Digital Images
1. The Bitmap Concept
Most images on a computer are stored as Bitmaps. A bitmap image is composed of a grid of tiny dots called Pixels (short for Picture Elements).
In a 1-bit image, 0 = White, 1 = Black.
2. Key Terminology
- Pixel
- The smallest addressable element of a digital image.
- Resolution
- The number of pixels that make up an image (Width × Height). Higher resolution means more detail but larger file size.
- Color Depth (Bit Depth)
- The number of bits used to represent the color of a single pixel.
3. Color Depth Calculations
The number of colors available is calculated by 2n, where n is the bit depth.
| Bit Depth | Colors Available | Usage |
|---|---|---|
| 1-bit | $2^1 = 2$ | Monochrome (Black/White) |
| 8-bit | $2^8 = 256$ | Basic web graphics |
| 24-bit | $2^{24} \approx 16.7$ Million | "True Color" (8 bits each for R, G, B) |
4. Estimating Image File Size
The Formula:
Example: An image is 1000 pixels wide, 500 pixels high, and uses 24-bit color.
File Size (bits) = Resolution (W × H) × Color Depth
Example: An image is 1000 pixels wide, 500 pixels high, and uses 24-bit color.
- Pixels: $1000 \times 500 = 500,000$
- Size in bits: $500,000 \times 24 = 12,000,000 \text{ bits}$
- Size in MiB: $12,000,000 \div 8 \div 1024 \div 1024 \approx 1.43 \text{ MiB}$
5. Metadata
An image file doesn't just contain pixel data. It also contains Metadata (data about data).
⚠️ Exam Alert: If you increase the Resolution OR the Color Depth, the file size will increase. This means it will take longer to download/upload and require more storage space.