OpenGL understanding opengl

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I realy dont understand this for some reason, ive simplified it below, and have the whole code below that. I understand it all, apart from these glLoadidentity() functions, take for example the render() function,
void Render()
{
glEnable(GL_DEPTH_TEST);				// enable depth testing

// do rendering here
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);					// clear to black
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);		// clear screen and depth buffer

angle = angle + 0.5f;					// increase our rotation angle counter
if (angle >= 360.0f)					// if we've gone in a circle, reset counter
angle = 0.0f;

glPushMatrix();							// put current matrix on stack
glTranslatef(0.0f, 0.0f, -30.0f);	// move to (0, 0, -30)
//glRotatef(angle, 0.0f, 1.0f, 0.0f);	// rotate the robot on its y-axis
//glRotatef(angle, 1.0f, 0.0f, 0.0f);	// rotate the robot on its y-axis

DrawBoxes(0.0f, 0.0f, 0.0f);		// draw the robot
glPopMatrix();							// dispose of current matrix

glFlush();

SwapBuffers(g_HDC);			// bring backbuffer to foreground
}
1.clears the colour to black 2.clears screen 3. i believ here it sets everything back to 0, so x,y,z are now back to the centre of the screen, x=0,y=0, z=0 4.sets angle for rotation. 5.puts current matrix on stack, which is currently x=0,y=0, z=0 6. loads current matrix to x=0,y=0, z=0, (which isn't it that allready??) 7. move the camear back 30 in z 8. rotate camera 9. draw boxes now howcome we see the boxes from 30 steps backwards, because after the we move 30 steps backwards, we draw the boxes, so shouldn't they be drawn where we currently are, in which case the camera and boxex would be in the same z position. Because after we say glTranslatef(0.0f, 0.0f, -30.0f); isn'nt saying move to DrawBoxes(0.0f, 0.0f, 0.0f); going to put you in the same position, as open gl is a state machine, so that once glTranslatef(0.0f, 0.0f, -30.0f); is carried out the, the current position is now the origan. so DrawBoxes(0.0f, 0.0f, 0.0f); is'nt going to move you anywhere glTranslatef(0.0f, 0.0f, -30.0f); // move to (0, 0, -30) DrawBoxes(0.0f, 0.0f, 0.0f); // draw the robot
#define WIN32_LEAN_AND_MEAN				// trim the excess fat from Windows

/*****************************************************

Chapter 5: Robot Example

OpenGL Game Programming
Kevin Hawkins, Dave Astle

Animates a walking robot-like figure.

******************************************************/

////// Includes
#include <windows.h>					// standard Windows app include
#include <gl/gl.h>						// standard OpenGL include
#include <gl/glu.h>						// OpenGL utilties
#include <gl/glaux.h>					// OpenGL auxiliary functions

////// Global Variables
float angle = 0.0f;						// current angle of the rotating triangle
HDC g_HDC;								// global device context
bool fullScreen = false;

////// Robot variables
float legAngle[2] = { 0.0f, 0.0f };		// each leg's current angle
float armAngle[2] = {0.0f, 0.0f };

// DrawCube
// desc: since each component of the robot is made up of
//       cubes, we will use a single function that will
//		 draw a cube at a specified location.
void DrawCube(float xPos, float yPos, float zPos)
{

glTranslatef(xPos, yPos, zPos);
glBegin(GL_POLYGON);

glColor3f(255,0,0);
glVertex3f(0.0f, 0.0f, 0.0f);	// top face
glVertex3f(0.0f, 0.0f, -1.0f);
glVertex3f(-1.0f, 0.0f, -1.0f);
glVertex3f(-1.0f, 0.0f, 0.0f);

glColor3f(0,255,0);
glVertex3f(0.0f, 0.0f, 0.0f);	// front face
glVertex3f(-1.0f, 0.0f, 0.0f);
glVertex3f(-1.0f, -1.0f, 0.0f);
glVertex3f(0.0f, -1.0f, 0.0f);

glColor3f(0,0,255);
glVertex3f(0.0f, 0.0f, 0.0f);	// right face
glVertex3f(0.0f, -1.0f, 0.0f);
glVertex3f(0.0f, -1.0f, -1.0f);
glVertex3f(0.0f, 0.0f, -1.0f);

glColor3f(255,0,255);
glVertex3f(-1.0f, 0.0f, 0.0f);	// left face
glVertex3f(-1.0f, 0.0f, -1.0f);
glVertex3f(-1.0f, -1.0f, -1.0f);
glVertex3f(-1.0f, -1.0f, 0.0f);

glColor3f(255,255,0);
glVertex3f(0.0f, 0.0f, 0.0f);	// bottom face
glVertex3f(0.0f, -1.0f, -1.0f);
glVertex3f(-1.0f, -1.0f, -1.0f);
glVertex3f(-1.0f, -1.0f, 0.0f);

glColor3f(100,0,0);
glVertex3f(0.0f, 0.0f, 0.0f);	// back face
glVertex3f(-1.0f, 0.0f, -1.0f);
glVertex3f(-1.0f, -1.0f, -1.0f);
glVertex3f(0.0f, -1.0f, -1.0f);
glEnd();

}

// DrawRobot
// desc: draws the robot located at (xpos,ypos,zpos)
void DrawBoxes(float xPos, float yPos, float zPos)
{

DrawCube(0,0,0);

DrawCube(10,0,0);

DrawCube(0,10,0);

DrawCube(10,10,0);

}

// Render
// desc: handles drawing of scene
void Render()
{
glEnable(GL_DEPTH_TEST);				// enable depth testing

// do rendering here
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);					// clear to black
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);		// clear screen and depth buffer

angle = angle + 0.5f;					// increase our rotation angle counter
if (angle >= 360.0f)					// if we've gone in a circle, reset counter
angle = 0.0f;

glPushMatrix();							// put current matrix on stack
glTranslatef(0.0f, 0.0f, -30.0f);	// move to (0, 0, -30)
//glRotatef(angle, 0.0f, 1.0f, 0.0f);	// rotate the robot on its y-axis
//glRotatef(angle, 1.0f, 0.0f, 0.0f);	// rotate the robot on its y-axis

DrawBoxes(0.0f, 0.0f, 0.0f);		// draw the robot
glPopMatrix();							// dispose of current matrix

glFlush();

SwapBuffers(g_HDC);			// bring backbuffer to foreground
}

// function to set the pixel format for the device context
void SetupPixelFormat(HDC hDC)
{
int nPixelFormat;					// our pixel format index

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, non-palettized mode
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 auxiliary buffer
PFD_MAIN_PLANE,					// main drawing plane
0,								// reserved
0, 0, 0 };						// layer masks ignored

nPixelFormat = ChoosePixelFormat(hDC, &pfd);	// choose best matching pixel format

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

// the Windows Procedure event handler
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
static HGLRC hRC;					// rendering context
static HDC hDC;						// device context
int width, height;					// window width and height

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

hDC = GetDC(hwnd);			// get current window's 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:					// windows is closing

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

// send WM_QUIT to message queue
PostQuitMessage(0);

return 0;
break;

case WM_SIZE:
height = HIWORD(lParam);		// retrieve width and height
width = LOWORD(lParam);

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

glViewport(0, 0, width, height);		// reset the viewport to new dimensions
glMatrixMode(GL_PROJECTION);			// set projection matrix current matrix

//// calculate aspect ratio of window
gluPerspective(54.0f,(GLfloat)width/(GLfloat)height,1.0f,1000.0f);

glMatrixMode(GL_MODELVIEW);				// set modelview matrix

return 0;
break;

default:
break;
}

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

// the main windows entry point
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd)
{
WNDCLASSEX windowClass;		// window class
HWND	   hwnd;			// window handle
MSG		   msg;				// message
bool	   done;			// flag saying when our app is complete
DWORD	   dwExstyle;						// Window Extended style
DWORD	   dwstyle;						// Window style
RECT	   windowRect;

// temp var's
int width = 800;
int height = 600;
int bits = 32;

//fullScreen = TRUE;

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);	// default icon
windowClass.hCursor			= LoadCursor(NULL, IDC_ARROW);		// default arrow
windowClass.hbrBackground	= NULL;								// don't need background
windowClass.lpszClassName	= "MyClass";
windowClass.hIconSm			= LoadIcon(NULL, IDI_WINLOGO);		// windows logo small icon

// register the windows class
if (!RegisterClassEx(&windowClass))
return 0;

if (fullScreen)								// fullscreen?
{
DEVMODE dmScreenSettings;					// device mode
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;
}
}

if (fullScreen)								// Are We Still In Fullscreen Mode?
{
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
}

// class registered, so now create our window
hwnd = CreateWindowEx(NULL,									// extended style
"MyClass",							// class name
"OpenGL Robot",		// app name
dwstyle | WS_CLIPCHILDREN |
WS_CLIPSIBLINGS,
0, 0,								// x,y coordinate
windowRect.right - windowRect.left,
windowRect.bottom - windowRect.top, // width, height
NULL,									// handle to parent
hInstance,							// application instance
NULL);								// no extra params

// check if window creation failed (hwnd would equal NULL)
if (!hwnd)
return 0;

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

done = false;						// intialize the loop condition variable

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

if (msg.message == WM_QUIT)		// do we receive a WM_QUIT message?
{
done = true;				// if so, time to quit the application
}
else
{
Render();

TranslateMessage(&msg);		// translate and dispatch to event queue
DispatchMessage(&msg);
}
}

if (fullScreen)
{
ChangeDisplaySettings(NULL,0);					// If So Switch Back To The Desktop
ShowCursor(TRUE);						// Show Mouse Pointer
}

return msg.wParam;
}



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Quote:
 Original post by wforl1.clears the colour to black2.clears screen3. i believ here it sets everything back to 0, so x,y,z are now back to the centre of the screen, x=0,y=0, z=04.sets angle for rotation.5.puts current matrix on stack, which is currently x=0,y=0, z=06. loads current matrix to x=0,y=0, z=0, (which isn't it that allready??)7. move the camear back 30 in z8. rotate camera9. draw boxesnow howcome we see the boxes from 30 steps backwards, because after the we move 30 steps backwards, we draw the boxes, so shouldn't they be drawn where we currently are, in which case the camera and boxex would be in the same z position.Because after we say glTranslatef(0.0f, 0.0f, -30.0f); isn'nt saying move to DrawBoxes(0.0f, 0.0f, 0.0f); going to put you in the same position, as open gl is a state machine, so that once glTranslatef(0.0f, 0.0f, -30.0f); is carried out the, the current position is now the origan. so DrawBoxes(0.0f, 0.0f, 0.0f); is'nt going to move you anywhereglTranslatef(0.0f, 0.0f, -30.0f); // move to (0, 0, -30) DrawBoxes(0.0f, 0.0f, 0.0f); // draw the robot

Except that there is no "camera", camera is just an illusion. You have one world transformation which transforms your vertices. You can think of the "camera" as stuck at the origin looking in one direction and the rest of the world moving around it. You don't actually move the "camera" you change how the vertices are transformed making it seem as if there is a camera.

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It's as Omid says.
When you do a glTranslate, you move the "origin of the world", and not the viewpoint/view frustum.

If you grab a pen and hold it in front of you (I'm serious... grab that pen, it'll help), and the tip of the pen is the origin of the world. A translate would move the tip of the pen, but you will stay in the same position. You stay fixed in your chair and all you are doing is move the tip of the pen around the screen.

So if you do a glTranslate(0,0, -30), you move the tip of the pen 30 units into the screen. Your view/camera/eyes will stay where they are. Whatever you draw now at position 0,0,0, the tip of the pen, will "appear" to be 30 units away from you.

Now, things get more complicated if you start doing multiple translations and rotations because they are additive, and it will become important that you understand how the world transformation matrix is manipulated, and how glPushMatrix and glPopMatrix are your best friends.

But I'll leave it with that. Try to understand how a translate and rotate affect the world, rather than the "view".

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