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PART 11 : Crossfade

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
                           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!

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.

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! :-)

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.

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 :

║BBS Name                  ║Telephone No.   ║Open ║
║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  ║

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…)

back2root/archives/denthor/part-11.txt · Dernière modification : 2021/09/02 12:01 de frater