1.1 Image Processing and Computer Graphics: Colors, Human Vision, and Color Models

The human eye is sensitive to electromagnetic waves with specific wavelengths (400-700 nanometers).
400 nm is perceived as blue light, and 700 nm as red light; other wavelengths appear as different colors (green, yellow, orange, etc.).

Light enters through the lens and is projected onto the retina.
Retina contains light-sensitive cells that produce electric signals.
These signals are transmitted through the optic nerve to the human brain, which analyzes them to understand shapes and objects.The retina consists of two types of photoreceptor cells: rods and cones, which are responsible for vision under low light conditions and color vision, respectively. Rods are highly sensitive to light but do not detect color, while cones are essential for perceiving hues and detail, providing the basis for our color vision.

The fovea is the area where the image is projected when looking straight at something.
Other parts of the retina handle peripheral vision.
The area where the optic nerve exits the eyeball lacks light-sensitive cells, creating a blind spot.

Rods are sensitive to the entire spectrum of visible light, enabling black and white vision.
Cones are of three types, each sensitive to different light wavelengths: blue, green, and red.

There are 100-120 million rods in each eye, responsible for peripheral vision.
The fovea has a high density of cones, allowing detailed and colorful vision when looking directly at something.
Peripheral retina contains mostly rods and few cones, resulting in less detailed and colorless vision on the sides.
Rods are very sensitive to light, enabling perception of shades in dark environments.

When looking at something, the fovea provides a bright, colorful, and detailed image, while peripheral vision captures a blurred black and white image.
The blind spot, where the optic nerve starts, results in no vision in that area.
Using two eyes compensates for the blind spots, allowing a complete view of the surroundings.

Normalized plots show the sensitivity of red, green, and blue cones, as well as rods, with maxima at the same level.
Blue cones are distinctly sensitive to blue light, while red and green cones have similar characteristics.
Relative sensitivity plots reveal that green cones are the most sensitive, followed by red, and then blue cones.

Most mammals have dichromatic vision, with only two kinds of cones, unable to differentiate between red and green.
Some primates, including humans, have trichromatic vision with three kinds of cones.
Color blindness affects some humans' ability to distinguish between green and red.

Dogs and cats are not very interested in colorful television sets due to their limited color vision.
Some animals like fish and birds have more complex color vision than humans; birds have tetrachromacy with a fourth cone for ultraviolet vision.
Bees can see in ultraviolet, which helps them find flowers.
Mantis shrimp (stomatopods) have up to 12 different cells sensitive to different wavelengths of light, enabling an incredibly colorful view of the world.

Three primary colors that humans can see are red, green, and blue.
Every other color can be composed by the mixture of the three.
Green and blue combine to create cyan, blue and red to create magenta, and red and green to create yellow.
Mixing all three primary colors with the highest intensity creates white.

This is an additive model because we add these colors together to create other ones.

A subtractive model uses color filters or complementary colors like cyan, magenta, and yellow.
Filters subtract components from white light:
- Cyan filters remove red.
- Magenta filters remove green.
- Yellow filters remove blue.
Equations to compute components in the complementary model:
- $c = 1 - r$
- $m = 1 - g$
- $y = 1 - b$
One means maximum intensity with values ranging from $0$ to $1$ .
Every color can be represented by a vector or a point in a three-dimensional space.

Every color can be represented in 3D space with axes for green, red, and blue components.
Mixing blue and red yields magenta, and mixing maximum amounts of red, blue, and green produces white.
The subtractive model starts from the opposite corner, showing how to remove components using yellow, magenta, and cyan filters.
Mixing yellow and magenta filters results in red, as yellow removes blue and magenta removes green.

Using RGB or CMY models is not always useful; some technologies benefit from a rotated coordinate system.
One main axis defines luminance (brightness or intensity), representing grayscale image information.

The YIQ model has luminance (Y) and two other components (I and Q), with all axes perpendicular to each other.
RGB values can be recomputed to the YIQ model using these weights:
- $Y = 0.30r + 0.59g + 0.11b$
Green has the highest weight because that’s what Human eye is most sensitive to and blue has the lowest weight.

Luminance represents gray level, while chrominance adds color information.
Cutting the space perpendicular to the Y direction creates a plane where the center represents gray shades.
Distance from the center indicates color saturation, and the angle around the center is the hue.

This model uses hue (angle), saturation (color saturation), and brightness/value (B/V).
The brightness axis is the same as the Y axis.
Distance from this axis is color saturation, and the angle around it is hue.

This model was applied in color television in Europe.
U and V (sometimes called PBPR or CBCR) are computed by subtracting brightness (Y) from blue and red components:
- $U = B - Y$
- $V = R - Y$
These axes are more or less perpendicular to the Y axis and define additional chrominance information.

YIQ and YUV models represent chrominance information with slight axis rotations.
In image processing and analysis, different color models are used depending on the needs.
Examples include:
- RGB (additive).
- CMY (subtractive).
- YUV/YIQ (luminance and chrominance split).
- HSL/HSB/HSV (hue, saturation, luminance/brightness/value).
HSL, HSB, and HSV models define chrominance in terms of angle (hue) and distance (saturation) but have slight differences.