Reflections

Posted on April 7, 2008. Filed under: Lake water shader, Technical background | Tags: , , , , |

Static cube-map reflections

If the water does not need to reflect everything, it is possible to use a pre-generated cube-map to calculate reflected colors. Cube-maps are a kind of hardware-accelerated texture maps (other approaches are for example sphere mapping and dual paraboloid mapping). Just imagine a normal cube with six images on its sides. These images are taken as a photo from the center point of the cube, and they show what is visible from the surrounding terrain through the points of the sides. An example is shown on the next figure:

Cub map sides

As shown on the following figure, the six sides of the cube are named after the three axle of the coordinate-system: x, y and z in positive and in negative directions:

Cube map center

So we have a cube map and the reflecting surface of the water. We can calculate the vector to each point of the water that points into the direction of the reflected object. Using this 3-dimensional vector (the red one on the last figure) the points of the cube-texture can be addressed from the center of the cube. This vector aims exactly one point of the cube, which has the same color as the reflected object in the original environment. But this calculations are much more efficient and hardware-accelerated to match the real-time requirements, while calculating global illuminations for every reflecting point needs much more time. Using cube-maps has one more advantage: the cube has sides which represent the environment that is not visible by the camera, so even points behind the camera can be reflected. On the other hand, cube-maps needs to be pre-rendered, so it is impossible to reflect changing environment (for instance, with moving objects) if we want to meet the real-time conditions. Using this technique, sky can be easily reflected on the water surface, but a moving boat needs to be handled in another way. Additionally, artifacts can be discovered at the edges of the cube, that are really hard to avoid.

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