Your First OpenGL Program

http://www.cprogramming.com/tutorial/opengl_first_opengl_program.html

 

Your First OpenGL Program

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
[e-mail]

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 revieve 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!!