Description

This repository contains raytracer programs written in Turbo Pascal for MS-DOS.

1. Text mode (default is 80x25)

2. Graphics mode (640x480)

Scene parameters are the following:

• Rays start at (0, 0.23, -2)
• Sphere of radius 1 is placed at the origin
• Floor plane is at Y=-2

3. More complex raytracer with two sphere bjects and multiple reflections

The algorithm is based on the BASIC programs for BBC Micro by Steve McCrea.

Text mode raytracer

Full source code: RTTXT.PAS

At the beginning get console mode dimensions. Usually, they are 80x25 but why not support other options.

  MaxX := Lo(WindMax);
  MaxY := Hi(WindMax);

Set parameters of rays' origin at (0, 0.23, -2):

  H := 0.23;
  J := H * H;

Loops enumerate all character position aka 'pixels':

  for V := 0 TO MaxY do
    for U := 0 to MaxX do

Convert screen coordinates to bounds [-1..1, -0.8..0.6]:

      X := (U - 40) / 40;
      Y := 0.6 - 1.4 * V / 25;

Calculate ray’s direction (X,Y,Z). Here the perspective is provided by calculation of Z with inverset square root function Z=1/Sqrt(L+1). Then Normalize the rest of vector's components X and Y:

      Z := 1 / Sqrt(X * X + Y * Y + 1);
      X := X * Z;
      Y := Y * Z;

Check intersection of a sphere with the ray. Parameter B is a flag for choosing fileal colour later:

      D := 4*Z*Z + J*Y*Y - (3+J);
      B := 0;

If the ray intersects with the sphere occurs, reflect the ray from the sphere""

      if D>0 then
      begin
        T := 2 * Z - H * Y - Sqrt(D);
        X := T * X;
        Y := T * Y + H;
        Z := T * Z - 2;
        B := 4;
      end;

If the ray is headed down (Y<0), intersect it with the floor plane at Y=-2. Choose tile colour using (Int(X)+Int(Z)) And 1. If the ray hit the sphere add B to the colour for switching to reflection color:

      if Y<0 then
      begin
        P := (Y + 2) / Y;
        C := (Round(Int(X * P) + Int(Z * P)) And 1) + B;
      end

If the ray is headed up (Y>=0), set sky color (11, Cyan)

        C := 11;

If the coordinate is near (1,1,-1) and the ray bounced off the sphere (D>0), randomly choose white (15) colour for dithering. Note that two checks and not united into one condition because otherwise Turbo Pascal will handle Random result as Integer and the code won't compile.

        if D>0 then
          if Random>(1-X-Y+Z) then
            C := 15;

Put current 'pixel' to position U,V. Since enumeration is sequential, just put whitespace character of specific color C.

      TextBackground(C);
      Write(' ');

Graphics mode raytracer

Full source code: RTVGA.PAS

This program is the same as text mode version with an addition of VGA initialization.

One particular note is that the constant BgiPath must specify the directory path where the graphics drivers can be found. For current value the friver files must be in the current directory. Driver files (*.BGI and *.CHR) usually can be found inside the BGI folder of the Turbo Pascal installation.

const
  BgiPath = '.';

More complex raytracer with a pair of spheres

Full source code: RTPAIR.PAS

This program displays a scene with two spheres and simulates multiple reflections of a ray before it reaches the ground or floor. This creates complex patterns on the surfaces of the spheres.

The following new code is added to the previous VGA graphics mode program. I is the horizontal direction of ray based on whether it is in left (U<0) or right (U>0) half of the screen.

      I := Sign(U);
      G := 1;

Start the reflection cycle. A ray may reflect between one sphere and another multiple times before hitting floor or sky.

      repeat
        stopReflection := True;
		...
      until stopReflection;

Check condition if a ray reflects from a sphere one more time.

        if D>0 then
        begin
          T := -P - Sqrt(D);
          if T>0 then
          begin
		    ...
		  end;
		end;

Invert ray's direction and continue the reflection cycle.

            I := -I;
            stopReflection := False;

If Y<0 (V<0) rays hit the floor. Display checkers floor with Black (0) and Red (4) tiles with the help of grid formula (1 And (Round(X - U*P) + Round(Z + W*P))):

      if V<0 then
      begin
        P := (Y+2)/V;
        C := (1 And (Round(X - U*P) + Round(Z + W*P))) * 4;
	  end

If Y>0 (V>0) rays hit the sky, Default sky color is Cyan (11). Additional condition (V<R) is added to create fancy Cyan-White horizon adjusted by the curve 0.2+0.1*Cos(3*R)*Abs(Sin(5*R)):

        C := 11;
        R := ArcTan(U/W);
        R := 0.2+0.1*Cos(3*R)*Abs(Sin(5*R));
        if Abs(G)<0.35 then
          R := R + 1;
        if V<R then
          C := 15;

Links

• Tweets by Steve McCrea:
  • Text mode raytracer
  • Graphics mode raytracer
  • Annotated graphics raytracer
  • Pair of spheres raytracer