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# Illumination models in computer graphics

## EScience Lectures Notes Illumination and Shading

4.17.2017 | Jennifer Bargeman

Most computer graphic rendering systems only attempt to model the direct illumination from the emitters to the reflectors of the scene. On the other hand most.

Reflection is a very special case of Snell's Law where the incident light's medium and the reflected rays medium is the same. Thus we can simplify the expression to:.

Slide 25 : 25 / 32 : Flat Shading.

This approach is fast and very simple, but it gives quite unrealistic results and non-smooth surfaces. This is highlighted by the Mach effect: the intensity at the vicinities of the edges is overestimated for light values and underestimated for dark values.

Careful : nshiny should introduce a factor 2 with the previous expression.

Slide 24 : 24 / 32 : Where do we Illuminate ?

Slide 17 : 17 / 32 : Effect of nshiny.

This shading is just a working hypothesis, that is not usually used.

At the microscopic level a specular reflecting surface is very smooth, and usually these microscopic surface elements are oriented in the same direction as the surface itself.

If both the normal vector and the incoming light vector are normalized (unit length) then diffuse shading can be computed as follows:.

## Phong reflection model

12.25.2017 | Jennifer Bargeman

The Phong reflection model is an empirical model of the local illumination of points on a surface. In 3D computer graphics, it is sometimes ambiguously referred.

Their alignment is measured by the α power of the cosine of the angle between them. The cosine of the angle between the normalized vectors R ^ m {\displaystyle {\hat }_ } and V ^ {\displaystyle {\hat }} is equal to their dot product. The specular term is large only when the viewer direction ( V ^ {\displaystyle {\hat }} ) is aligned with the reflection direction R ^ m {\displaystyle {\hat }_ }. and the hats indicate that the vectors are normalized. When α is large, in the case of a nearly mirror-like reflection, the specular highlight will be small, because any viewpoint not aligned with the reflection will have a cosine less than one which rapidly approaches zero when raised to a high power.

## Simple Illumination Model

7.20.2017 | Logan Blare

Computer Graphics. Illumination Models and Surface Rendering. Simple illuminated model is unable to directly accommodate all scattered light. · It is grouped.

3. Scale to frame buffer values between 0 and 255.

Use dot product formula:. i.

o This produces more localized highlights.

0 q p /2.

Below Figure: Only ambient illumination.

o some angular scattering of light.

Determine length of P and Q. iii.

Sometimes this look can be advantageous though, such as in modeling boxy objects. Disadvantage - it gives low-polygon models a faceted look. Artists sometimes use flat shading to look at the polygons of a solid model they are creating.

Normals are then averaged for all the polygons that meet at each vertex to produce a vertex normal.

## A Simple Local Illumination Model (Phong Model)

5.18.2017 | Nathan Becker

Illumination models model the interaction of light with the surface and range from simple to very complex. In Computer Graphics, we use.

Phong Model for Specular Reflection.

In Computer Graphics, we use physics to derive an illumination model and then fudge it to make the picture look good. More realistic and complex models adhere more closely to the underlying physics. In this section, we will look at only a simple illumination model and consider approximations for two types of light (ambient and point light sources) and two types of light reflection (diffuse and specular). Illumination models model the interaction of light with the surface and range from simple to very complex. Remember that we have to handle both specular and diffuse reflections and to solve the "pixel equation" as given below:.

Blinn Model for Specular Reflection.

Phong Illumination Model Demonstration program.

Despite its lack of a theoretical grounding, it produces quite good results and has been the basis for most computer graphics imagery produced since it was developed in 1975. A local illumination model must handle diffuse and specular reflections. Both of these models assume that the specular highlights are the color of the light source and make no attempt to estimate the color contribution from the material itself. An even more advanced model by Cook and Torrance does estimate the color contribution from the material and is more asccurate for some substances, especially metals. The Phong model is not based in physics, but on empirical observation. Much research has gone into the specular reflection component. The Blinn model, developed in 1977, is based on research results from Physics and corrects some of the deficiencies of the Phong model. Diffuse reflections are fairly simple and easily handled. Diffuse Reflection. Below, two different models are discussed.

## Illumination Models

6.19.2017 | Logan Miers

The contribution from the light that goes directly from the light source and is reflected from the surface is called a "local illumination model". So, for a local illumination model, the shading of any surface is independent from the shading of all other surfaces.

Many current graphics images and commercial systems are in this category, but many systems are becoming global illumination based. Scan-line rendering methods use only local illumination models, although they may use tricks to simulate global illumination.

It is also more computationally expensive. A global illumination model is more comprehensive, more physically correct, and produces more realistic images. A "global illumination model" adds to the local model the light that is reflected from other surfaces to the current surface. We will first look at some basic properties of light and color, the physics of light-surface interactions, local illumination models, and global illumination models.

From Physics we can derive models, called "illumination models", of how light reflects from surfaces and produces what we perceive as color. Image Reference. In general, light leaves some light source, e.g. a lamp or the sun, is reflected from many surfaces and then finally reflected to our eyes, or through an image plane of a camera.

The contribution from the light that goes directly from the light source and is reflected from the surface is called a "local illumination model". So, for a local illumination model, the shading of any surface is independent from the shading of all other surfaces.

HyperGraph Home page. The two major types of graphics systems that use global illumination models are radiosity and ray tracing. These produce more realistic images but are more computationally intensive than scan-line rendering systems. Main Illumination page.