OpenGL is a low-level graphics library specification originally developed by Silicon Graphics Inc. Your system's particular implementation of this specification, often called OpenGL driver, allows you to use a set of geometric primitives (points, lines, polygons, images, and so on) to describe the scene you wish to draw. Visualizing a scene of moderate complexity takes mere milliseconds, which means the library has sufficient performance to support you in the creation of animations and virtual worlds.

The OpenGL driver is generally provided as a library in binary format. It can be linked to your program dynamically. On the Windows platform, it will be a DLL (check for opengl.dll in your system directory). Since Delphi can use any DLL, it is as simple as with any other language to program OpenGL 3D graphics. This article will help you to get acquainted with OpenGL techniques in Delphi.

MATHEMATICAL FUNDAMENTALS

OpenGL is based on strong mathematical fundamentals so you are limited only by your imagination. Rather then getting into axioms and lemmas, let's just proceed with a simple 3D coordinate system, as used by most 3D programmers. Here it is:

This is how you should imagine the placement of your screen (the blue square) in your scene. The point from which the four rays extend to create the screen is your "point of view" in the imaginary world.OpenGL lets you define this situation with two simple function calls

glMatrixMode(GL_PROJECTION);
glFrustum(-0.1, 0.1, -0.1, 0.1, 0.3, 25.0);

In these calls, -0.1, 0.1, -0.1, 0.1 define the size of virtual screen in left, right, bottom, top sequence; 0.3 defines viewpoint-to-screen distance (same as near clipping plane) and 25.0 defines the far clipping plane. Anything in front of the near clipping plane or behind the far clipping plane will be invisible. Of course, you can play with these numbers to suit your needs for your scene dimensions.

FROM PRIMITIVES TO OBJECTS

Now comes the challenging part: the objects. OpenGL supports only the following geometrical primitives: points, lines and polygons. No surface of higher order (such as sphere) can be drawn as a primitive. But it can be perfectly approximated with polygons. Take a look at any modern 3D game and you'll find it is completely built from triangles. So we won't be limited by this restriction.

The object drawing is very similar to Pascal programming. Each block should be surrounded by a begin-end pair...or actually, a glBegin() and glEnd() pair. Take a look at this:

const S=1.0; D=5.0;
  ...
glBegin(GL_TRIANGLES);
  glVertex3f( -S, 0, D); glVertex3f(S, 0, D); glVertex3f(0, S, D);
glEnd;

It's a simple triangle. It is five units away from your viewpoint; it is one unit tall and two units wide.

Here is a screenshot:

A few words should be said prior to letting you dig into the code. Every OpenGL program contains some OS-specific code that initializes the output device. If you use Win32, you'll need to set up the pixel format and create a rendering context out of the windows device context. If you are not so good in system-level Windows programming, just use following program as a template. For more info read the help on each function called in FormCreate.

FILE: Tri.pas

unit Tri;

interface

uses
  OpenGL, Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, ExtCtrls, ComCtrls;

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
    procedure FormPaint(Sender: TObject);
  private
    procedure Draw; //Draws an OpenGL scene on request
  public
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure setupPixelFormat(DC:HDC);
const
  pfd:TPIXELFORMATDESCRIPTOR = (
      nSize:sizeof(TPIXELFORMATDESCRIPTOR); // size
      nVersion:1;                       // version
      dwFlags:PFD_SUPPORT_OPENGL or PFD_DRAW_TO_WINDOW or
      PFD_DOUBLEBUFFER;                 // support double-buffering
      iPixelType:PFD_TYPE_RGBA;         // color type
      cColorBits:24;                    // preferred color depth
      cRedBits:0; cRedShift:0;          // color bits (ignored)
      cGreenBits:0; cGreenShift:0;
      cBlueBits:0; cBlueShift:0;
      cAlphaBits:0; cAlphaShift:0;      // no alpha buffer
      cAccumBits: 0;
      cAccumRedBits: 0;                 // no accumulation buffer,
      cAccumGreenBits: 0;               // accum bits (ignored)
      cAccumBlueBits: 0;
      cAccumAlphaBits: 0;
      cDepthBits:16;                    // depth buffer
      cStencilBits:0;                   // no stencil buffer
      cAuxBuffers:0;                    // no auxiliary buffers
      iLayerType:PFD_MAIN_PLANE;        // main layer
    bReserved: 0;
    dwLayerMask: 0;
    dwVisibleMask: 0;
    dwDamageMask: 0;                    // no layer, visible, damage masks
    );
var pixelFormat:integer;
begin
    pixelFormat := ChoosePixelFormat(DC, @pfd);
    if (pixelFormat = 0) then
        exit;
    if (SetPixelFormat(DC, pixelFormat, @pfd) <> TRUE) then
        exit;
end;

procedure GLInit;
begin
    // set viewing projection
    glMatrixMode(GL_PROJECTION);
    glFrustum(-0.1, 0.1, -0.1, 0.1, 0.3, 25.0);
    // position viewer
    glMatrixMode(GL_MODELVIEW);
    glEnable(GL_DEPTH_TEST);
end;

procedure TForm1.FormCreate(Sender: TObject);
var DC:HDC;
    RC:HGLRC;
    i:integer;
begin
    DC:=GetDC(Handle);               //Actually, you can use any windowed control here
    SetupPixelFormat(DC);
    RC:=wglCreateContext(DC);        //makes OpenGL window out of DC
    wglMakeCurrent(DC, RC);          //makes OpenGL window active
    GLInit; //initialize OpenGL
end;

procedure TForm1.Draw;
const S=1.0; D=5.0;
begin
    glClear(GL_COLOR_BUFFER_BIT or GL_DEPTH_BUFFER_BIT);
    glLoadIdentity;
    glTranslatef(0.0, 0.0, -12.0);
    glBegin(GL_TRIANGLES);
    glVertex3f( -S, 0, D); glVertex3f(S, 0, D); glVertex3f(0, S, D);
    glEnd;
    SwapBuffers(wglGetCurrentDC);
 end;

procedure TForm1.FormPaint(Sender: TObject);
begin
    Draw;
end;

end.

FILE: Tri.dfm

object Form1: TForm1
    BorderStyle = bsDialog
    Caption = 'BASIC OpenGL Program'
    ClientHeight = 318
    ClientWidth = 373
    OnCreate = FormCreate
    OnPaint = FormPaint
end

ADVENTURES IN THE THIRD DIMENSION

OK, lets do real 3D now. Using the previous program as a skeleton, we add a few lines of code to create a flat shaded tetrahedron. How can we build it out of available primitives? We will use four triangles. One for the base and three on the sides. Here is the code that produces it:

procedure TForm1.Draw;
const D=1.5;
    H1=D/1.732;
    H2=D*1.732-H1; // D/H = tg(30) = 1/sqrt(3)
    HY=3.0;
const //vertexes
    a1:TGLArrayf3=(-D, 0, -H1);   //bootom left
    a2:TGLArrayf3=( D, 0, -H1);   //bootom right
    a3:TGLArrayf3=( 0, 0, H2);    //bootom back
    a4:TGLArrayf3=( 0, HY, 0);    //top
begin
    glClear(GL_COLOR_BUFFER_BIT or GL_DEPTH_BUFFER_BIT);
        glLoadIdentity;
        glTranslatef(0.0, 0.0, -12.0);
        glBegin(GL_TRIANGLES);
        glVertex3fv(@a1); glVertex3fv(@a3); glVertex3fv(@a2);
        glVertex3fv(@a1); glVertex3fv(@a2); glVertex3fv(@a4);
        glVertex3fv(@a2); glVertex3fv(@a3); glVertex3fv(@a4);
        glVertex3fv(@a3); glVertex3fv(@a1); glVertex3fv(@a4);
    glEnd;
    SwapBuffers(wglGetCurrentDC);
end;

Even if it looks a little complex, it is easier to understand when you have a picture in front of you.

Just replacing TForm1.Draw() in Tri.pas will not make a lot of changes. It still does not look three-dimensional. That's because we have not defined any lights yet.

LIGHTS! CAMERA! OPENGL!

The lighting model in OpenGL has two parts: the lights themselves (color, intensity etc.) and materials of the bodies. Material, in turn, contains colors, some physical parameters (like opacity or glossiness) and textures. It is a huge world to play with, we'll go step by step.

Defining a light source is rather easy.

procedure GLInit;
const
    light0_position:TGLArrayf4=( -8.0, 8.0, -16.0, 0.0);
    ambient: TGLArrayf4=( 0.3, 0.3, 0.3, 0.3);
begin
    // set viewing projection
    glMatrixMode(GL_PROJECTION);
    glFrustum(-0.1, 0.1, -0.1, 0.1, 0.3, 25.0);
    // position viewer */
    glMatrixMode(GL_MODELVIEW);
    glEnable(GL_DEPTH_TEST);

    // set lights
    glEnable(GL_LIGHTING);
    glLightfv(GL_LIGHT0, GL_POSITION, @light0_position);
    glLightfv(GL_LIGHT0, GL_AMBIENT, @ambient);
    glEnable(GL_LIGHT0);
end;

Two constants were needed. One defines the light position (to the left, top, back from viewer) and another defines ambient lighting. It produces some small amount of scattered light, which allows you to see something even if it is completely in shadow.

Although you enabled lighting and specified a light source, the body is still not drawn shaded. This is because OpenGL needs to know "normal" to each polygon you specify to do lighting math. (Normal is a vector perpendicular to the surface.) If you have no library of vector functions of your own, just use the following method to calculate a normal having 3 points of triangle. This function needs them to be specified counterclockwise, because the normal is produced as a cross product of the vectors and it will point inside the tetrahedron if you don't obey this rule.

function getNormal(p1,p2,p3:TGLArrayf3):TGLArrayf3;
var a,b:TGLArrayf3;
begin
    //make two vectors
    a[0]:=p2[0]-p1[0]; a[1]:=p2[1]-p1[1]; a[2]:=p2[2]-p1[2];
    b[0]:=p3[0]-p1[0]; b[1]:=p3[1]-p1[1]; b[2]:=p3[2]-p1[2];
    //calculate cross-product
    result[0]:=a[1]*b[2]-a[2]*b[1];
    result[1]:=a[2]*b[0]-a[0]*b[2];
    result[2]:=a[0]*b[1]-a[1]*b[0];
end;

Using this function, you can now specify all the information needed to calculate the lighting:

procedure TForm1.Draw;
const  D=1.5;
       H1=D/1.732;
       H2=D*1.732-H1; // D/H = tg(30) = 1/sqrt(3)
       HY=3.0;
const //vertexes
       a1:TGLArrayf3=(-D, 0, -H1);
       a2:TGLArrayf3=(D, 0, -H1);
       a3:TGLArrayf3=(0, 0, H2);
       a4:TGLArrayf3=(0, HY, 0);
var n1, n2, n3, n4: TGLArrayf3; //normals
begin
    n1 := getNormal(a1,a3,a2);
    n2 := getNormal(a1,a2,a4);
    n3 := getNormal(a2,a3,a4);
    n4 := getNormal(a3,a1,a4);
    glClear(GL_COLOR_BUFFER_BIT or GL_DEPTH_BUFFER_BIT);
    glEnable(GL_NORMALIZE);
    glShadeModel(GL_FLAT);
    glCullFace(GL_BACK);
    glLoadIdentity;
    glTranslatef(0.0, 0.0, -12.0);
    glBegin(GL_TRIANGLES);
    glNormal3fv(@n1);
    glVertex3fv(@a1); glVertex3fv(@a2); glVertex3fv(@a3);
    glNormal3fv(@n2);
    glVertex3fv(@a1); glVertex3fv(@a2); glVertex3fv(@a4);
    glNormal3fv(@n3);
    glVertex3fv(@a2); glVertex3fv(@a3); glVertex3fv(@a4);
    glNormal3fv(@n4);
    glVertex3fv(@a3); glVertex3fv(@a1); glVertex3fv(@a4);
    glEnd;
    SwapBuffers(wglGetCurrentDC);
end;

That's it. The code produces a view like this:



And now let us use something offered by Delphi VCL. Put the timer on your window, specify class member "angle:single" and increment it by 1.0 each time the timer ticks:

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  angle:=angle+1.0;
  Draw;
end;

You are just one line away from making an OpenGL animation:

glRotatef(angle, 0.0, 1.0, 0.0);

Put this line just before you start your triangles in a glBegin() and your rotating shaded-body application is finished.

If you like this stuff, you can play a little with this application. You can download it from CodeCentral: Step1, 2.6K Try to understand how it works and why you use this code or that. If you want to go further, there are plenty of books and tutorials. Check out www.opengl.org if you haven't done so yet.