6.21.6 Turbulence

To create an illusion of turbulent flow, first-derivative discontinuities are introduced into the noise function by taking the absolute value of the function. Although OpenGL does not include an absolute value operator for framebuffer contents, the same effect can be achieved by the following:

1. glAccumGL_ LOAD,1.0(GL_ LOAD,1.0);
2. glAccumGL_ ADD,-0.5(GL_ ADD,-0.5);
3. glAccumGL_ MULT,2.0(GL_ MULT,2.0);
4. glAccumGL_ RETURN,1.0(GL_ RETURN,1.0);
5. Save the image in the color buffer to a texture, main memory, or other color buffer.
6. glAccumGL_ RETURN,-1.0(GL_ RETURN,-1.0);
7. Draw the saved image from Step 5 using GL_ ONE as both the source blend factor and the destination blend factor.

The calls with GL_ ADD and GL_ MULT map the values in the accumulation buffer from the range [0,1] to [-1,1]; this is needed because values retrieved from the color buffer into the accumulation buffer are positive. Since values from the accumulation buffer are clamped to [0,1] when returned, the first GL_ RETURN clamps all negative values to 0 and returns the positive values intact. The second GL_ RETURN clamps the positive values to 0, and negates and returns the negative values. The color buffer needs to be saved after the first GL_ RETURN because the second GL_ RETURN overwrites the color buffer; OpenGL does not define blending for accumulation buffer operations.