I am in the process of separating direct and indirect illumination. I will only store indirect illumination in the photon map. So far everything seems simple: Just sum the indirect illumination from the photon map to the direct illumination evaluated through distributed raytracing. There is one important thing I am unsure about, however, and that is the lighting power. For example, in photon mapping, if there is a light with a power of 40 watts, and we emit 1 million photons, we simply divide the 40 watts among the million photons, and each photons will carry 40 microwatts of energy. At the surface we then divide the total energy of the photons within a certain radius by that radius to obtain the energy per square unit. This constitutes the irradiance estimate. But what about direct lighting? Simply doing DirectIrradiance = LightColor * LightPower does not give the right intensity at all. The direct illumination is much too strong. How would I go about computing the *right* intensity, so that it would be just as intense as if the direct illumination were computed through photon mapping?

What would be a good approximation for an arbitrary shaped light then? It seems something like this would be extremely diffucult to solve for any n-gon light... And why is the dot product involved in there, is that radiance or irradiance?

But why do you include the dot product in there:

Power density (power per area, e.g. w/m^2) will be
-dot_product(surface_normal,light_direction)*power/(4*pi*r^2)

Is that meant to estimate radiance or irradiance?

Quote:Original post by Eelco
you do realize that direct illumination IS orders of magnitude bigger than indirect?

Of course. My problem is that using photon mapping for indirect illumination, I was getting a decent lighting level, but when I switched to separate direct illumination, it looked like the scene was aside a nuclear explosion.

Quote:also, light energy is not conserved in a typical scene, so your reasoning for indirect light doesnt hold i believe.

I already have that working fine through photon mapping.

Quote:as for direct illumination: youd need the radiation transfer factor between a circle with correct gathering radius and normal, and the pixel on the film of your imaginary camera. calculating that exactly is pretty damned hard i believe.

if your interested i could look up the formula for two facing circles, it would be a reasonable approximation i think.

I think Dmytry is on the right track... He's just not quite explaining it precisely enough for my taste.

"DirectIrradiance = LightColor * LightPower" is not the correct formula. The light's power (energy transferred per unit time) is a total in all directions. For a point light, the power is equally distributed over a sphere surrounding the light.

At any given point, when you want to estimate the radiance at that point, you need to estimate the irradiance of each individual light source. However, a ray strikes at a discrete point; it makes no sense to talk about the irradiance only for a certain point. Instead, you estimate the irradiance for the local region around that point. For instance, say you choose the estimation area of A units^2. For an estimation area R units from a point light source, the energy received is: ((Total light energy) / (4 * pi * R^2)) * A. This will give you an energy value which you can feed into the BRDF for the surface at the incident point to find the radiance.

Note that there is only one real variable in the energy equation: R. Rearrange the equation a tiny bit: ((Total light energy * A) / (4 * pi)) / R^2. Now, the numerator of this is a constant. We can simply take this constant and store it as the "direct illumination strength" factor for the point light. A common trick is to let this factor be "fudged" a little bit (independently of the power used for photon generation) in the scene data to get better looking results.