Nambafa
Graphics tutorials
Downloads: TUT11.zip
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│ W E L C O M E │
│ To the VGA Trainer Program │ │
│ By │ │
│ DENTHOR of ASPHYXIA │ │ │
╘═══════════════════════════════╛ │ │
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--==[ PART 11 ]==--
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■ Contents
- Introduction
- What is a "Crossfade"?
- How does a crossfade work?
- Problems with crossfades
- In closing
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■ Introduction
Hello again everybody!
The reason _this_ one is delayed (every single trainer has been so far ;))
is mainly due to a birthday (my 19th), and numerous tests at the
university (ugh!). But anyway, here it is. The sample program this time
is on cross-fading. The reason for this is that many people have
commented that I should be moving over to a few basic demo effects now
that we have most of the basics of VGA programming. I was also thinking
of either doing sound in a future version of this trainer, or starting a
separate "ASPHYXIA Sound Tutorial" series. Comments?
One major difference between this trainer and previous ones is that I am
including binary files (pictures in this case). This means that it will
not be available in the message bases of selected boards anymore, and it
must be obtained from the file base. Notice will however be given of
it's existence in the message base.
Asphyxia has formalised things a bit, and we now have a few official
distribution sites for all our demos and trainers. If you would like
your BBS to become a distribution site, please email me at
smith9@batis.bis.und.ac.za and I will send you the necessary info.
If you would like to contact me, or the team, there are many ways you
can do it : 1) Write a message to Grant Smith/Denthor/Asphyxia in private mail
on the ASPHYXIA BBS.
2) Write to Denthor, EzE, Goth, Fubar or Nobody on Connectix.
3) Write to : Grant Smith
P.O.Box 270 Kloof
3640
Natal
South Africa
4) Call me (Grant Smith) at (031) 73 2129 (leave a message if you
call during varsity). Call +27-31-73-2129 if you call
from outside South Africa. (It's YOUR phone bill ;-))
5) Write to smith9@batis.bis.und.ac.za in E-Mail.
NB : If you are a representative of a company or BBS, and want ASPHYXIA
to do you a demo, leave mail to me; we can discuss it.
NNB : If you have done/attempted a demo, SEND IT TO ME! We are feeling
quite lonely and want to meet/help out/exchange code with other demo
groups. What do you have to lose? Leave a message here and we can work
out how to transfer it. We really want to hear from you!
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■ What is a "Crossfade"?
This is a simple question. When you are watching a TV program, you will
often see one picture on the screen, which slowly fades to a new
picture, with the new picture becoming more and more prominent and the
old one becoming less and less prominent. This is a crossfade. Easy huh?
Perhaps, but it is not that easy to code on a computer...
In most demos, there is a crossfade of two colors, black and white, for
example : The words 'MYDEMOTEAM' appears in large with letters, then
crossfades to 'PRESENTS' in large white letters.
I decided to allow the programmer to have a bit of color to his
crossfade, and the sample program can handle a many color crossfade.
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■ How does a crossfade work?
Here comes the clever bit.
To do a crossfade, we load in two pictures, FROM and TO. Then, for every
pixel in which they are different, put a new pixel in a third screen.
For example, wherever there is pixel color 9 on screen 1 and pixel color
45 on screen 2, put pixel color 1 on the third screen. You then repeat
this for all combinations of pixels on screen one and two, and put the
results into screen 3. Here it is in ascii ...
Screen 1 Screen 2 Screen 3
.1... .3... .1...
..... ..2.. ..2..
...8. + ...1. = ...3.
.1... ....2 .4..2
Note how the values on screen 3 are sequential? We keep a count for
this... The two "2"'s on screen 3 are identical, so we do not use a new
color for it...
We also keep to pallettes ... source and dest.
For the above example source[1] would be the pallette of 1 in screen 1,
and dest[1] would be the pallette of 3 in screen 2 (Note that screen 1
and screen 2 have different pallettes)
When we are finished with the picture, we flip screen 3 to the vga and
do the following : move the pallette from source to dest or vice versa.
Thats it. No fancy screen manipulations for the crossfade, we just
change the pallette. Cool, huh? It also means that you can be doing fun
stuff in the foreground with unused pallette colors without your program
executing at two frames per second ;)
The sample program is fully documented, and you shouldn't have a problem
deciphering it... If you ever use this effect in a demo or game, greet
me! :-)
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■ Problems with crossfades
The main problem with crossfading is this : there may only be 256 colors
on screen 3, in other words, only 256 combinations of colors. In the
sample program, if you load up two pics with more then 256 combinations,
the program gives an error message and exits to dos. To sort this
problem out, you can do two things : reduce the number of places where
the two pictures intersect, or squeeze down the pallette, using
Autodesk Animators "SQUEEZE" command. This reduces the number of colors
used by the picture, and therefore reduces the number of combinations.
The picture does however lose a bit of quality.
The second problem with crossfading is this : It hogs most of the
colors. Whatever you want to do in the foreground, make sure you do it
with as few colors as possible.
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■ In closing
So, what do you think? Should I continue with demo effects, or should I
find more basic things to do? Perhaps I should stop and just do sound
coding from now on? It is up to you, so leave me mail.
You will notice that the sample program can load in any .CEL files, you
needn't be restricted by the ones I have given you. Try crossfading a
few of your own pictures and see how it turns out. The picture of the
robot was draw by Fubar for our demo Psycho Neurosis, and I then
squeezed down the pallette somewhat in order for the crossfade to work.
The word "ASPHYXIA" was drawn by me, also in Autodesk Animator.
Oh well, I had better get this off quickly, today is the last time for
the next few days that I can get on to the Net. I will also be voting
tomorrow! If I see a CNN camera, I'll wave (Thats me, the one on the
left in the red shirt! ;-)) The next trainer will be coming from the
New South Africa (TM)
See you next time!
- Denthor
- 9:16, 26 April, 1994
PS. Does anyone in Holland with net access want to act as a courier
between myself and the Accidental Connection BBS? Please leave me mail
at smith9@batis.bis.und.ac.za ....
The following are official ASPHYXIA distribution sites :
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║BBS Name ║Telephone No. ║Open ║
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║ASPHYXIA BBS #1 ║+27-31-765-5312 ║ALL ║
║ASPHYXIA BBS #2 ║+27-31-765-6293 ║ALL ║
║Connectix BBS ║+27-31-266-9992 ║ALL ║
║POP! ║+27-12-661-1257 ║ALL ║
║Pure Surf BBS ║+27-31-561-5943 ║A/H ║
║Wasted Image ║407-838-4525 ║ALL ║
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Leave me mail if you want to become an official Asphyxia BBS
distribution site.
(I will find out the country code for Wasted Image later...)
Downloads: TUT11.zip