ABCs of DEW Software chapter 2:
The term pixel is a truncation of the phrase “picture element” which is exactly what a pixel is. A pixel is the smallest block of color in a digital picture. The term is also used for the smallest block of color on your computer monitor. In fact, to run these activities we recommend that you have the display setting for screen area on your monitor set to at least 1024x768 pixels. What that means is that your monitor has 786,432 blocks of colors arranged in rectangle with 1024 columns and 768 rows.
The resolution of an image refers to the number of pixels used to display an image. A higher resolution image uses more pixels and allows for more detail to be seen in the picture. Scanners and printers will often advertise their resolution in dots per inch (dpi), which is the number of pixels per inch that they are capable of recording or depositing. A document printed at low resolution (fewer dpi) has jagged steps of dots that make up a curve like the letter “O”. From a high resolution printer (more dpi), that same letter looks like a smooth circle.
Color Within A Digital Picture
You see color on things around you because light shines into your eye, is received by the cones and rods of your retina, and is converted to electrochemical signals that are then processed through your brain. A digital camera detects color because light shines on sensors in your camera which are sensitive to red, green, and blue. The number of sensors in the camera will define the highest resolution possible for that camera.
A traditional film-based camera records an image onto a chemically treated plastic. Digital cameras record the red, green, and blue intensities for each pixel into a numerical file – the values of color and position of the pixel are defined with numbers. A number of different file types are used to compress the data so the file takes up a minimal amount of computer memory. To display the image on the computer screen, the computer takes the red intensity value for a particular pixel from the file and shines the red component of the pixel at that amount at that place on the computer screen. It does the same thing for the other two primary colors (green and blue) for every pixel in the image.
Seeing Only One Color of Light
When we talk about seeing only one color we are not referring to the condition known as color blindness. People who are colorblind have difficulty distinguishing between certain colors, for example red and green, but they are still capable of perceiving light of both colors. What we are talking about is if you could only see the small range of light wavelengths that is called “red”. You would be unable to perceive the spectrum of light including orange, yellow, green, blue, and violet.
In satellite images, each color is data of the image that provides specific information. By turning off all of the light that is not red, we can look solely at the information that red light provides. If we were physically only able to see the range of light that is red, it would appear more like a black and white image than a red and black image. The use of black and white to view such information allows us to make out details and differences in shade and intensity more easily.