In this section we'll look at how digital images work. (Hat tip to Mark Guzdial and Barbara Ericson for promoting the media computation idea of using images, sounds etc. to introduce computing.)
You see images on computers all the time. Here we will look behind the curtain, seeing how images are put together. What looks like a whole image to us, in the computer is a structure made of many little numbers.
Here is a digital image of some yellow flowers:
Zooming in on the upper left flower, we can see that it is actually made of many square "pixels", each showing one color.
We will not work with individual x/y coordinates too much. You just need to appreciate that there is this x/y coordinate system, so that every pixel in an image has some x/y coordinate that identifies its location within the image grid.
Now let's talk about colors.
Sir Isaac Newton did the famous prism experiment in 1665, showing that white light is made up of a spectrum pure colored light. Here is a picture of the experiment on my floor. White sunlight is coming in from the left into the glass triangular prism which splits up the light. Coming out of the prism we have a continuous range of pure colors, and a few are picked out by name: red, orange, yellow, green, blue, indigo, violet (ROY G BIV).
How to represent the color of a pixel? The red/green/blue (RGB) scheme is one popular way of representing a color in the computer. In RGB, every color is defined as a particular combination of pure red, green, and blue light.
The best way to see how RGB works is to just play with. This RGB Explorer page which shows how any color can be made by combining red, green, and blue light. Each light is encoded as a number from 0 (off) to 255 (brightest).
So essentially, any color can be encoded as three numbers .. one each for red, green, and blue.
| Color | Red number | Green number | Blue number |
|---|---|---|---|
| red | 255 | 0 | 0 |
| purple | 255 | 0 | 255 |
| yellow | 255 | 255 | 0 |
| dark yellow | 100 | 100 | 0 |
| white | 255 | 255 | 255 |
| black | 0 | 0 | 0 |
In RGB, a color is defined as a mixture of pure red, green, and blue lights of various strengths. Each of the red, green and blue light levels is encoded as a number in the range 0..255, with 0 meaning zero light and 255 meaning maximum light.
So for example (red=255, green=100, blue=0) is a color where red is maximum, green is medium, and blue is not present at all, resulting in a shade of orange. In this way, specifying the brightness 0..255 for the red, blue, and green color components of the pixel, any color can be formed.
Pigment Note -- you may have mixed color paints, such as adding red and green paint together. That sort of "pigment" color mixing works totally differently from the "light" mixing we have here. Light mixing is, I think, easier to follow, and in any case, is the most common way that computers store and manipulate images.
It's not required that you memorize, say, what blue=137 looks like. You just need to know the most common RGB patterns we use.
1. In the RGB Explorer, play with the sliders to make the light tan color of a cafe latte.
2. Figure out how to make orange
3. Figure out how to make brown. Brown is basically dark orange.
Demo! MaxM RGB Module
Incandescent lights can pass through a filter to give colored light. In contrast, LEDs intrinsically emit light of a specific color.
We started with a whole image, and reduced it to a big collection of small elements. This a common theme in computer science -- what looks like a complicated whole is "atomized" in the computer (made up of a very large collection of very simple elements).
So we can start with a whole, textured digital image of something. Then break it down into small square pixels. Then each pixel breaks down to 3 numbers in the range 0-255. This is a typical computer pattern -- something large and complicated is broken down and represented as a lot of little numbers if you look behind the scenes.
How to change the image? Changes to the image are done by looking at and changing the numbers that make up the image.