16. Fixing Problems with the Image.
OK, so you've got your X configuration numbers. You put them in
Xconfig with a test mode label. You fire up X, hot-key to the new
mode, ... and the image doesn't look right. What do you do? Here's a
list of common video image distortions and how to fix them.
(Fixing these minor distortions is where
xvidtune(1) really shines.)
You move the image by changing the sync
pulse timing. You scale it by changing the frame
length (you need to move the sync pulse to keep it in the same
relative position, otherwise scaling will move the image as well).
Here are some more specific recipes:
The horizontal and vertical positions are independent. That is,
moving the image horizontally doesn't affect placement vertically, or
vice-versa. However, the same is not quite true of scaling. While
changing the horizontal size does nothing to the vertical size or vice
versa, the total change in both may be limited. In particular, if
your image is too large in both dimensions you will probably have to
go to a higher dot clock to fix it. Since this raises the usable
resolution, it is seldom a problem!
16.1. The image is displaced to the left or right
To fix this, move the horizontal sync pulse. That is, increment or
decrement (by a multiple of 8) the middle two numbers of the
horizontal timing section that define the leading and trailing edge of
the horizontal sync pulse.
If the image is shifted left (right border too large, you want
to move the image to the right) decrement the numbers. If the image
is shifted right (left border too large, you want it to move left)
increment the sync pulse.
16.2. The image is displaced up or down
To fix this, move the vertical sync pulse. That is, increment
or decrement the middle two numbers of the vertical timing section
that define the leading and trailing edge of the vertical sync pulse.
If the image is shifted up (lower border too large, you want to
move the image down) decrement the numbers. If the image is shifted
down (top border too large, you want it to move up) increment the
numbers.
16.3. The image is too large both horizontally and vertically
Switch to a higher card clock speed. If you have multiple modes
in your clock file, possibly a lower-speed one is being activated by
mistake.
16.4. The image is too wide (too narrow) horizontally
To fix this, increase (decrease) the horizontal frame length.
That is, change the fourth number in the first timing section. To
avoid moving the image, also move the sync pulse (second and third
numbers) half as far, to keep it in the same relative position.
16.5. The image is too deep (too shallow) vertically
To fix this, increase (decrease) the vertical frame length.
That is, change the fourth number in the second timing section. To
avoid moving the image, also move the sync pulse (second and third
numbers) half as far, to keep it in the same relative position.
Any distortion that can't be handled by combining these
techniques is probably evidence of something more basically wrong,
like a calculation mistake or a faster dot clock than the monitor can
handle.
Finally, remember that increasing either frame length will decrease your
refresh rate, and vice-versa.
Occasionally you can fix minor distortions by fiddling with the
picture controls on your monitor. The disadvantage is that if you
take your controls too far off the neutral (factory) setting to fix
graphics-mode problems, you may end up with a wacky image in text
mode. It's better to get your modeline right.