Jagged Lines

This is not actually a good way to draw things. Usually you don't want to change the color of pixels you have already painted, except for the pixels at the corners of figures that have been painted in several colors.

Say you wrote a program to draw a line from (x=0, y=3) to (x=11, y=6):

' Diagonal Line
'
SCREEN 12
COLOR 4                ' Pen color 4 (red)
LINE (0, 3) - (11, 6)   
END

The program will fill in the start and end pixels with red and fill in the pixels in-between. But the line will not be smooth because sometimes there will be abrupt changes between one red pixel and the next:

You can't get a smooth line because a pixel is like a square of graph paper that is either completely filled with one color or completely filled with another.

With less expensive computer graphics, diagonal lines can be very jagged. Sometimes in newspapers and magazines you can tell when a picture has come out of a computer because of its rough, grainy appearance. Here are some magnified jaggy lines on part of a graphics screen:

With expensive computers, the squares of the graph paper of the graphics screen are so small that they are hard to notice.