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Computer Graphics

Where

Conversion from HIS to RGB is as follows:
L=S sin H,

M=S cos H,
N=1/√3,
Where,

National Television System Committee (NTSC) YIQ Color Model

NTSC YIQ Color model is used to represent the colors in systems that use color screens where Y stands
for the luminosity, which is also the black and white signal, I stands for red minus the luminosity, and
Q stands for blue minus the luminosity.

NTSC YIQ model is considered as a linear transformation of the RGB color model. The Y component of
YIQ model is purposely made same as the Y component of the CIE XYZ color model. Since, Y is meant
to carry luminance information, black and white television systems can use it to display color images as
gray scale pictures. The formula for NTSC YIQ model is as follows:

The elements in the transformation matrix are obtained through the standard NTSC RGB phosphors
whose chromaticity co-ordinates are R (0.67, 0.33), G (0.21, 0.71), and B (0.14, 0.08). In YIQ model, it is
assumed that the white point is at x w=0.31, yw=0.316, and Yw=1.0.
12.2 Phong Model

Phong model is generally used to imitate the reflection of light from object surfaces to the viewer’s eye.
Phong model is known as an observation approach because it relies on some basic principle of physics,
and is mainly based on our observation of the phenomenon. It is also called as a local illumination
model because the Phong model gives much importance to the direct impact of the light coming from
the light source. On the other hand, a global illumination model may include such secondary effects as
light passing through transparent/translucent material and light passing from one object to another.

Now, let us consider a point light source as shown in figure 12.6, which is an idealized light with a
single point in space as the source of energy. Our eye is looking at point Q that is present on the surface
of an object. The point Q can appear in different colors depending on the type of reflection.

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