SuperHack/SuperHack CD.bin/CODING/DEMOS/OTEFFEX1.ZIP/EFFECT#1.DOC

Text File | 1996-09-20 | 8.1 KB | 165 lines

                     - THE OUTLAW TRIAD DEMO-SERIES -

───────────────────────────────■ PART XI ■────────────────────────────────────

                         Written by : Vulture/OT
                         Code in    : Pascal
                         Topics     : 3d glenzing, flatshading

──────────────────────────────■ 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.
 Right, we will continue our discussions on 3d graphics. This time we'll be
 talking about some relatively simple concepts. 3d glenzing and flatshading!
 These effects are pretty easy to implement if you know your 3d. This doc is
 an add-on to part 10 in which we created a basic 3d engine. Ya really should
 read that text first before reading this. Otherwise you might get a little
 confused. Or maybe not... or maybe you will... naah, enough said... Enjoy!

─────────────────────────────────■ Theory ■───────────────────────────────────

 - GLENZING -

 First of all, let me explain what glenzing means. To put it simply, imagine
 a cube made out of colored glass. Whenever you look through a side, you see
 the sides lying behind it. But... in a slightly different color than that
 side really has. Why? Obviously, because the color of the side which you are
 looking through effects the color of the sides behind it.

 Ok, with that clear, how do we code this stuff? It's real easy! Here we go:

 Most important, in the previous tutorial (on face sorting) we sorted all our
 polygons on their average z value. We don't do this when showing a glenzing
 3d object because it isn't necessary! It doesn't matter in which order you
 draw the polygons. Ok, as you probably know, a polygon is drawn by drawing
 each horizontal scanline it is consists of. In the previous tutorial (#10)
 we just used one constant color for each polygon. This time, while drawing,
 we don't just put one color of the polygon on the screen but we'll also use
 the background color of the pixel of the current scanline we are plotting.
 So, the algorithm for drawing one pixel of one scanline looks like:

    - grab background color
    - add polygon color
    - plot pixel on the screen
    - point to next vga spot

 All you have to do is change a few lines of code in your horizontal line
 routine and you're done. Of course this is quite a bit slower than just
 plotting a row of pixels of one constant color.

 Ok, here's a part of the horizontal line routine. Examine it. Shouldn't
 be too hard to understand. Of course you have to calculate the exact vga
 position first before using this code. Check the complete source code for
 the total routine.

      mov     cx,x2
      sub     cx,x1                         { Calculate # of bytes to draw }
      cmp     cx,0
      jle     @No_Draw                      { Quit if no pixels to draw }
   @Draw_Loop:
      mov     al,[byte ptr es:di]           { Grab background color }
      add     al,Color                      { Add color }
      stosb                                 { And store the byte }
      loop    @Draw_Loop
   @No_Draw:

 There you go! Easy, eh? First check to see if pixels have to be drawn and
 if so, get the backgroundcolor, add the polygon color and store the byte!
 Do this for all pixels in the horizontal line. That's all! The source code
 is simply the 3d system used in the previous trainer so if you got that one
 already, this should be a breeze.

 - FLATSHADING -

 Ok, next step in 3d is of course shading of the objects. There are many
 different ways of shading, some of which are very complicated and difficult
 to understand. We will start with the most simple and basic shading method
 around and that's flatshading (also called Lambart shading). What's that?

 Imagize a cube rotating in front of the sun. When a face turns towards the
 sun, it will get a brighter color. And when the face turns away from the sun,
 it will get a darker color. Important: the entire face will get another color
 using this method. This is called flatshading and is of course more realistic
 than just having faces with one constant color. The light is coming from the
 viewers side in this example (and in most demos anyway).

 How do we code this stuff... Well, there are different ways of implementing
 this effect and I'm going to show the easiest one. We simply use the average
 z-value of the polygon to determine the color of this polygon. If you read
 part 10 of these series, you know we need to determine the drawing order of
 the polygons. This is done by adding the z-values of the 4 3d points of the
 polygons. The resulting values are then sorted and the object is drawn from
 back to front. For a complete explanation, go and read part 10. Anyway, the
 z-values we sorted can also be used to determine the color of the polygons.
 Again, the values in the list are the 4 z-values of each polygon added. The
 resulting values are then sorted from small to large. Now, while drawing,
 grab the z-value of the poly (the absolute z-value) from the list and divide
 this by a certain number to get it into the color range you want (I divided
 the value by 8). This is the color of the face. So, for each frame:

    - Rotate polygons
    - Store added z-values
    - Sort polygons on these z-values
    - While drawing:
      - Get absolute z-value from list
      - Divide it by color factor
      - Draw the polygon with resulting color

 For example: if I reserved colors 0 - 18 for the shading and the z-value can
 be 280 or less (the 4 z-values of the polygon added), the dividing factor
 should be around 16. Because 280/16=17.5 and that fits into the color range
 we wanted. Before this, you should have setup a nice palette for the shading.
 In the source I used a gray-scale palette. And of course, the more colors you
 use, the more realistic the shading will be.

 This method is very easy to implement but it's also the most unrealistic one.
 It's good enough for cubes and other simple objects but for more complicated
 objects you'll need a better method of calculating the color of the polygon.
 Flatshading can also be realised by using face-normals and other complicated
 vector calculations. This method will result in better shading. Hmm, yeah, so
 you would like to know about that too, rite? Be prepared for future trainers
 from OT including textfiles on improved flatshading, gouraud, texture, phong,
 z-buffering and a lot more!

 For questions, chatting or for suggestions for future trainers, mail me.
 Ya know where to find me... I'm always in for a chat so don't hesitate to
 contact me (or any other member of OT).

 That's all for now. Happy coding!

     -Vulture/Outlaw Triad-

───────────────────────────────■ Distro Sites ■───────────────────────────────

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

  BlueNose      World HQ          [phone]
  The Force     Distrosite        [phone]
  Bugs'R'Us     Distrosite        [phone]    More distros wanted!
  The 7 Angels  Distrosite        [phone]    (preferably outside
  ShockWave     South African HQ  [phone]     of the Netherlands)
  Society HQ    United States HQ  +1  [phone]
  ACe World     Brazilian HQ      [phone]
  Corps Elite   Canadian HQ      +403 (ITS)-PRIVATE

 Also check the major FTP/WWW 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)   [e-mail]

 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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