A raster grid can be made simply by overlaying
a piece of graph paper over an image. Now, with the graph paper
in place one can see that each small square section of the image
has an exact location. The small sections are called pixels. The
pixel's location is identified by giving its x and y coordinates.
When working with various software packages, the point of origin
for the coordinates may vary. Even though the point of origin
varies, each pixel will still have a specific location.
Each of the pixels is assigned a number value
that corresponds to the amount of energy that is reflected by the
surface located under that pixel. The number values range from 0
(very low reflectivity) to 255 (very high reflectivity).