Fixing the lighting

Normals on displaced terrain

 
While trying to find out what's the problem with ambient lighting on objects, which was making them too dark, I discovered that there's another problem with terrain lighting. Parts of the terrain that are subject to horizontal displacement were having incorrectly computed normals. For some reason the displacement effect was not affecting the terrain normal vector fully, resulting in contrast being lower than appropriate. The reason was probably a forgotten coefficient from an earlier debugging session, because when I derived the Jacobian again, there was this value of 0.5 that didn't belong anywhere.

It wasn't the first time when a surgical tool from a year-old operation was found inside our code. On the other hand, maybe one day we'll discover a hidden sleep() call and get unbelievable speedup by removing it :-)

Here are some comparison screens, old one on the left, fixed on the right side.

Shadow artifacts

Another issue that was solved along the way were the artifacts on ATI cards in the shadowed areas. On ATI cards the floating point values that are output from shader may apparently end up slightly modified when written to a floating point render target, because of the blending unit. It does not matter when these values are supposed to be normal floating point values, but if the exact bit representation is important because these values are actually reinterpreted, a floating point render target cannot be used. Fortunately, the fix was easy once the reason for the artifacts was known.

Here's also the updated ambient lighting:

Adjusted ambient lighting on objects. Previously the downfacing surfaces were almost black.

The ambient lighting isn't finished yet, there will be a better one that will reflect the surrounding environment.

White balance

When implementing the fog mentioned in the previous post, I observed a weird thing happening: the fog wasn't white, as I expected, but it had a dirty Beige tint making it look a bit like a smog or something. But since the implementation didn't use different absorption and scattering coefficients for RGB components, and thus the color of the sun light shouldn't have been modified, I thought it was a bug, and neglected it until most of other issues were solved.
But then, after inspecting all the code paths, I came to the only conclusion that the computation is right and the problem must be in the interpretation. So I tried to convince myself that the fog must be white, and the tint actually isn't there. Almost made it, too.

But the machine coldly asserted that the color wasn't white as well. Didn't bother with any hinting as to why, though.
Apparently the incoming light that was scattering on fog particles was already this color, even though the sun color was not modified in any way, unlike in the previous experiments.

Interpretation?

The thing is that sunlight really gets modified a bit until it arrives to the planet surface. The same thing that is responsible for blue sky causes this: a small part of the blue light (and a smaller part of the green light too) gets scattered away from the sun ray. What comes down here has a slightly shifted spectrum.
But how come we see the fog white in real life?
Turns out, everything is fake.

The way we perceive colors is purely subjective interpretation of a part of the electromagnetic spectrum.
And as it is easier for the brain to orient in the environment when the sensors don't move, it is also simpler to stick with constant properties on objects. Our brain "knows" that a sheet of paper is white, and so it will make it appear white in wildly varying lighting conditions. This becomes apparent when you use a digital camera without adjusting for the white color - the results will be ugly.

So basically that's why we have to implement an automatic white balancing, at least until we all have full surround displays and our brains magically adapt by themselves. By the way, playing in fullscreen in the dark room with uncorrected colors slowly makes it adapt too.

Implementation

Our implementation tries to mimic what the perception actually does. By definition, a white sheet appears to be white under a wide range of lighting conditions. So we are running a quick computation that uses the existing atmospheric code on GPU, that computes what light reflects off a white horizontal surface. The light has two components - sun light that reflects at an angle and its illuminative power diminishes as the sun recedes from zenith, and the second one is the aggregated light from the sky. Once this compound color is known, we could perform the color correction as a post-process, but there's another way - adjusting the color of sun so that the resulting surface color is white. This has an advantage of not affecting the performance at all, since the sun color is already taken into equation.

While this algorithm doesn't mimic the human perception precisely, i.e. the actual process is more complex and depends on other things, it seems to be pretty satisfactory, though I expect further tuning.

Some of the properties: it extends the period of day that seems to have a "normal" lighting, and removes the unnatural greenish tint on the sky:


During the day it compensates for the brownish light color by making the blue things bluer. Can't say the old colors were entirely bad though.

So long, and thanks for all the fish

Alien planet Earth

Rendering our planet "alienized", using a different set of basic materials for fractal mixer, with changed parameters of atmosphere, sun and water.

Scattering of light in the atmosphere determines both the color of sky and sunsets. We can see a blue sky because the blue light is more likely to bounce off the air molecules than the green and even more than the red components of sun light. As the light from sun travels through the atmosphere above us, some of it gets scattered away from the ray and towards our eyes. The same effect is responsible for red sunsets - as the sun sets, light from it has to travel a longer way through a denser layers of atmosphere. By the time it reaches us, most of the blue and green light gets scattered away from the ray, leaving only the most persistent red component.

This effect is simulated in Outerra, and so we are able to play with it. What if the atmosphere consisted of different gases and the scattering characteristic was different?

In the following video we are showing planet Earth that was "alienized". The atmosphere in it scatters the green light best, which you can see not only on the sky itself but also on the shaded parts that are not lighted by sun but only by a portion of the sky.
The sun has got an orange shade, which you can see mainly on the horizon (the sun itself is too bright so looking at it directly saturates the color to white).

The absorption of light in the water has been altered as well - normally, the red light gets only so far in the water, when it almost entirely disappears. Here, the medium absorbs the green and blue light instead, letting the red one to penetrate into depths. Of course, since the water surface largely reflects the sky at an angle, the ocean appears to be green in the distance.

At the end there's also a short sequence with a red-orange atmosphere.

Here are some screens showing it under various settings:

Milk water & yellow skies:

Violet atmosphere:

No atmosphere (or no atmospheric scattering). This is what you'd get for example on the Moon: