*Generalizing Alert*
my notes continued.
So say you have a point on a surface and you want to find out how much illuminance it is getting. What we do, is look at the light source and see how big of an angle it covers. We take that angle, multiply it by the amount of luminance the light has and you get the illuminance value at that point. If the light source is bigger, it covers a larger angle. If the same light is farther away, it covers a smaller angle which means the illuminance of that point is lower. Distance is irrelevant actually. All that matters is the size of that angle. A large light from a far distance and a small light at a near distance that covers the same angle and same luminance value will give the same illuminance.
You have heard the term “light falls of quadratically” or at least seen the option for the light in Maya (distance^2). This is actually a fairly inaccurate representation of the above subject. It is only accurate for distant lights. Example. You have a plane of light that has a length of X meters at a distance of Y from object P covering an angle of Z. If you were to half the distance between the light source and the object, the angle of course gets bigger, and at far distances it’s really close to doubling the size of that angle Z. However, since the light source can never reach point P, that means the angle can never grow past 180 degrees. This means that for each time you half the distance, the degree that the angle changes will grow farther away from doubling and closer to staying the same.
Example. Walk up to a large light source such as a window or tv. Look at your hand as well as how bright the tv/window is and notice how bright it is. It has a specific luminance. Take two steps back and look at your hand, your hand will appear darker, but tv/window look to have exactly the same luminance. This is because when you were closer, the light source covered a larger angle compared to your hand and when you stood back, it covered a smaller angle which means it will illuminate your hand less.
There are many ways to imitate this in CG. For MR, you have your final gather, photometric lights, and of course the beautiful and amazing Portal Light.
There is a big problem with traditional cg lights because they are point lights. Points cannot cover an angle to a point on a surface because it has no area which means it cannot emit any illuminance. This means the objects in a scene does not know what luminance the light has so it is faked. It also has an unclear mythical 0-1 value of color which is of course unrealistic (value of light is from 0-whatever). Lights also have strange falloffs (none, linear, or tries to do a distance^2 blindly). They are also rarely ever visible in a scene like they are in real life. When they are visible, their intensity is computed incorrectly. Even old MR. lights are buggy and hacks.
The reflections of the light source is generated by a fuzzy blob. This is a big problem because our eye uses highlights to judge an object and figure out most of it’s features. If we make it visible so it can be seen in reflections we run into major sampling issues.
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