In computer graphics , the rendering equation is an integral equation in which the equilibrium radiance leaving a point is given as the sum of emitted plus reflected radiance under a geometric optics approximation. It was simultaneously introduced into computer graphics by David Immel et al. The various realistic rendering techniques in computer graphics attempt to solve this equation. The physical basis for the rendering equation is the law of conservation of energy. Assuming that L denotes radiance , we have that at each particular position and direction, the outgoing light L o is the sum of the emitted light L e and the reflected light. The reflected light itself is the sum from all directions of the incoming light L i multiplied by the surface reflection and cosine of the incident angle.
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There are many such equations which do have an analytic solution; even some forms of the rendering equation have one e. It is also not necessary to bias the estimated solution by bounding the number of recursions. Russian Roulette provides an useful tool for giving us an unbiased solution for an infinitely recursive rendering equation. The main difficulty lies in the fact that the functions for reflectance BRDF , emitted radiance and visibility are highly complex and often contain many discontinuities.
In these cases there often is no analytic solution, or it is simply unfeasible to find such a solution. This is also true in the one dimensional case; most integrals lack analytic solutions. Finally I'd like to note that even though most cases of the rendering equation do not have analytic solutions, there is a lot of research in forms of the rendering equation which do have an analytic solution.
Using such solutions as approximations when possible can significantly reduce noise and can speed up rendering times. You integrate over some attenuated light, incoming from every direction.
But how much light comes in? You cannot solve it exactly and analytically because it has infinite integrals over infinite integration domains. But since light gets weaker each time it is reflected, at some point a human simply cannot notice the difference any more.
And so you do not actually solve the rendering equation, but you limit the number of recursions say reflections to something that is 'close enough'. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered.
Rendering equation - why unsolvable directly? Ask Question. Asked 3 years, 11 months ago. Active 2 years, 9 months ago. Viewed 2k times. Blongphong Blongphong 99 2 2 bronze badges. Are you asking about an analytical solution to it? Active Oldest Votes. Tom van Bussel Tom van Bussel 1 1 silver badge 5 5 bronze badges. In short, the rendering equation is infinitely recursive. Dragonseel Dragonseel 1, 1 1 gold badge 7 7 silver badges 22 22 bronze badges.
And possibly you could get an analytical solution for the entire projected image if your scene is even simpler than that. But that would be useless Since there is the integration about the whole sphere even if there are any two points which can see each other the rendering equation gets infinite. Each point includes the other in the integration domain.
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