Caustics are defined as "The light patterns generated on a surface by reflected or refracted light rays." Imaging that you are in a swimming pool and swimming beneath the water. The sides of the pool are not evenly lit. As the light enters through the surface of the water it is refracted and causes areas of shadow to appear on any surface beneath the water line.
This implementation of caustics is physically fake. I do not represent light beams entering and refracting on the water surface. This effect is achieved by tiling a sequence of textures on the terrain for the area under the surface of the water.
Sequence of textures which are tiled over the terrain
This does require the graphics card to support a 3rd texture unit. If you remember from previous blog entries, the terrain already requires 2 texture units, 1 for the terrain texture and 1 for the detail map to add more realism to the scene. From some quick research it looks as though most modern graphics cards support at least 4 texture units so this should not be a problem.
With both my planned implementations of water nearing completion it seemed appropriate to run some comparisons.
CPUWater was coded so that Decade could render a terrain with water on PCs which do not support the relevant OpenGL Extensions required for GPUWater. Decade will automatically detect which it can use.
The main surprise to come from running this comparison is non technical. After a few friends and programming peers viewed my comparison video (below) approximately 80% if them commented on how good CPUWater looked and that they thought it looked more realistic than GPUWater.
Scene Rendered with CPU Water
Scene Rendered with GPU Water
The main reason for this is because CPU water moves. Its height at each point varies depending on the tessellation of the mesh therefore it looks like waves lapping the shore. Since GPU water is comprised on only 1 quad and the waves are simulated using bump mapping the shore line is static.
Perhaps I should write a CPUGPUWater. The water will comprise of a tessellating mesh like CPU water, but it will be rendered using bump mapping like GPU water. This might achieve a nice result of having water which moves naturally however also appears to be highly tessellated despite its low poly count.
My second attempt at water rendering uses a vertex/pixel shader along with normal and dudv map's. Using the normal and dudv map's along with the reflection texture, which is dynamically generated each frame, the final effect is achieved.
The image above is the normal map. Instead of using each pixel for a color, the nomal map stores a normal (between -1 and 1). The normals naturally give the image a purple, pink and blue color. What we do with this image is basically use it to bump map our water to give it the appearance of being highly tessellated and realistic. This realism comes when we use per-pixel lighting on the water. If we didn't have a normal map we would just depend on the normal of the water surface, which is pointing straight up, the water would look like a flat mirror.
The image above is the dudv map. It is called that because it's actually the derivative of a normal map. In mathematics the notation for a derivative is du/dv. What this texture does is give us a way to calculate refraction and how the light will react and bend on the water. Notice how the dudv map looks like waves of water.
The image above is the reflection texture. This is caculated each frame by inverting the scene, setting up a clipping plane and rendering the reflective data. Projective texture mapping is then used to apply this texture to the water.
Although I am happy with this implementation there are a few changes that need to be made. Memories of my first implementation of CPU water are coming back. My lake again looks like a mirror. It is too bright and too reflective. It should be possible to see some way under the surface of the water. I have tried to implement the simple changes which were required by CPU water, after I changed to using a reflection texture rather than drawing the inverted scene to the screen, however there is no differnce. Adding Alpha Blending with a fixed function does not seem to make any difference when the pixel shader is used. If I remove the shader from the rendering, a correct flat blue alpha blended quad is rendered. Since the GPU water is effected by per pixel lighting prehaps I need to modify the pixel shader in some way. I have read a few tutorials on OpenGL shaders, however I feel that I need to complete a few more before I have the confidence to say for sure.
As all can see its been a while since I updated the blog so yesterday I recorded a small video of decade. You can now see the engine in motion rather than just static screenshots.
Unfortunately YouTube has really lowered the quality of the video when it was uploaded but it should still give a rough idea about Decade.
The video shows a simple terrain with random trees and my CPU water with vertex projected reflections.
Other features that I am still working on are
GPU Water Shadow Maps, Parallel Split Shadow Maps, Shadow Volumes Atmopheric Scattering Priority Rendering Queue Octree Scriptable Particle System
Sequence of textures which are tiled over the terrain
