A color model (also called a color space or color system) is a method for representing colors in a standardized and numerical way. It defines:
- A coordinate system, and
- A subspace where each color is represented by a single point.
Color models are classified based on:
- Hardware orientation (monitors, printers, cameras), or
- Application orientation (color editing, animation, image processing).
1. RGB Color Model (Red, Green, Blue):
- Used mainly in color monitors, scanners, and digital cameras.
- It is a hardware-oriented model based on Cartesian coordinates.
- Colors are represented as combinations of red, green, and blue components.
- The RGB model forms a cube, with:
- Black at (0,0,0),
- White at (1,1,1),
- And grayscale on the main diagonal.
- Each pixel is defined as a triplet (R, G, B).
- In an 8-bit system for each channel, we get 24-bit color:
Total colors = 2⁸ × 2⁸ × 2⁸ = 16,777,216 colors. - Application: Image acquisition and display.
2. CMY and CMYK Color Models:
- CMY (Cyan, Magenta, Yellow) is used for printers. These are subtractive colors, derived from the RGB model:
- CMYK (Cyan, Magenta, Yellow, Black) model adds black (K) to overcome the impurity in pigments and produce deeper blacks.
- Black (K) is calculated as:
K = min(C, M, Y)
and then the CMY values are adjusted accordingly. - Used in 4-color printing for magazines, packaging, etc.
3. HSI Color Model (Hue, Saturation, Intensity):
- The HSI model is designed to match how humans perceive and describe color.
- Components:
- Hue (H): Actual color type (e.g., red, green).
- Saturation (S): Purity of the color.
- Intensity (I): Brightness (average of R, G, B).
- Advantage: Separates color (H, S) from intensity (I), making it easier to apply grayscale techniques.
- Conversions:
- From RGB to HSI:
- From HSI to RGB: Based on the hue sector (RG, GB, BR) with trigonometric formulas.
- Application: Color-based segmentation, image editing, feature extraction.
4. CIE L*a*b* Color Model (Device Independent Model):
- Also called CIELAB, it is a colorimetric and perceptually uniform model.
- Designed for device-independent color management, especially useful for ensuring color consistency across different devices (monitors, printers, etc.).
- Components:
- L* = Lightness (intensity)
- a* = Red-Green axis
- b* = Blue-Yellow axis
- Equations involve a function h(q) based on cube roots and reference white values.
- Application: Professional printing, color correction, tone adjustments.
Conclusion:
Each color model serves a specific purpose:
| Model | Use Case | Type |
|---|---|---|
| RGB | Image display, monitors | Hardware-oriented |
| CMY/CMYK | Printing | Hardware-oriented |
| HSI | Image analysis, segmentation | Application-oriented |
| CIE L*a*b* | Cross-device color consistency | Device-independent |