18.2. Understanding Image Representation

Understanding images requires understanding a set of abstractions:

1. Images are made up of little pixels (objects of the Pixel class), laid out on an (x,y) grid.

2. Each pixel contains a red value, a green value, and a blue value.

3. Each color value is actually a number between 0 and 255.

4. If a pixel has red=0, green=0, and blue=0 then the color is black since this is an absence of any color of light.

5. If a pixel as red=255, green=255, and blue=255 then the color is white. Try tilting the bottom of a cd in white light to see the colors in the light.

Figure: A grid with horizontal (x) and vertical (y) dimensions

There are some excellent CS Unplugged activities for understanding image and color representation.

11-9-1: Where is x = 0 y = 0 on an image?

• Middle of the image in width on the left side
• y is 0 at the top left corner
• Top right corner of the image
• x is 0 at the left corner
• Middle of the image in width and height
• y is 0 at the top left corner and x is 0 on the left
• Top left corner of the image
• Correct! x is 0 and y is 0 at the top left corner of the image
• Bottom left corner of the image
• y is 0 at the top and increases towards the bottom

11-9-2: What color would red = 255, green = 0, blue = 0 be?

• black
• Black is an absence of light (all values are 0).
• white
• White is when all values are 255.
• red
• If the green and blue are 0 and the red is 255 this will be red.
• green
• This would be true if the green was 255 and the other values were 0.
• blue
• This would be true if the blue was 255 and the other values were 0.

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