Sample OpenGL Program in C or C++

By RoD
In this lesson I shall introduce several functions and show you actual OpenGL rendering in a program. Prior to showing you the code, however, I want to go over a few things with you. This will give you a better understanding of what is going on when you do see the code, so you don't stare at the screen wondering what you're looking at. So, on with the show.



Transformations in OpenGL rely on the matrix for all mathematical computations. No, not the movie. Concentrate grasshopper. OpenGL has what is known as a matrix stack, which comes in handy for constructing models composed of many simple objects.

The modelview matrix defines the coordinate system that is being used to place and orient objects. It is a 4x4 matrix that is multiplied by vertices and transformations to create a new matrix that reflects the result of any transformations that have been applied to the vertices. When we want to modify the modelview matrix we use the command glMatrixMode(). We define this as
void glMatrixMode(GLenum mode);
Before you call any transformation commands you MUST specify whether you want to modify the modelview matrix or the projection matrix. The argument for modifying the modelview matrix is GL_MODELVIEW. So the complete line would appear as:
void glMatrixMode(GL_MODELVIEW);
Now we will look at translation. Translation allows you to move an object from one location to another within a 3D environment. The functions for this in OpenGL are glTranslatef() and glTranslated(). Here are their descriptions:
void glTranslatef(GLfloat x, GLfloat y, GLfloat z);
void glTranslated(GLdouble x, GLdouble y, GLdouble z);
Note that you must pass float types to glTranslatef() and double types to glTranslated(). X, Y, and Z represent the amount of translation on that axis.

Rotation in OpenGL is accomplished through the glRotate*() function, which is defined as
void  glRotatef(GLfloat angle, GLfloat x, GLfloat y, GLfloat z);
void glRotated(GLdouble angle, GLdouble x, GLdouble y, GLdouble z);
Now let's take a look at these, and a few others, functions mentioned in a program. The following code is taken from OpenGL Game Programming and is commented by myself. If you have any problems building and using this code, feel free to contact me.
/*      Steven Billington
        January 13, 2003
        May 26, 2003 - UPDATE
        RobotOGL.cpp
        rod@cprogramming.com

        The following program creates a window and then
        uses multiple OpenGL functions to display a
        animated robot constructed of different size
        cubes. To compile this code you must make the
        proper library links in project--->settings.

        I apologize for any amount of scattered code or
        mis-formatting that printing this may occur. Please
        feel free to email me at the address above for the .cpp
        file.
*/

/*      These are what we refer to as Pre-processor
        Directives. In order for certain functions in
        C++ to operate you must include certain header
        files. Each header file below contains different
        functions needed throughout this program.
*/

#pragma comment(linker, "/subsystem:windows")

#include <windows.h>
#include <gl/gl.h>
#include <gl/glu.h>
#include <gl/glaux.h>

/*      Here we find a few global variables. While
        i don't really like to use global variables,
        i found them very handy for this particular
        program. These variables will control angles,
        fullscreen, and the global device context.
*/

HDC g_HDC;
float angle = 0.0f;
float legAngle[2] = {0.0f, 0.0f};
float armAngle[2] = {0.0f, 0.0f};
bool fullScreen = false;

/*      Function:       DrawCube
        Purpose:        As the name would suggest, this is
                                the function for drawing the cubes.
*/

void DrawCube(float xPos, float yPos, float zPos)
{
        glPushMatrix();
        glBegin(GL_POLYGON);

                /*      This is the top face*/
                glVertex3f(0.0f, 0.0f, 0.0f);
                glVertex3f(0.0f, 0.0f, -1.0f);
                glVertex3f(-1.0f, 0.0f, -1.0f);
                glVertex3f(-1.0f, 0.0f, 0.0f);

                /*      This is the front face*/
                glVertex3f(0.0f, 0.0f, 0.0f);
                glVertex3f(-1.0f, 0.0f, 0.0f);
                glVertex3f(-1.0f, -1.0f, 0.0f);
                glVertex3f(0.0f, -1.0f, 0.0f);

                /*      This is the right face*/
                glVertex3f(0.0f, 0.0f, 0.0f);
                glVertex3f(0.0f, -1.0f, 0.0f);
                glVertex3f(0.0f, -1.0f, -1.0f);
                glVertex3f(0.0f, 0.0f, -1.0f);

                /*      This is the left face*/
                glVertex3f(-1.0f, 0.0f, 0.0f);
                glVertex3f(-1.0f, 0.0f, -1.0f);
                glVertex3f(-1.0f, -1.0f, -1.0f);
                glVertex3f(-1.0f, -1.0f, 0.0f);

                /*      This is the bottom face*/
                glVertex3f(0.0f, 0.0f, 0.0f);
                glVertex3f(0.0f, -1.0f, -1.0f);
                glVertex3f(-1.0f, -1.0f, -1.0f);
                glVertex3f(-1.0f, -1.0f, 0.0f);

                /*      This is the back face*/
                glVertex3f(0.0f, 0.0f, 0.0f);
                glVertex3f(-1.0f, 0.0f, -1.0f);
                glVertex3f(-1.0f, -1.0f, -1.0f);
                glVertex3f(0.0f, -1.0f, -1.0f);

        glEnd();
        glPopMatrix();
}

/*      Function:       DrawArm
        Purpose:        This function draws the arm
                                for the robot.
*/

void DrawArm(float xPos, float yPos, float zPos)
{
        glPushMatrix();

                /*      Sets color to red*/
                glColor3f(1.0f, 0.0f, 0.0f);
                glTranslatef(xPos, yPos, zPos);

                /*      Creates 1 x 4 x 1 cube*/
                glScalef(1.0f, 4.0f, 1.0f);
                DrawCube(0.0f, 0.0f, 0.0f);

        glPopMatrix();
}

/*      Function:       DrawHead
        Purpose:        This function will create the
                                head for the robot.
*/

void DrawHead(float xPos, float yPos, float zPos)
{
        glPushMatrix();

                /*      Sets color to white*/
                glColor3f(1.0f, 1.0f, 1.0f);
                glTranslatef(xPos, yPos, zPos);

                /*      Creates 2 x 2 x 2 cube*/
                glScalef(2.0f, 2.0f, 2.0f);
                DrawCube(0.0f, 0.0f, 0.0f);

        glPopMatrix();
}

/*      Function:       DrawTorso
        Purpose:        Function will do as suggested
                                and draw a torso for our robot.
*/

void DrawTorso(float xPos, float yPos, float zPos)
{
        glPushMatrix();

                /*      Sets color to blue*/
                glColor3f(0.0f, 0.0f, 1.0f);
                glTranslatef(xPos, yPos, zPos);

                /*      Creates 3 x 5 x 1 cube*/
                glScalef(3.0f, 5.0f, 1.0f);
                DrawCube(0.0f, 0.0f, 0.0f);

        glPopMatrix();
}

/*      Function:       DrawLeg
        Purpose:        Not to sound repetitve, but as suggested
                                this function will draw our robots legs.
*/

void DrawLeg(float xPos, float yPos, float zPos)
{
        glPushMatrix();

                /*      Sets color to yellow*/
                glColor3f(1.0f, 1.0f, 0.0f);
                glTranslatef(xPos, yPos, zPos);

                /*      Creates 1 x 5 x 1 cube*/
                glScalef(1.0f, 5.0f, 1.0f);
                DrawCube(0.0f, 0.0f, 0.0f);

        glPopMatrix();
}

/*      Function:       DrawRobot
        Purpose:        Function to draw our entire robot
*/

void DrawRobot(float xPos, float yPos, float zPos)
{
        /*      Variables for state of robots legs. True
                means the leg is forward, and False means
                the leg is back. The same applies to the
                robots arm states.
        */
        static bool leg1 = true;
        static bool leg2 = false;
        static bool arm1 = true;
        static bool arm2 = false;

        glPushMatrix();

                /*      This will draw our robot at the
                        desired coordinates.
                */
                glTranslatef(xPos, yPos, zPos);

                /*      These three lines will draw the
                        various components of our robot.
                */
                DrawHead(1.0f, 2.0f, 0.0f);
                DrawTorso(1.5f, 0.0f, 0.0f);
                glPushMatrix();


                /*      If the arm is moving forward we will increase
                        the angle; otherwise, we will decrease the
                        angle.
                */
                if (arm1)
                {
                        armAngle[0] = armAngle[0] + 1.0f;
                }
                else
                {
                        armAngle[0] = armAngle[0] - 1.0f;
                }

                /*      Once the arm has reached its max angle
                        in one direction, we want it to reverse
                        and change direction.
                */
                if (armAngle[0] >= 15.0f)
                {
                        arm1 = false;
                }
                if (armAngle[0] <= 15.0f)
                {
                        arm1 = true;
                }


                /*      Here we are going to move the arm away
                        from the torso and rotate. This will
                        create a walking effect.
                */
                glTranslatef(0.0f, -0.5f, 0.0f);
                glRotatef(armAngle[0], 1.0f, 0.0f, 0.0f);
                DrawArm(2.5f, 0.0f, -0.5f);

        glPopMatrix();

        glPushMatrix();


                /*      If the arm is moving forward we will increase
                        the angle, otherwise we will decrease the
                        angle
                */
                if (arm2)
                {
                        armAngle[1] = armAngle[1] + 1.0f;
                }
                else
                {
                        armAngle[1] = armAngle[1] - 1.0f;
                }

                /*      Here we are going to move the arm away
                        from the torso and rotate. This will
                        create a walking effect.
                */
                glTranslatef(0.0f, -0.5f, 0.0f);
                glRotatef(armAngle[1], 1.0f, 0.0f, 0.0f);
                DrawArm(-1.5f, 0.0f, -0.5f);

        glPopMatrix();

        /*      Now its time to rotate the legs relative to the
                robots position in the world, this is the first
                leg, ie the right one.
        */
        glPushMatrix();

                /*      If the leg is moving forward we will increase
                        the angle; otherwise, we will decrease the
                        angle.
                */
                if (leg1)
                {
                        legAngle[0] = legAngle[0] + 1.0f;
                }
                else
                {
                        legAngle[0] = legAngle[0] - 1.0f;
                }

                /*      Once the leg has reached its max angle
                        in one direction, we want it to reverse
                        and change direction.
                */
                if (legAngle[0] >= 15.0f)
                {
                        leg1 = false;
                }
                if (legAngle[0] <= -15.0f)
                {
                        leg1 = true;
                }


                /*      Here we are going to move the leg away
                        from the torso and rotate. This will
                        create a walking effect.
                */
                glTranslatef(0.0f, -0.5f, 0.0f);
                glRotatef(legAngle[0], 1.0f, 0.0f, 0.0f);


                /*      Time to draw the leg.
                */
                DrawLeg(-0.5f, -5.0f, -0.5f);

        glPopMatrix();

        /*      Same as above, for the left leg.
        */
        glPushMatrix();

                /*      If the leg is moving forward we will increase
                        the angle, otherwise we will decrease the
                        angle
                */
                if (leg2)
                {
                        legAngle[1] = legAngle[1] + 1.0f;
                }
                else
                {
                        legAngle[1] = legAngle[1] - 1.0f;
                }

                /*      Once the leg has reached its max angle
                        in one direction, we want it to reverse
                        and change direction.
                */
                if (legAngle[1] >= 15.0f)
                {
                        leg2 = false;
                }
                if (legAngle[1] <= -15.0f)
                {
                        leg2 = true;
                }

                /*      Here we are going to move the leg away
                        from the torso and rotate. This will
                        create a walking effect.
                */
                glTranslatef(0.0f, -0.5f, 0.0f);
                glRotatef(legAngle[1], 1.0f, 0.0f, 0.0f);
                DrawLeg(1.5f, -5.0f, -0.5f);

        glPopMatrix();

        glPopMatrix();

}

/*      Function:       Render
        Purpose:        This function will be responsible
                                for the rendering, got to love my
                                descriptive function names : )
*/
void Render()
{
        /*      Enable depth testing
        */
        glEnable(GL_DEPTH_TEST);

        /*      Heres our rendering. Clears the screen
                to black, clear the color and depth
                buffers, and reset our modelview matrix.
        */
        glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        glLoadIdentity();

        /*      Increase rotation angle counter
        */
        angle = angle + 1.0f;

        /*      Reset after we have completed a circle
        */
        if (angle >= 360.0f)
        {
                angle = 0.0f;
        }

        glPushMatrix();
                glLoadIdentity();

                /*      Move to 0,0,-30 , rotate the robot on
                        its y axis, draw the robot, and dispose
                        of the current matrix.
                */
                glTranslatef(0.0f, 0.0f, -30.0f);
                glRotatef(angle, 0.0f, 1.0f, 0.0f);
                DrawRobot(0.0f, 0.0f, 0.0f);
        glPopMatrix();

        glFlush();

        /*      Bring back buffer to foreground
        */
        SwapBuffers(g_HDC);
}

//function to set the pixel format for the device context
/*      Function:       SetupPixelFormat
        Purpose:        This function will be responsible
                                for setting the pixel format for the
                                device context.
*/
void SetupPixelFormat(HDC hDC)
{
        /*      Pixel format index
        */
        int nPixelFormat;

        static PIXELFORMATDESCRIPTOR pfd = {
                sizeof(PIXELFORMATDESCRIPTOR),          //size of structure
                1,                                      //default version
                PFD_DRAW_TO_WINDOW |                    //window drawing support
                PFD_SUPPORT_OPENGL |                    //opengl support
                PFD_DOUBLEBUFFER,                       //double buffering support
                PFD_TYPE_RGBA,                          //RGBA color mode
                32,                                     //32 bit color mode
                0, 0, 0, 0, 0, 0,                       //ignore color bits
                0,                                      //no alpha buffer
                0,                                      //ignore shift bit
                0,                                      //no accumulation buffer
                0, 0, 0, 0,                             //ignore accumulation bits
                16,                                     //16 bit z-buffer size
                0,                                      //no stencil buffer
                0,                                      //no aux buffer
                PFD_MAIN_PLANE,                         //main drawing plane
                0,                                      //reserved
                0, 0, 0 };                              //layer masks ignored

                /*      Choose best matching format*/
                nPixelFormat = ChoosePixelFormat(hDC, &pfd);

                /*      Set the pixel format to the device context*/
                SetPixelFormat(hDC, nPixelFormat, &pfd);
}

/*      Windows Event Procedure Handler
*/
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
        /*      Rendering and Device Context
                variables are declared here.
        */
        static HGLRC hRC;
        static HDC hDC;

        /*      Width and Height for the
                window our robot is to be
                displayed in.
        */
        int width, height;

        switch(message)
        {
                case WM_CREATE: //window being created

                        hDC = GetDC(hwnd);  //get current windows device context
                        g_HDC = hDC;
                        SetupPixelFormat(hDC); //call our pixel format setup function

                        /*      Create rendering context and make it current
                        */
                        hRC = wglCreateContext(hDC);
                        wglMakeCurrent(hDC, hRC);

                        return 0;
                        break;

                case WM_CLOSE:  //window is closing

                        /*      Deselect rendering context and delete it*/
                        wglMakeCurrent(hDC, NULL);
                        wglDeleteContext(hRC);

                        /*      Send quit message to queue*/
                        PostQuitMessage(0);

                        return 0;
                        break;

                case WM_SIZE:

                        /*      Retrieve width and height*/
                        height = HIWORD(lParam);
                        width = LOWORD(lParam);

                        /*      Don't want a divide by 0*/
                        if (height == 0)
                        {
                                height = 1;
                        }

                        /*      Reset the viewport to new dimensions*/
                        glViewport(0, 0, width, height);

                        /*      Set current Matrix to projection*/
                        glMatrixMode(GL_PROJECTION);
                        glLoadIdentity(); //reset projection matrix

                        /*      Time to calculate aspect ratio of
                                our window.
                        */
                        gluPerspective(54.0f, (GLfloat)width/(GLfloat)height, 1.0f, 1000.0f);

                        glMatrixMode(GL_MODELVIEW); //set modelview matrix
                        glLoadIdentity(); //reset modelview matrix

                        return 0;
                        break;

                default:

                        break;
        }

        return (DefWindowProc(hwnd, message, wParam, lParam));
}

int APIENTRY WinMain(HINSTANCE hInstance,
                     HINSTANCE hPrevInstance,
                     LPSTR     lpCmdLine,
                     int       nCmdShow)
{
        WNDCLASSEX windowClass; //window class
        HWND    hwnd;                   //window handle
        MSG             msg;                    //message
        bool    done;                   //flag for completion of app
        DWORD   dwExStyle;              //window extended style
        DWORD   dwStyle;                //window style
        RECT    windowRect;

        /*      Screen/display attributes*/
        int width = 800;
        int height = 600;
        int bits = 32;

        windowRect.left =(long)0;               //set left value to 0
        windowRect.right =(long)width;  //set right value to requested width
        windowRect.top =(long)0;                //set top value to 0
        windowRect.bottom =(long)height;//set bottom value to requested height

        /*      Fill out the window class structure*/
        windowClass.cbSize                      = sizeof(WNDCLASSEX);
        windowClass.style                       = CS_HREDRAW | CS_VREDRAW;
        windowClass.lpfnWndProc         = WndProc;
        windowClass.cbClsExtra          = 0;
        windowClass.cbWndExtra          = 0;
        windowClass.hInstance           = hInstance;
        windowClass.hIcon                       = LoadIcon(NULL, IDI_APPLICATION);
        windowClass.hCursor                     = LoadCursor(NULL, IDC_ARROW);
        windowClass.hbrBackground       = NULL;
        windowClass.lpszMenuName        = NULL;
        windowClass.lpszClassName       = "MyClass";
        windowClass.hIconSm                     = LoadIcon(NULL, IDI_WINLOGO);

        /*      Register window class*/
        if (!RegisterClassEx(&windowClass))
        {
                return 0;
        }

        /*      Check if fullscreen is on*/
        if (fullScreen)
        {
                DEVMODE dmScreenSettings;
                memset(&dmScreenSettings, 0, sizeof(dmScreenSettings));
                dmScreenSettings.dmSize = sizeof(dmScreenSettings);
                dmScreenSettings.dmPelsWidth = width;   //screen width
                dmScreenSettings.dmPelsHeight = height; //screen height
                dmScreenSettings.dmBitsPerPel = bits;   //bits per pixel
                dmScreenSettings.dmFields = DM_BITSPERPEL | DM_PELSWIDTH | DM_PELSHEIGHT;

                if (ChangeDisplaySettings(&dmScreenSettings, CDS_FULLSCREEN !=
                                                                  DISP_CHANGE_SUCCESSFUL))
                {
                        /*      Setting display mode failed, switch to windowed*/
                        MessageBox(NULL, "Display mode failed", NULL, MB_OK);
                        fullScreen = false;
                }
        }

        /*      Check if fullscreen is still on*/
        if (fullScreen)
        {
                dwExStyle = WS_EX_APPWINDOW;    //window extended style
                dwStyle = WS_POPUP;                             //windows style
                ShowCursor(FALSE);                              //hide mouse pointer
        }

        else
        {
                dwExStyle = WS_EX_APPWINDOW | WS_EX_WINDOWEDGE; //window extended style
                dwStyle = WS_OVERLAPPEDWINDOW;                                  //windows style
        }

        AdjustWindowRectEx(&windowRect, dwStyle, FALSE, dwExStyle);

        /*      Class registerd, so now create our window*/
        hwnd = CreateWindowEx(NULL, "MyClass",  //class name
                                                  "OpenGL Robot",       //app name
                                                  dwStyle |
                                                  WS_CLIPCHILDREN |
                                                  WS_CLIPSIBLINGS,
                                                  0, 0,                         //x and y coords
                                                  windowRect.right - windowRect.left,
                                                  windowRect.bottom - windowRect.top,//width, height
                                                  NULL,                 //handle to parent
                                                  NULL,                 //handle to menu
                                                  hInstance,    //application instance
                                                  NULL);                //no xtra params

        /*      Check if window creation failed (hwnd = null ?)*/
        if (!hwnd)
        {
                return 0;
        }

        ShowWindow(hwnd, SW_SHOW);      //display window
        UpdateWindow(hwnd);                     //update window

        done = false;   //initialize loop condition variable

        /*      Main message loop*/
        while (!done)
        {
                PeekMessage(&msg, hwnd, NULL, NULL, PM_REMOVE);

                        if (msg.message == WM_QUIT)     //did we receive a quit message?
                        {
                                done = true;
                        }

                        else
                        {
                                Render();
                                TranslateMessage(&msg);
                                DispatchMessage(&msg);
                        }
        }

        if (fullScreen)
        {
                ChangeDisplaySettings(NULL, 0);
                ShowCursor(TRUE);
        }

        return msg.wParam;

}
That's a lot of code! Spend some time studying the example, practice a bit, and then we will proceed to lesson 7, where more of this code will be explained. Also, in lesson 7 we will cover Projections. Happy Coding!!
Previous: WGL functions
Next: Projections in OpenGL
Back to OpenGL tutorial index