It's certainly doable and heightmaps make the implementation easier since both ray tracing and the unique texture mapping are simplified. Terrains are also mainly lit by the sky/sun which are very far away which is another assumption of PRT, so that also makes it suitable. However, you'll have poor results trying to get sharp shadows from the sun on a clear day with PRT so you'll probably have to combine it with simple direct lighting perhaps utilising shadow mapping for shadows, to get good results.

I agree with mixing in a shadow-map. The sun subtends 0.5 degrees in sky or something like that so it gives quite sharp shadows for all but casters that are far away from the receiver - the sharpness decreases with distance between the two.

For sun in the sky I would treat the two separately - I think Sloan (author of PRT) also suggested this in a GDalgorithms post somewhere. He suggested either using direct shadow maps or a steerable basis (see "steerable illumination textures") for the sun and using SH basis for the sky (since the sky, minus the sun, is alot smoother and low-frequency). I looked into steerable basis but I prefer shadow maps, but you miss out on sunlight interreflections (you get direct sunlight, direct skylight and skylight interreflections). Maybe add polynomial texture maps to capture sunlight interreflections (ie, simulate sunlight, minus surrouding skylight, and subtract the direct contribution, ie keep the 2nd, 3rd, etc bounces) and store in a polynomial texture.

If you've ever been out walking on a sunny day then you would notice that the shadow is pretty sharp. One thing to remember though is that PRT is not just Spherical Harmonics. You could also you Haar Wavelets to store your transfer functions, resulting in 'All frequency' Shadows, I have to admit I have not implemented Haar PRT but that would solve the hard shadow problem.

I agree that the effect would be incredible, especially if you are going to implement sky-light illumination models to create the incident lighting coefficients.

One advantage of mixing the SH represenation with e.g. shadow map is that dynamic objects can be supported (although this can also be done with neighbourhood transfer)

Hard shadows aren't really realistic in terrains. As Soiled pointed out, sunlight produces sharp shadows, but they grow less sharp as the distance between the shadow caster and the reciever increases. A mountain doesn't cast hard shadows on the plains below, it just looks that way when seen from far away. The transition from full shadow to full sunlight stretches over several meters, in some cases as much as 100 meters. A filtered lightmap or even vertex colors is enough for this.
I'm not sure how much it applies to PRT/SH in terrain rendering though.

AFAIK the standard way of lighting the shaded parts of terrain is by using an ambient light value, and while it's "ok", it could be better. I think SHL could be a great idea in terrain rendering, but a SHL probe of only the indirect/skylight part could be a better option. This could be used in the shaded areas (or entire terrain when overcast, while areas with direct sunlight would use a blend between SHL and a directional light. I don't know how/if retransmitted light from the rest of the terrain could be factored in.

Perhaps HDR would also be useful, since the SHL probe would otherwise be very dark (5-10% of full brightness perhaps... dunno) and thus have very little dynamic range. It might also be possible to use a normal 24bit image for the probe and change the value in the shader. I've never used either, but I noticed that the HDR example in the DX SDK spent 100% of my CPU (2800+ AMD).

The thing you must consider is wether the perhaps indesceranble increase in visual quality and realism is worth the extra cycles. I'd be interested in seeing the results though, and will be watching this topic :)

NOTE: I've never used PRT and my knowledge is minimal. I may way off. IIRC SH is a texture that is looked up in based on the normal of the pixel/vertex. The way I've interpreted this is that this texture is a half sphere view of the sky (light blue near sun that darkens and goes towards purple as the angle/distance from the sun increases). This is also the reason I think that disregarding the sun itself (just keeping the athmospheric scattering) is a good idea.

Quote:Original post by EvilDecl81

The only thing is - with a terrain, there is usually only 1 infinite light object, the sun. If the sun isn't moving, well, there isn't any point to PRT - you would just bake the lighting into the texture. PRT is used to make an object's enviroment have influence on it, since this will change based on the objects position and orientation.

Since an enviroment is static, it doesn't face this problem - the enviroments interaction with itself is static, or a function of the time of day and cloud cover. Terrain should be paramertized with these functions. That is, if you wanted to make a setting sun, then using SH wouldn't be the best way to do it since the light can only move along one arc - a simpler basis function would work much better.

Thats a good point, but I think your neglecting the fact that although the sun moves along one arc the position of the sun has a big impact on the incident light from the entire sky. I think PRT would be useful here because of this, especially if you wanted the lighting of your terrain to match PRT lit objects(even if you just use it for self-shadowing and produce dynamic shadows using
some other method)

Check out :

Check out this for daylight models.

If you do implement this then please let us know I'd be very interested in seeing your results!