## 1.7 DITHERING

• Sometimes we want to display color images in black and white. (a very common use is when printing shaded images).

• The average pixel is composed of the Additive colors Red-Green-Blue (RGB)

• We assume that the intensity (brightness) of the RGB value can be used as a reasonable approximation. Unfortunately, colors like RED and BLUE may appear to be the same with this method.

• The intensity is then used to set a few pixels ON or OFF in a grid. This is assuming a Black, and White device.

• Say the Dither Map is 2 by 2, In this scheme there are 5 different intensity levels.

• In dithering the dot patterns are made to be at 45 degree angles. This is because if the pattern were horizontal, or vertical, the human eye would detect the repeated pattern easily (this is also done with newspaper pictures).

### 1.7.1 A Model for Light Ray Reflection

• When light strikes a surface it is often reflected. The reflection model is quite simple. In this case a simple fraction of the incoming light will be used. More sophisticated models may be constructed using information about particular materials.

Reflection of an Incident Light Beam

### 1.7.2 A Model for Light Ray Refraction:

• When light enters or exits material, its path changes. There are also optical adjustments based on the transparency of the specific material. The path change is determined by Snell’s law, and the change in optical properties are considered by a simplified formula.

Refraction of an Incident Light Beam

• After the collision vector has been calculated, the object’s transparency must be taken into account. This is done by using a transparency coefficient ‘t’. When ‘t’ has a value of 1 the object is opaque, and all light originates from the surface. If the object has a ‘t’ value of 0, then all light is from refraction.

Light Transmission in the Presence of Transparency

### 1.7.3 A Model for Specular Reflection of Point Light

• The highlights which often appear on an illuminated object can be estimated also. In this case the Phong model is used. This model includes an estimate of how ‘fuzzy’ the light patch appears. This is not done if the collision point lies in a shadow.

Specular Reflection of Light