Blinn–Phong shading model
Encyclopedia
The Blinn–Phong shading model (also called Blinn–Phong reflection model or modified Phong reflection model) is a modification to the Phong reflection model
developed by Jim Blinn
.
Blinn–Phong is the default shading model used in OpenGL
and Direct3D
's fixed-function pipeline (before Direct3D 10 and OpenGL 3.1), and is carried out on each vertex as it passes down the graphics pipeline
; pixel
values between vertices are interpolated by Gouraud shading
by default, rather than the more computationally-expensive Phong shading
.
between a viewer (V) and the beam from a light-source (L) reflected (R) on a surface.
If, instead, one calculates a halfway vector between the viewer and light-source vectors,
we can replace
with 
, where N is the normalized surface normal. In the above equation, L and V are both normalized vectors.
This dot product represents the cosine of an angle that is half of the angle represented by Phong's dot product if V, L, N and R all lie in the same plane. This relation between the angles remains approximately true when the vectors don't lie in the same plane, especially when the angles are small. The angle between N and H is therefore sometimes called the halfway angle.
The halfway angle is smaller than the angle desired in Phong's model, but considering that
Phong is using
, an exponent can be set 
such
that
is closer to the former expression. The size of the specular highlights can be matched in this way very closely to a corresponding Phong reflection, but they will always retain a subtly different shape.
Additionally, while it can be seen as an approximation to the Phong model, it produces more accurate models of empirically determined bidirectional reflectance distribution function
s than Phong for many types of surfaces. (See: Experimental Validation of Analytical BRDF Models, Siggraph 2004)
However, Blinn-Phong will be faster in the case where the viewer and light are treated to be at infinity. This is the case for directional lights. In this case, the half-angle vector is independent of position and surface curvature. It can be computed once for each light and then used for the entire frame, or indeed while light and viewpoint remain in the same relative position. The same is not true with Phong's original reflected light vector which depends on the surface curvature and must be recalculated for each pixel of the image (or for each vertex of the model in the case of vertex lighting).
In most cases where lights are not treated to be at infinity, for instance when using point lights, the original Phong model will be faster.
is a method of determining the diffuse and specular light from a point light. The light structure, position in space of the surface, view direction vector and the normal of the surface are passed through. A Lighting structure is returned;
Phong reflection model
The Phong reflection model is an empirical model of the local illumination of points on a surface...
developed by Jim Blinn
Jim Blinn
James F. Blinn is a computer scientist who first became widely known for his work as a computer graphics expert at NASA's Jet Propulsion Laboratory , particularly his work on the pre-encounter animations for the Voyager project, his work on the Carl Sagan Cosmos documentary series and the research...
.
Blinn–Phong is the default shading model used in OpenGL
OpenGL
OpenGL is a standard specification defining a cross-language, cross-platform API for writing applications that produce 2D and 3D computer graphics. The interface consists of over 250 different function calls which can be used to draw complex three-dimensional scenes from simple primitives. OpenGL...
and Direct3D
Direct3D
Direct3D is part of Microsoft's DirectX application programming interface . Direct3D is available for Microsoft Windows operating systems , and for other platforms through the open source software Wine. It is the base for the graphics API on the Xbox and Xbox 360 console systems...
's fixed-function pipeline (before Direct3D 10 and OpenGL 3.1), and is carried out on each vertex as it passes down the graphics pipeline
Graphics pipeline
In 3D computer graphics, the terms graphics pipeline or rendering pipeline most commonly refers to the current state of the art method of rasterization-based rendering as supported by commodity graphics hardware. The graphics pipeline typically accepts some representation of a three-dimensional...
; pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
values between vertices are interpolated by Gouraud shading
Gouraud shading
Gouraud shading, named after Henri Gouraud, is an interpolation method used in computer graphics to produce continuous shading of surfaces represented by polygon meshes...
by default, rather than the more computationally-expensive Phong shading
Phong shading
Phong shading refers to an interpolation technique for surface shading in 3D computer graphics. It is also called Phong interpolation or normal-vector interpolation shading. Specifically, it interpolates surface normals across rasterized polygons and computes pixel colors based on the interpolated...
.
Description
In Phong shading, one must continually recalculate the angle between a viewer (V) and the beam from a light-source (L) reflected (R) on a surface.If, instead, one calculates a halfway vector between the viewer and light-source vectors,
we can replace
with , where N is the normalized surface normal. In the above equation, L and V are both normalized vectors.This dot product represents the cosine of an angle that is half of the angle represented by Phong's dot product if V, L, N and R all lie in the same plane. This relation between the angles remains approximately true when the vectors don't lie in the same plane, especially when the angles are small. The angle between N and H is therefore sometimes called the halfway angle.
The halfway angle is smaller than the angle desired in Phong's model, but considering that
Phong is using
, an exponent can be set suchthat
is closer to the former expression. The size of the specular highlights can be matched in this way very closely to a corresponding Phong reflection, but they will always retain a subtly different shape.Bidirectional reflectance distribution function
The bidirectional reflectance distribution function is a four-dimensional function that defines how light is reflected at an opaque surface...
s than Phong for many types of surfaces. (See: Experimental Validation of Analytical BRDF Models, Siggraph 2004)
Efficiency
This rendering model is less efficient than pure Phong shading in most cases, since it contains a square root calculation. While the original Phong model only needs a simple vector reflection, this modified form takes more into consideration. However, as many CPUs and GPUs contain single and double precision square root functions (as standard features) and other instructions that can be used to speed up rendering -- the time penalty for this kind of shader will not be noticed in most implementations.However, Blinn-Phong will be faster in the case where the viewer and light are treated to be at infinity. This is the case for directional lights. In this case, the half-angle vector is independent of position and surface curvature. It can be computed once for each light and then used for the entire frame, or indeed while light and viewpoint remain in the same relative position. The same is not true with Phong's original reflected light vector which depends on the surface curvature and must be recalculated for each pixel of the image (or for each vertex of the model in the case of vertex lighting).
In most cases where lights are not treated to be at infinity, for instance when using point lights, the original Phong model will be faster.
Code sample
This sample in High Level Shader LanguageHigh Level Shader Language
The High Level Shader Language or High Level Shading Language is a proprietary shading language developed by Microsoft for use with the Microsoft Direct3D API. It is analogous to the GLSL shading language used with the OpenGL standard...
is a method of determining the diffuse and specular light from a point light. The light structure, position in space of the surface, view direction vector and the normal of the surface are passed through. A Lighting structure is returned;
struct Lighting
{
float3 Diffuse;
float3 Specular;
};
struct PointLight
{
float3 position;
float3 diffuseColor;
float diffusePower;
float3 specularColor;
float specularPower;
};
Lighting GetPointLight(PointLight light, float3 pos3D, float3 viewDir, float3 normal)
{
Lighting OUT;
if(light.diffusePower > 0)
{
float3 lightDir = light.position - pos3D; // FIND THE VECTOR BETWEEN THE 3D POSITION IN SPACE OF THE SURFACE
float distance = length(lightDir); // GET THE DISTANCE OF THIS VECTOR
distance = distance * distance; // USES INVERSE SQUARE FOR DISTANCE ATTENUATION
lightDir = normalize(lightDir); // NORMALIZE THE VECTOR
// INTENSITY OF THE DIFFUSE LIGHT
// SATURATE TO KEEP WITHIN THE 0-1 RANGE
// DOT PRODUCT OF THE LIGHT DIRECTION VECTOR AND THE SURFACE NORMAL
float i = saturate(dot(lightDir, normal));
OUT.Diffuse = i * light.diffuseColor * light.diffusePower / distance; // CALCULATE THE DIFFUSE LIGHT FACTORING IN LIGHT COLOUR, POWER AND THE ATTENUATION
//CALCULATE THE HALF VECTOR BETWEEN THE LIGHT VECTOR AND THE VIEW VECTOR. THIS IS CHEAPER THAN CALCULATING THE ACTUAL REFLECTIVE VECTOR
float3 h = normalize(lightDir + viewDir);
// INTENSITY OF THE SPECULAR LIGHT
// DOT PRODUCT OF NORMAL VECTOR AND THE HALF VECTOR TO THE POWER OF THE SPECULAR HARDNESS
i = pow(saturate(dot(normal, h)), specularHardness);
OUT.Specular = i * light.specularColor * light.specularPower / distance; // CALCULATE THE SPECULAR LIGHT FACTORING IN LIGHT SPECULAR COLOUR, POWER AND THE ATTENUATION
}
return OUT;
}
See also
- List of common shading algorithms
- Phong reflection modelPhong reflection modelThe Phong reflection model is an empirical model of the local illumination of points on a surface...
for Phong's corresponding model - Specular highlightSpecular highlightA specular highlight is the bright spot of light that appears on shiny objects when illuminated . 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:The...