Expert Answer
John Michell answered .
2025-11-20
Reason 1: Saving Space (Smaller File Size)
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Normal (True Color) Images: For every single dot, your computer stores its exact color by mixing Red, Green, and Blue (RGB). This is like saying, "This dot is (255, 0, 0) for red, this dot is (0, 255, 0) for green," and so on. This takes up a lot of space in memory or on your hard drive because each dot needs three pieces of color information.
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Indexed Images: If your image doesn't use a huge variety of colors (say, only 256 different colors), you can be smarter about it.
- You create a "color lookup table" (or "palette"). This is like a small dictionary where each of your 256 unique colors gets a number (from 0 to 255). So, "red" might be number 1, "blue" might be number 2, etc.
- Instead of storing the full RGB color for every dot, you just store the number from your palette that corresponds to that dot's color.
- This saves a lot of space! Storing a single number (like 1, 2, or 3) for each dot is much smaller than storing three color values (Red, Green, Blue) for each dot.
Example: A 500x500 image.
- Normal: Each dot needs 3 units of info. Total = 75 units.
- Indexed: Each dot needs 1 unit of info, PLUS a tiny extra bit for the color palette itself. Total = around 25 units.
- You can see the indexed image is much smaller!
Reason 2: Easy Color Changes (Flexibility)
- Imagine you have an image that shows different areas, like a map with countries or a diagram with different parts (triangle, circle, square). You want to give each area a different color, but the exact colors might change later.
- With Indexed Images:
- Instead of directly coloring the triangle red, you say "the triangle should be color number 1." The circle is "color number 2," and the square is "color number 3." These numbers are stored in the image.
- Separately, in your color palette, you say: "Color number 1 is Red, Color number 2 is Blue, Color number 3 is Green."
- Now, if you decide you want the square to be pink instead of green, you don't have to change every single dot that makes up the square in the image. You just go to your color palette and change what "Color number 3" means (from Green to Pink). All squares in your image will instantly turn pink because they all refer to "color number 3."
This makes it super easy to change the look of your image without actually redrawing or re-encoding the image itself.
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