Specular highlight

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Specular highlights on a pair of spheres.

A specular highlight is the bright spot of light that appears on shiny objects when illuminated (for example, see image at right). Specular highlights are important in 3D computer graphics, as they provide a strong visual cue for the shape of an object and its location with respect to light sources in the scene.

Microfacets[edit]

The term specular means that light is perfectly reflected in a mirror-like way from the light source to the viewer. Specular reflection is visible only where the surface normal is oriented precisely halfway between the direction of incoming light and the direction of the viewer; this is called the half-angle direction because it bisects (divides into halves) the angle between the incoming light and the viewer. Thus, a specularly reflecting surface would show a specular highlight as the perfectly sharp reflected image of a light source. However, many shiny objects show blurred specular highlights.

This can be explained by the existence of microfacets. We assume that surfaces that are not perfectly smooth are composed of many very tiny facets, each of which is a perfect specular reflector. These microfacets have normals that are distributed about the normal of the approximating smooth surface. The degree to which microfacet normals differ from the smooth surface normal is determined by the roughness of the surface. At points on the object where the smooth normal is close to the half-angle direction, many of the microfacets point in the half-angle direction and so the specular highlight is bright. As one moves away from the center of the highlight, the smooth normal and the half-angle direction get farther apart; the number of microfacets oriented in the half-angle direction falls, and so the intensity of the highlight falls off to zero.

The specular highlight often reflects the color of the light source, not the color of the reflecting object. This is because many materials have a thin layer of clear material above the surface of the pigmented material. For example plastic is made up of tiny beads of color suspended in a clear polymer and human skin often has a thin layer of oil or sweat above the pigmented cells. Such materials will show specular highlights in which all parts of the color spectrum are reflected equally. On metallic materials such as gold the color of the specular highlight will reflect the color of the material.

Models of specular highlight[edit]

A number of different models exist to predict the distribution of microfacets. Most assume that the microfacet normals are distributed evenly around the normal; these models are called isotropic. If microfacets are distributed with a preference for a certain direction along the surface, the distribution is anisotropic.

NOTE: In most equations, when it says it means

Phong distribution[edit]

In the Phong reflection model, the intensity of the specular highlight is calculated as:

Where R is the mirror reflection of the light vector off the surface, and V is the viewpoint vector.

In the Blinn–Phong shading model, the intensity of a specular highlight is calculated as:

Where N is the smooth surface normal and H is the half-angle direction (the direction vector midway between L, the vector to the light, and V, the viewpoint vector).

The number n is called the Phong exponent, and is a user-chosen value that controls the apparent smoothness of the surface. These equations imply that the distribution of microfacet normals is an approximately Gaussian distribution (for large ), or approximately Pearson type II distribution, of the corresponding angle.[1] While this is a useful heuristic and produces believable results, it is not a physically based model.

Another similar formula, but only calculated differently:
where R is an eye reflection vector, E is an eye vector (view vector), N is surface normal vector. All vectors are normalized (). L is a light vector. For example, then:
Approximate formula is this:
If vector H is normalized then

Gaussian distribution[edit]

A slightly better model of microfacet distribution can be created using a Gaussian distribution.[citation needed] The usual function calculates specular highlight intensity as:

where m is a constant between 0 and 1 that controls the apparent smoothness of the surface.[2]

Beckmann distribution[edit]

A physically based model of microfacet distribution is the Beckmann distribution:[3]

where m is the rms slope of the surface microfacets (the roughness of the material).[4] Compared to the empirical models above, this function "gives the absolute magnitude of the reflectance without introducing arbitrary constants; the disadvantage is that it requires more computation".[5] However, this model can be simplified since . Also note that the product of and a surface distribution function is normalized over the half-sphere which is obeyed by this function.

Heidrich–Seidel anisotropic distribution[edit]

The Heidrich–Seidel.[6] distribution is a simple anisotropic distribution, based on the Phong model. It can be used to model surfaces that have small parallel grooves or fibers, such as brushed metal, satin, and hair.

Parameters[edit]

Input parameters:

D - Thread direction ( In original papers this appear as T )
s - Shininess exponent. Values are between 0 and infinity
N - Real surface normal
L - Vector from point to light
V - Vector from point to viewer
T - Thread direction based on real surface normal.
P - Projection of vector L on to plane with normal T ( in original paper this appear as N ).
R - Reflected incoming light ray against T. Incoming light ray is equal to negative L.

All vectors are unit.

Conditions[edit]

If some of the conditions are not satisfied from the list color is zero

Note: Thins list is not optimized.

Formula[edit]

First we need to correct original direction of fiber D to be perpendicular to real surface normal N. This can be done by projection fiber direction on to plane with normal N:

It is expected that fiber is cylindrical. Note the fact that normal of fiber depends of light position. Normal of fiber at given point is:

Reflected ray needed for specular calculation:

Final calculation[edit]

Optimization[edit]

Calculation of R and P are expensive operation. To avoid their calculation original formula can be rewritten in next form:

Diffuse[edit]
Specular[edit]

Comments[edit]

T can be observed as bump normal and after that it is possible to apply other BRDF than Phong. The anisotropic should be used in conjunction with an isotropic distribution like a Phong distribution to produce the correct specular highlight

Ward anisotropic distribution[edit]

The Ward anisotropic distribution [2] uses two user-controllable parameters αx and αy to control the anisotropy. If the two parameters are equal, then an isotropic highlight results. The specular term in the distribution is:

The specular term is zero if N·L < 0 or N·V < 0. All vectors are unit vectors. The vector V is the viewing direction, L is the direction from the surface point to the light, H is the half-angle direction between V and L, N is the surface normal, and X and Y are two orthogonal vectors in the normal plane which specify the anisotropic directions.

Cook–Torrance model[edit]

The Cook–Torrance model[5] uses a specular term of the form

.

Here D is the Beckmann distribution factor as above and F is the Fresnel term,

For performance reasons in real-time 3D graphics Schlick's approximation is often used to approximate Fresnel term.

G is the geometric attenuation term, describing selfshadowing due to the microfacets, and is of the form

.

In these formulas V is the vector to the camera or eye, H is the half-angle vector, L is the vector to the light source and N is the normal vector, and α is the angle between H and N.

Using multiple distributions[edit]

If desired, different distributions (usually, using the same distribution function with different values of m or n) can be combined using a weighted average. This is useful for modelling, for example, surfaces that have small smooth and rough patches rather than uniform roughness.

GLSL Implementation[edit]

An implementation of the Phong shading equation that provides specular highlighting is given as follows:

#version 440

out vec4 color;

in vec3 frag_pos;  
in vec3 normal;  

uniform float ambient_intensity;
uniform float specular_intensity; 
uniform vec3 specular_factor;
 
uniform vec3 light_pos; 
uniform vec3 view_pos;
uniform vec3 light_color;
uniform vec3 object_color;
void main()
{
    /* Calculate the ambient compoonent. */
    float ambient_intensity                                     = ambient_intensity;
    vec3 ambient                                                = ( ambient_intensity * light_color );
        
    /* Calculate the diffuse component. */
    vec3 norm                                                   = normalize ( normal);
    vec3 light_direction                                        = normalize ( light_pos - frag_pos );
    float diff                                                  = max ( dot ( norm, light_direction ), 0.0 );
    vec3 diffuse                                                = ( diff * light_color );
    
    /* Calculate the specular component. */
    vec3 view_direction                                         = normalize ( view_pos - frag_pos );
    float spec                                                  = pow ( ( dot ( norm, view_direction ) ), specular_factor );
    vec3 specular                                               = ( specular_intensity * spec * light_color );  
        
    vec3 result                                                 = ( ambient + specular + diffuse ) * object_color;
    color                                                       = vec4 ( result, 1.0f );
} 

This shader assumes that the provided 'normal' argument has been multiplied by the normal matrix, e.g a matrix which preserves vertex normals, and is calculated as the inverse of the transpose of the model-view matrix.

See also[edit]

References[edit]

  1. ^ Richard Lyon, "Phong Shading Reformulation for Hardware Renderer Simplification", Apple Technical Report #43, Apple Computer, Inc. 1993 PDF
  2. ^ Glassner, Andrew S. (ed). An Introduction to Ray Tracing. San Diego: Academic Press Ltd, 1989. p. 148.
  3. ^ Petr Beckmann, André Spizzichino, The scattering of electromagnetic waves from rough surfaces, Pergamon Press, 1963, 503 pp (Republished by Artech House, 1987, ISBN 978-0-89006-238-8).
  4. ^ Foley et al. Computer Graphics: Principles and Practice. Menlo Park: Addison-Wesley, 1997. p. 764.
  5. ^ a b R. Cook and K. Torrance. "A reflectance model for computer graphics". Computer Graphics (SIGGRAPH '81 Proceedings), Vol. 15, No. 3, July 1981, pp. 301–316.
  6. ^ Wolfgang Heidrich and Hans-Peter Seidel, "Efficient Rendering of Anisotropic Surfaces Using Computer Graphics Hardware", Computer Graphics Group, University of Erlangen [1]