- THE OUTLAW TRIAD DEMO-SERIES -

----------------------- PART VII -----------------------------------

                Written by : Vulture/OT
                Code in    : Assembler
                Topic      : Plasmas

                

--------------------- Introduction ---------------------------------

 Welcome to the Outlaw Triad demo-series! In these series we will be
 talking about programming demo-effects in either pascal or assembler.
 Theory behind the effects shall be discussed while a full sourcecode
 is also provided.
 This time a VERY wellknown type of effect is discussed and that is
 plasmas! Plasmas are great for background effects. For example, when
 displaying some text, you could run a plasma screen in the background.
 Let's see how plasma screens are made. Enjoy!

------------------------ Theory ------------------------------------

 Note: I know plasmas are very old and shouldn't be used too much
 nowadays but I think plasmas are great for learning purposes.
 That's the reason why I decided to put a plasma trainer into these
 series anyway.

 Let's start by mentioning the diverence between 2 sorts of plasmas.
 Static plasma and realtime plasma. The first variant simply creates
 a screen on which pixels fluently flow into other colors. Then the
 palette is rotated.
 Realtime plasma uses cosine values to create magnificient curves on
 the vga thus being much more fun to watch than static plasma, IMHO.
 But, realtime plasma is more time consuming than static plasma
 because each pixel on the screen has to be refreshed each frame!

 To create a plasma screen you should use an unchained tweaked vga
 mode. Because every pixel on the screen is affected each frame,
 it is usually best to reprogram the vga to a mode in which you have
 large pixels, let's say 4x4. This will result in mode 80x50 so we
 only have to process 80*50= 4000 pixels each frame. Also create a
 good palette. The colors should run smoothly from one to the other.

 To create realtime plasma, we need cosine values. Why? Because cosine
 values represent a nice curve and that's what we want, nice curves.
 Take a look at the source to see a good cosine chart. Oh, that chart
 was generated by Jare/VangelisTeam (always give credit where credit
 is due), but it's pretty easy to generate your own charts. The values
 in this example range from 0 to 64 but of course you can go and
 experiment with other values/ranges. The range in this example is
 pretty clever. Think about it! When you add 4 cosine values in range
 0..64, the maximum value you can get is 4*64=256. And that's the
 exact amount of colors we have to our disposal in plain mode 13h.
 In this example I only used 128 colors. The cosine values go up to 64
 so I had to implement code to handle additions above 128.

 Anyway, now we have setup our vga and created a good cosine chart.
 Let's go and code the plasma. We want complex curves on the screen.
 To create plasma screens, we simply add 4 cosine values and put the
 resulting value on the vga. We use 2 cosine values for vertical
 movement and 2 for the horizontal movement. For each frame we use 4
 pointers to the cosine chart. For each horizontal pixel, we update
 our 2 horizontal cosine pointers to point to another cosine value in
 the chart! For each horizontal line we go down we update our
 2 vertical pointers. When updating the pointers, don't save the new
 values to the pointer variables but into temporary variables
 (in asm I just used some registers). We update the original pointers
 when we've done the entire screen (frame). Complex? Observe this:

  - Set vertical cosine start values (save into temp variables)
  - Go into vertical loop (50 times in this example)
     - Set horizontal cosine start values (save into temp variables)
     - Go into horizontal loop (80 times in this example)
        - Add 4 temporary cosine values
        - Put result on vga
        - Update the temporary horizontal cosine values
     - End horizontal loop
     - Update the temporary vertical cosine values
  - End vertical loop
  - Update the original cosine start values
  - Back to start

 Just take a look at the source to see what I mean. Go and change some
 of the values and see what happens. Also, try to implement this in
 hi-resolutions to see what you get. Allthough it's a rather old
 effect, it still is good enough to put in demos when you use it
 clever enough. I know this text is a bit cryptic but once you've
 experimented with the source, all will become clear. Trust me ...
 I know ... :-)

 Ok, this is all for now. Happy coding!

     -Vulture/Outlaw Triad-

---------------------- Distro Sites --------------------------------

 Call our distrobution sites! All our releases are available at:

 BlueNose    World HQ          +31 (0)345-619401
 The Force   Distrosite        +31 (0)36-5346967 More distros wanted!
 Bugs'R'Us   Distrosite        +31 (0)252-686092 (preferably outside
 MagicWare   Italian HQ        +39  6-52355532    of The Netherlands)
 ShockWave   South African HQ  +27 (011)888-6345
 Society HQ  United States HQ  +1  (518)465-6721

 Also check the major FTP sites for Outlaw Triad productions.

------------------------- Contact ----------------------------------

 Want to contact Outlaw Triad for some reason? You can reach us at our
 distrosites in Holland. Or if you have e-mail access, mail us:

   Vulture   (coder/pr)   comma400@tem.nhl.nl

 Our internet homepage:

   http://www.tem.nhl.nl/~comma400/vulture.html

 These internet adresses should be valid at least till june 1997.

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      Quote: It is dangerous to be sincere unless you are also stupid.