
Cím:
BRDF  Interactive 3D Graphics

Leírás:

I want to give you a taste of how materials can be represented in a more

general way. You'll also learn a great term to impress your friends and

confound your enemies. Think about a surface and how it reflects light. The two

variables we use are the light's incoming direction and the amount of light

reflecting towards the eye. So at it's simplest, a material can be represented

by this function,given a light and eye direction, give back an intensity. This

function is called the BRDF which stands for Bidirectional Reflectance

Distribution Function. Let's look at that phrase. First, it's a function. The

inputs are the light and the eye. The function depends on two directions so

it's bidirectional. These directions are normally given with respect to the

surface itself, that is, each vector is often given as two numbers, the

altitude angle and the azimuth. The altitude is the angle away from the normal,

and the azimuth is the angle of the vector when projected onto the plain. The

phrase reflectance distribution means how the light is spread. One simple

example is a perfect mirror. The reflectance distribution in this case is that

when the eyes direction is exactly equal to the lights reflection direction,

all light is reflected towards the eye. Every other eye direction gets no

light. Another basic distribution is diffuse reflection. For some given

incoming light direction, the direction to the eye doesn't matter. That's the

definition of diffuse reflection. Since this value is constant, diffuse is then

represented by the surface of a hemisphere. Specular highlights are represented

by lobes. This distribution represents a glossy surface, where light is

reflected in a general direction. The lights direction determines where most of

the lights energy is reflected. If the load gets wider, the specular

reflections spreads out. Written this way, our BRDF needs four numbers, two for

the light and two for the eye. But if you think about it, most materials really

only need three. These two altitude angles and this azimuth between them. For

example if you put a sheet of paper on a table top and rotate it. Both the

light azimuth and eye azimuth angles change with respect to the paper, but the

angle between the two remains the same. Most materials are fine with three

numbers.