Mail Archives: djgpp/1997/10/13/00:08:01
Thomas Charron wrote:
> ... supposed to merely slowly rotate a box,
> just so I could see it.. Only one problem, it's definitely NOT doing
> THAT.. It seems to be moving at an angle, changing it's size, getting
> thinner on the x, and longer on the Y.. Here's a copy of the source,
> and I've attached the exe so you can see..
First of all, I'll also start by scolding you about posting a binary
on a non-binary newsgroup. But I won't go balistic about it either.
Second, what graphic mode are you running in? The VGA is capable
of many non-square pixel aspect ratio modes, and that can definately
account for a lot of weird problems like "thinner on the x, and longer
on the Y...".
Third, you also realize that this square you have defined is definately
not rotating about *its* center, it's rotating about the screen 0,0
location, which is probably in the upper or lower left corner of
your screen. Now, put your finger in the middle of your screen,
and rotate it around the corner of your screen, keeping the same
distance from that corner, like a compass with the pointy part at
the corner of the screen. Looks mostly like it's moving across or
down, because your only seeing a part of that rotation.
Fourth, you cannot cheap out by using only two x values and two y
values...you need 4 distinct pairs of points. If you only rotate
the box around it's center, you can cheap out with 2 pairs, but
that's more work and so much less flexible and so unexpandable that
it's not worth even trying to do it that way.
The very next thing you should do is find the graphics faq, which
must be lying around somewhere...I don't know where, however.
But it has the answers to many questions for beginner to intermediate
graphics programming...maybe more these days.
Then the next thing you should get accustomed to doing is using the
old transformation matrix. There are speedier ways to do things, but
it's the most general and most common way to represent a *series*
of transformations.
If you wanted that box to rotate around it's center, for example, you
would translate the box to the "world" center, (0,0), then rotate it,
then translate it back. Matrices are a wonderful thing, and by
compounding a series of transformation allow you to do wonderful
things. For example, if you wanted to both rotate around zero and
around it's center, you could compound the rotation - tranlation -
rotation - translation into one matrix. I can't express enough
how important it is to find a book that gives this basic overview.
Another thing is to distinguish between "world" and "screen" space.
That'll be described in any book.
One last thing...I know you say you only dabble in programming
"just hacks to do something real quick", but your programming
style is very freshmen, and leads to all sorts of programming
dilemas down the road. You should at least, for example, have
a "rotate" function so that that code isn't repeated. Then
having 4 pairs of points wouldn't be so bad, would it? Again,
it's not like you're trying to change careers, but one good
book would probably give you all the functions (at least in
psuedo-code) that you would need.
Good luck, have fun exploring the wonderful world of computer
graphics.
Fred
--
- Frederick Haab - Software Developer - Turner Production -
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