dit * t
• So to generate a ray we need to find the origin point and the direction vector.

This can be done be rendering a cube where the colors represent coordinates, and let OpenGL’s interpolation take care of the rest. The way to do this is to render the front and the backside of a unitcube that is illustrated just below.

If we subtract the backface (on the right) from the front we get at a direction vector for each pixel. This is the direction of our ray. The origin is just the frontface values of the cube. So we have to do two renderpasses one for the front and one for the back. To render the backside we enable frontface culling. In my implementation i use an OpenGL framebuffer object (fbo) to store the result from the rendering of the backside, and use the frontface rendering to generate the fragments that starts the raycasting process. If you are unfamiliar with framebuffer objects check out this link.

So to calculate the raycasting we need to create the ray and then step through the volumetexture. This is all done in a single fragmentprogram and calculated on the GPU. The fragment program is fairly simple, the only real issue is to calculate texturecoordinates used to index the backface buffer in order to get the ray exit point. These texture coordinates is refered to as normalized device coordinate, and in the implementation we find a corresponding pixel in the backface buffer by this calculation.

float2 texc = ((IN.Pos.xy / IN.Pos.w) + 1) / 2;

Where IN.Pos is the modelviewprojected position. This calulation gives us the fragments screen position in the interval between [0,1]. Then the ray exitposition is found be using texc to index in the backface buffer like this:

float4 exit_position  = tex2D(backface_buffer, texc);

Now we create the ray and use the shader model 3.0 looping capabilities to create a for loop. This loop will step through the volume with a certain stepsize delta and we can accumulate opacity and color value according to the nature of our volume data set. In the demo implementation the ray terminates when it leaves the volume or when the accumulated opacity reaches a high enough value. But there are many possibilities. For instance if we terminate the ray when a certain opacity threshold is first encountered then the result will be some kind of iso surface rendering.

Here is a link to the raycasting shader.

The raycasting technique can be used for many types of rendering problems where polygonbased rendering have a hard time. For instance effects like smoke and glass can be rendered in realtime. Maybe i will do a tut on these subjects in the near future.

To get you started with this cool rendering algorithm, i have made a simple GPU raycasting implementation that hopefully will clear up the details. That is just the kind of guy i am

The demo contains a windows executable and source code that implements the GPU raycasting algorithm. To compile you need Nvidias cg and Glew.

The demo just shows a volume of colors where some spheres have been subtracted. The stepsize of the ray can be adjusted be pressing the “w” and “e” buttons. Notice this demo might be hard on your gfx card, just try to press “w” a lot this will result in a big stepsize and the raycasting will update more rapidly.

To download windows demo and source code click here.

As a last comment: this is just a tutorial that will get you started with GPU ryacasting, the technique was invented back in 2003 by these guys:

J. Kr¨uger and R. Westermann. Acceleration techniques for
GPU-based volume rendering. In Proceedings of IEEE Visualization
2003, pages 287–292, 2003.

The GPU raycasting is an active research area and if you want to learn more about this, here is a couple of references:

A more recent article that explain a shader model 3.0 implementation can be fund at this url: http://www.vis.uni-stuttgart.de/ger/research/fields/current/spvolren/

VRVis has posted a lot of really good papers on the subject so that has been my greatest resource. Visit them at this address: http://medvis.vrvis.at/home/

If you find some errors or make some cool improvements please let me know at peter.trier@alexandra.dk. Have fun!!