Overview
Displaying pixels on a VGA display monitor is roughly the same
as displaying a standard TV screen from a broadcast station.
It works much the same way.
A small, narrow beam of electrons are propelled from a "gun"
at the back of the CRT towards the front face where a phosphor-based
material is uniformly deposited. The electrons are released from a
small filament of specially coated wire that is heated to help force
the electrons out of the wire and into the vacuum around it. They
are then propelled away from the wire and towards the screen by a high
positive voltage on back side of the CRT face.
If that's all there was, the electrons would simply stream
helter-skelter to the CRT face and there would only be a general
glow of light and no detailed image. However, there are some steering
elements to help guide the electrons into a narrow beam as well as
electron absorbers to pick up and remove electrons that aren't well
focused. By focusing some of the electrons and removing the
hard-to-focus remainder, a narrow beam results. By twiddling with
the steering voltages a bit, you can cause this narrow beam to move
about the front screen face kind of like an etch-a-sketch.
Now, color makes this just a bit more complicated. It's rather easy
to come up with various phosphor compounds that exhibit different
colors when they are struck by electrons. But once they are mixed,
their color doesn't really change much. So to get a color screen
three different color mixes are used, placing three tiny dots of
different phosphors on the CRT face in each small "pixel"
area. By carefully arranging them close together and in a fixed
relationship to each other so the electronics can pick out one from
another, a human will not see them as separate dots, but as a combined
color. This effect is enhanced by including a very black mask that
fills in the space between these color triads. They are colored red,
green, and blue. The color that your eye senses will be a complicated
formulation of the combined wavelengths and their relative intensities.
Image Details
A display monitor is operated to display an image by starting the beam
near the upper-lefthand corner of the screen and then to sweep it exactly
horizontally from the left side to the right, while modulating (varying)
the beam strength and position to light up the phosphor color dots as it
moves past them. It takes a little while to move the beam from one area
to another, so hitting all of the pixels in a line like this is efficient.
Trying to pick them out at random would mean all kinds of longer delays
and each screen full would take much longer, overall.
However, the beam does need to be turned as close to off as possible
when swinging it back to the left side of the screen to start on the
next line. I don't know the history as to why they didn't just choose
to sweep the next line backwards, but they just didn't design it that
way. So, this period of time required to swing the beam back to the
left side (by modifying the voltages on the beam steering elements)
while keeping the beam off is called the horizontal retrace period.
Since the beam isn't on and doesn't need to hit all the color dots on
the way back, it can be swung back into position much faster.
Actually, to bring the beam back to the left side of the screen, there
are two steps. The first step is to simply turn off the beam so that
more pixels can't be accidentally lit up. Then the voltages on the
steering elements are changed to start the return swing. Once the beam
has returned to the left side, the monitor will start scanning it back
in a rightward motion from the far left edge to the visible left edge
of the display, all the while still keeping the beam "blanked."
Once the beam has arrived at the visible left edge, it is turned on again
for the next display line.
So a horizontal line starts out at the left visible edge of the screen,
hitting each of the 3 color dots of a pixel with separate intensity values
for each color, advances to the next pixel and repeats this until the
right visible edge is reached. Then it starts the horizontal blanking while
preparing for the return swing to the left and begins the horizontal retrace
or swing back to some distance beyond the left visible edge. Once it gets
back past the left edge, it prepares for and begins to start sweeping back
to the right, thus ending the horizontal retrace period, but not the
horizontal blanking period until it reaches the left visible edge, when
the next scan line begins once again.
This line-by-line process continues until the entire image has been
scanned onto the CRT face. Once the entire image is complete, the
phosphor dots up near the top are already beginning to fade out from
last time, so the whole process is repeated once again by going back
up to the upper lefthand corner to start anew. Just like the horizontal
display method, there is a vertical blanking interval during which the
vertical retrace period occurs while the beam is brought back up to the
top left corner from the bottom right corner. Once the beam swings back
into position on the first line, the new image can start again.
TV sets use a method like this, called NTSC, which actually scans the odd
numbered horizontal lines first, then goes back and scans the even ones.
This is called "interlacing." Computer display monitors will
use this technique, too, when their speed just can't keep up with the pixels
if all the lines are scanned the first time. You can usually see this as a
kind of annoying flickering of the display, since the phosphor often manages
to dim down more by the time the beam gets back to restoring the image.
Summary
So that's the process that is used to fill out a display on the screen.
If you think a bit about it, you can see that changing the point at which
you start the horizontal retrace within the horizontal blanking period,
you can adjust where the left edge of the screen is at on the monitor. The
same is also true for the vertical timing signals in aligning the top and
bottom margins.
It's a pretty basic idea, but managing all of the little details in exactly
the right way is what makes the difference between success and failure.