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wforl

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
	glLoadIdentity();										// reset modelview matrix

	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
		glLoadIdentity();					// reset matrix
		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
	glLoadIdentity();										// reset modelview matrix

	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
		glLoadIdentity();					// reset matrix
		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
			glLoadIdentity();						// reset projection matrix

			//// calculate aspect ratio of 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));
}

// 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.lpszMenuName	= NULL;								// no menu
	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
	}

	AdjustWindowRectEx(&windowRect, dwstyle, FALSE, dwExstyle);		// Adjust Window To True Requested Size

	// 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
						  NULL,									// handle to menu
						  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 wforl
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


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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