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NeHe Productions: OpenGL Lesson #12

Date de publication : 31/03/2006 , Date de mise à jour : 31/03/2006

Par Jeff Molofee ( NeHe ) (Autres articles)

Lesson: 12 In this tutorial I'll teach you how to use Display Lists. Not only do display lists speed up your code, they also cut down on the number of lines of code you need to write when creating a simple GL scene.

For example. Lets say you're making the game asteroids. Each level starts off with at least 2 asteroids. So you sit down with your graph paper (grin), and figure out how to make a 3D asteroid. Once you have everything figured out, you build the asteroid in OpenGL using Polygons or Quads. Lets say the asteroid is octagonal (8 sides). If you're smart you'll create a loop, and draw the asteroid once inside the loop. You'll end up with roughly 18 lines or more of code to make the asteroid. Creating the asteroid each time it's drawn to the screen is hard on your system. Once you get into more complex objects you'll see what I mean.

So what's the solution? Display Lists!!! By using a display list, you create the object just once. You can texture map it, color it, whatever you want to do. You give the display list a name. Because it's an asteroid we'll call the display list 'asteroid'. Now any time I want to draw the textured / colored asteroid on the screen, all I have to do is call glCallList(asteroid). the premade asteroid will instantly appear on the screen. Because the asteroid has already built in the display list, OpenGL doesn't have to figure out how to build it. It's prebuilt in memory. This takes alot of strain off your processor and allows your programs to run alot faster!

So are you ready to learn? :) We'll call this the Q-Bert Display List demo. What you'll end up with is a Q-Bert type screen made up of 15 cubes. Each cube is made up of a TOP, and a BOX. The top will be a seperate display list so that we can color it a darker shade. The box is a cube without the top :)

This code is based around lesson 6. I'll rewrite most of the program so it's easier to see where I've made changes. The follow lines of code are standard code used in just about all the lessons.
#include	<windows.h>								 // Header File For Windows
#include	<stdio.h>								 // Header File For Standard Input/Output
#include	<gl\gl.h>								 // Header File For The OpenGL32 Library
#include	<gl\glu.h>								 // Header File For The GLu32 Library
#include	<gl\glaux.h>								 // Header File For The GLaux Library
HDC		hDC=NULL;								 // Private GDI Device Context
HGLRC		hRC=NULL;								 // Permanent Rendering Context
HWND		hWnd=NULL;								 // Holds Our Window Handle
HINSTANCE	hInstance;								 // Holds The Instance Of The Application
bool		keys[256];								 // Array Used For The Keyboard Routine
bool		active=TRUE;								 // Window Active Flag Set To TRUE By Default
bool		fullscreen=TRUE;							 // Fullscreen Flag Set To Fullscreen Mode By Default
Now we set up our variables. First we set up storage for one texture. Then we create two new variables for our 2 display lists. These variable will act as pointers to where the display list is stored in ram. They're called box and top.

After that we have 2 variables called xloop and yloop which are used to position the cubes on the screen and 2 variables called xrot and yrot that are used to rotate the cubes on the x axis and y axis.
GLuint	texture[1];									 // Storage For One Texture
GLuint	box;										 // Storage For The Display List
GLuint	top;										 // Storage For The Second Display List
GLuint	xloop;										 // Loop For X Axis
GLuint	yloop;										 // Loop For Y Axis
GLfloat	xrot;										 // Rotates Cube On The X Axis
GLfloat	yrot;										 // Rotates Cube On The Y Axis
Next we create two color arrays. The first one boxcol stores the color values for Bright Red, Orange, Yellow, Green and Blue. Each value inside the {}'s represent a red, green and blue value. Each group of {}'s is a specific color.

The second color array we create is for Dark Red, Dark Orange, Dark Yellow, Dark Green and Dark Blue. The dark colors will be used to draw the top of the boxes. We want the lid to be darker than the rest of the box.
static GLfloat boxcol[5][3]=								 // Array For Box Colors
	 // Bright: Red, Orange, Yellow, Green, Blue
static GLfloat topcol[5][3]=								 // Array For Top Colors
	 // Dark: Red, Orange, Yellow, Green, Blue
LRESULT	CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);					 // Declaration For WndProc
Now we build the actual Display List. If you notice, all the code to build the box is in the first list, and all the code to build the top is in the other list. I'll try to explain this section in alot of detail.
GLvoid BuildLists()									 // Build Box Display List
We start off by telling OpenGL we want to build 2 lists. glGenLists(2) creates room for the two lists, and returns a pointer to the first list. 'box' will hold the location of the first list. Whenever we call box the first list will be drawn.
	box=glGenLists(2);								 // Building Two Lists
Now we're going to build the first list. We've already freed up room for two lists, and we know that box points to the area we're going to store the first list. So now all we have to do is tell OpenGL where the list should go, and what type of list to make.

We use the command glNewList() to do the job. You'll notice box is the first parameter. This tells OpenGL to store the list in the memory location that box points to. The second parameter GL_COMPILE tells OpenGL we want to prebuild the list in memory so that OpenGL doesn't have to figure out how to create the object ever time we draw it.

GL_COMPILE is similar to programming. If you write a program, and load it into your compiler, you have to compile it every time you want to run it. If it's already compiled into an .EXE file, all you have to do is click on the .exe to run it. No compiling needed. Once GL has compiled the display list, it's ready to go, no more compiling required. This is where we get the speed boost from using display lists.
	glNewList(box,GL_COMPILE);							 // New Compiled box Display List
The next section of code draws the box without the top. It wont appear on the screen. It will be stored in the display list.

You can put just about any command you want between glNewList() and glEndList(). You can set colors, you can change textures, etc. The only type of code you CAN'T add is code that would change the display list on the fly. Once the display list is built, you CAN'T change it.

If you added the line glColor3ub(rand()%255,rand()%255,rand()%255) into the code below, you might think that each time you draw the object to the screen it will be a different color. But because the list is only CREATED once, the color will not change each time you draw it to the screen. Whatever color the object was when it was first made is the color it will remain.

If you want to change the color of the display list, you have to change it BEFORE you draw the display list to the screen. I'll explain more on this later.
		glBegin(GL_QUADS);							 // Start Drawing Quads
			 // Bottom Face
			glTexCoord2f(1.0f, 1.0f); glVertex3f(-1.0f, -1.0f, -1.0f);	 // Top Right Of The Texture and Quad
			glTexCoord2f(0.0f, 1.0f); glVertex3f( 1.0f, -1.0f, -1.0f);	 // Top Left Of The Texture and Quad
			glTexCoord2f(0.0f, 0.0f); glVertex3f( 1.0f, -1.0f, 1.0f);	 // Bottom Left Of The Texture and Quad
			glTexCoord2f(1.0f, 0.0f); glVertex3f(-1.0f, -1.0f, 1.0f);	 // Bottom Right Of The Texture and Quad
			 // Front Face
			glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f, -1.0f, 1.0f);	 // Bottom Left Of The Texture and Quad
			glTexCoord2f(1.0f, 0.0f); glVertex3f( 1.0f, -1.0f, 1.0f);	 // Bottom Right Of The Texture and Quad
			glTexCoord2f(1.0f, 1.0f); glVertex3f( 1.0f, 1.0f, 1.0f);	 // Top Right Of The Texture and Quad
			glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f, 1.0f, 1.0f);	 // Top Left Of The Texture and Quad
			 // Back Face
			glTexCoord2f(1.0f, 0.0f); glVertex3f(-1.0f, -1.0f, -1.0f);	 // Bottom Right Of The Texture and Quad
			glTexCoord2f(1.0f, 1.0f); glVertex3f(-1.0f, 1.0f, -1.0f);	 // Top Right Of The Texture and Quad
			glTexCoord2f(0.0f, 1.0f); glVertex3f( 1.0f, 1.0f, -1.0f);	 // Top Left Of The Texture and Quad
			glTexCoord2f(0.0f, 0.0f); glVertex3f( 1.0f, -1.0f, -1.0f);	 // Bottom Left Of The Texture and Quad
			 // Right face
			glTexCoord2f(1.0f, 0.0f); glVertex3f( 1.0f, -1.0f, -1.0f);	 // Bottom Right Of The Texture and Quad
			glTexCoord2f(1.0f, 1.0f); glVertex3f( 1.0f, 1.0f, -1.0f);	 // Top Right Of The Texture and Quad
			glTexCoord2f(0.0f, 1.0f); glVertex3f( 1.0f, 1.0f, 1.0f);	 // Top Left Of The Texture and Quad
			glTexCoord2f(0.0f, 0.0f); glVertex3f( 1.0f, -1.0f, 1.0f);	 // Bottom Left Of The Texture and Quad
			 // Left Face
			glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f, -1.0f, -1.0f);	 // Bottom Left Of The Texture and Quad
			glTexCoord2f(1.0f, 0.0f); glVertex3f(-1.0f, -1.0f, 1.0f);	 // Bottom Right Of The Texture and Quad
			glTexCoord2f(1.0f, 1.0f); glVertex3f(-1.0f, 1.0f, 1.0f);	 // Top Right Of The Texture and Quad
			glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f, 1.0f, -1.0f);	 // Top Left Of The Texture and Quad
		glEnd();								 // Done Drawing Quads
We tell OpenGL we're done making our list with the command glEndList(). Anything between glNewList() and glEndList is part of the Display List, anything before glNewList() or after glEndList() is not part of the current display list.
	glEndList();									 // Done Building The box List
Now we'll make our second display list. To find out where the second display list is stored in memory, we take the value of the old display list (box) and add one to it. The code below will make 'top' equal the location of the second display list.
	top=box+1;									 // top List Value Is box List Value +1
Now that we know where to store the second display list, we can build it. We do this the same way we built the first display list, but this time we tell OpenGL to store the list at 'top' instead of 'box'.
	glNewList(top,GL_COMPILE);							 // New Compiled top Display List
The following section of code just draws the top of the box. It's a simple quad drawn on the Z plane.
		glBegin(GL_QUADS);							 // Start Drawing Quad
			 // Top Face
			glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f, 1.0f, -1.0f);	 // Top Left Of The Texture and Quad
			glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f, 1.0f, 1.0f);	 // Bottom Left Of The Texture and Quad
			glTexCoord2f(1.0f, 0.0f); glVertex3f( 1.0f, 1.0f, 1.0f);	 // Bottom Right Of The Texture and Quad
			glTexCoord2f(1.0f, 1.0f); glVertex3f( 1.0f, 1.0f, -1.0f);	 // Top Right Of The Texture and Quad
		glEnd();								 // Done Drawing Quad
Again we tell OpenGL we're done building our second list with the command glEndList(). That's it. We've successfully created 2 display lists.
	glEndList();									 // Done Building The top Display List
The bitmap/texture building code is the same code we used in previous tutorials to load and build a texture. We want a texture that we can map onto all 6 sides of each cube. I've decided to use mipmapping to make the texture look real smooth. I hate seeing pixels :) The name of the texture to load is called 'cube.bmp'. It's stored in a directory called data. Find LoadBMP and change that line to look like the line below.
	if (TextureImage[0]=LoadBMP("Data/Cube.bmp"))					 // Load The Bitmap
Resizing code is exactly the same as the code in Lesson 6.

The init code only has a few changes. I've added the line BuildList(). This will jump to the section of code that builds the display lists. Notice that BuildList() is after LoadGLTextures(). It's important to know the order things should go in. First we build the textures, so when we create our display lists, there's a texture already created that we can map onto the cube.
int InitGL(GLvoid)									 // All Setup For OpenGL Goes Here
	if (!LoadGLTextures())								 // Jump To Texture Loading Routine
		return FALSE;								 // If Texture Didn't Load Return FALSE
	BuildLists();									 // Jump To The Code That Creates Our Display Lists
	glEnable(GL_TEXTURE_2D);							 // Enable Texture Mapping
	glShadeModel(GL_SMOOTH);							 // Enable Smooth Shading
	glClearColor(0.0f, 0.0f, 0.0f, 0.5f);						 // Black Background
	glClearDepth(1.0f);								 // Depth Buffer Setup
	glEnable(GL_DEPTH_TEST);							 // Enables Depth Testing
	glDepthFunc(GL_LEQUAL);								 // The Type Of Depth Testing To Do
The next three lines of code enable quick and dirty lighting. Light0 is predefined on most video cards, so it saves us the hassle of setting up lights. After we enable light0 we enable lighting. If light0 isn't working on your video card (you see blackness), just disable lighting.

The last line GL_COLOR_MATERIAL lets us add color to texture maps. If we don't enable material coloring, the textures will always be their original color. glColor3f(r,g,b) will have no affect on the coloring. So it's important to enable this.
	glEnable(GL_LIGHT0);								 // Quick And Dirty Lighting (Assumes Light0 Is Set Up)
	glEnable(GL_LIGHTING);								 // Enable Lighting
	glEnable(GL_COLOR_MATERIAL);							 // Enable Material Coloring
Finally we set the perspective correction to look nice, and we return TRUE letting our program know that initialization went OK.
	glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);				 // Nice Perspective Correction
	return TRUE;									 // Initialization Went OK
Now for the drawing code. As usual, I got a little crazy with the math. No SIN, and COS, but it's still a little strange :) We start off as usual by clearing the screen and depth buffer.

Then we bind a texture to the cube. I could have added this line inside the display list code, but by leaving it outside the display list, I can change the texture whenever I want. If I added the line glBindTexture(GL_TEXTURE_2D, texture[0]) inside the display list code, the display list would be built with whatever texture I selected permanently mapped onto it.
int DrawGLScene(GLvoid)									 // Here's Where We Do All The Drawing
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);				 // Clear The Screen And The Depth Buffer
	glBindTexture(GL_TEXTURE_2D, texture[0]);					 // Select The Texture
Now for the fun stuff. We have a loop called yloop. This loop is used to position the cubes on the Y axis (up and down). We want 5 rows of cubes up and down, so we make a loop from 1 to less than 6 (which is 5).
	for (yloop=1;yloop<6;yloop++)							 // Loop Through The Y Plane
We have another loop called xloop. It's used to position the cubes on the X axis (left to right). The number of cubes drawn left to right depends on what row we're on. If we're on the top row, xloop will only go from 0 to 1 (drawing one cube). The next row xloop will go from 0 to 2 (drawing 2 cubes), etc.
		for (xloop=0;xloop<yloop;xloop++)					 // Loop Through The X Plane
We reset our view with glLoadIdentity().
			glLoadIdentity();						 // Reset The View
The next line translates to a specific spot on the screen. It looks confussing, but it's actually not. On the X axis, the following happens:

We move to the right 1.4 units so that the pyramid is in the center of the screen. Then we multiply xloop by 2.8 and add the 1.4 to it. (we multiply by 2.8 so that the cubes are not on top of eachother (2.8 is roughly the width of the cubes when they're rotated 45 degrees). Finally we subtract yloop*1.4. This moves the cubes left depending on what row we're on. If we didn't move to the left, the pyramid would line up on the left side (wouldn't really look a pyramid would it).

On the Y axis we subtract yloop from 6 otherwise the pyramid would be built upside down. Then we multiply the result by 2.4. Otherwise the cubes would be on top of eachother on the y axis (2.4 is roughly the height of each cube). Then we subtract 7 so that the pyramid starts at the bottom of the screen and is built upwards.

Finally, on the Z axis we move into the screen 20 units. That way the pyramid fits nicely on the screen.
			 // Position The Cubes On The Screen
Now we rotate on the x axis. We'll tilt the cube towards the view by 45 degrees minus 2 multiplied by yloop. Perspective mode tilts the cubes automatically, so I subtract to compensate for the tilt. Not the best way to do it, but it works :)

Finally we add xrot. This gives us keyboard control over the angle. (fun to play around with).

After we've rotated on the x axis, we rotate 45 degrees on the y axis, and add yrot so we have keyboard control on the y axis.
			glRotatef(45.0f-(2.0f*yloop)+xrot,1.0f,0.0f,0.0f);		 // Tilt The Cubes Up And Down
			glRotatef(45.0f+yrot,0.0f,1.0f,0.0f);				 // Spin Cubes Left And Right
Next we select a box color (bright) before we actually draw the box portion of the cube. Notice we're using glColor3fv(). What this does is loads all three values (red, green, blue) from inside the {}'s at once and sets the color. 3fv stands for 3 values, floating point, v is a pointer to an array. The color we select is yloop-1 which gives us a different color for each row of the cubes. If we used xloop-1 we'd get a different color for each column.
			glColor3fv(boxcol[yloop-1]);					 // Select A Box Color
Now that the color is set, all we have to do is draw our box. Instead of writing out all the code to draw a box, all we do is call our display list. We do this with the command glCallList(box). box tells OpenGL to select the box display list. The box display list is the cube without its top.

The box will be drawn using the color we selected with glColor3fv(), at the position we translated to.
			glCallList(box);						 // Draw The Box
Now we select a top color (darker) before we draw the top of the box. If you actually wanted to make Q-Bert, you'd change this color whenever Q-Bert jumped on the box. The color depends on the row (yloop-1).
			glColor3fv(topcol[yloop-1]);					 // Select The Top Color
Finally, the only thing left to do is draw the top display list. This will add a darker colored lid to the box. That's it. Very easy!
			glCallList(top);						 // Draw The Top
	return TRUE;									 // Jump Back
The remaining changes have all been made in WinMain(). The code has been added right after our SwapBuffers(hDC) line. It check to see if we are pressing left, right, up or down, and moves the cubes accordingly.
		SwapBuffers(hDC);							 // Swap Buffers (Double Buffering)
		if (keys[VK_LEFT])							 // Left Arrow Being Pressed?
			yrot-=0.2f;							 // If So Spin Cubes Left
		if (keys[VK_RIGHT])							 // Right Arrow Being Pressed?
			yrot+=0.2f;							 // If So Spin Cubes Right
		if (keys[VK_UP])							 // Up Arrow Being Pressed?
			xrot-=0.2f;							 // If So Tilt Cubes Up
		if (keys[VK_DOWN])							 // Down Arrow Being Pressed?
			xrot+=0.2f;							 // If So Tilt Cubes Down
Like all the previous tutorials, make sure the title at the top of the window is correct.
		if (keys[VK_F1])							 // Is F1 Being Pressed?
			keys[VK_F1]=FALSE;						 // If So Make Key FALSE
			KillGLWindow();							 // Kill Our Current Window
			fullscreen=!fullscreen;						 // Toggle Fullscreen / Windowed Mode
			 // Recreate Our OpenGL Window
			if (!CreateGLWindow("NeHe's Display List Tutorial",640,480,16,fullscreen))
				return 0;						 // Quit If Window Was Not Created
By the end of this tutorial you should have a good understanding of how display lists work, how to create them, and how to display them on the screen. Display lists are great. Not only do they simplify coding complex projects, they also give you that little bit of extra speed required to maintain high framerates.

I hope you've enjoy the tutorial. If you have any questions or feel somethings not clear, please email me and let me know.

Jeff Molofee ( NeHe )

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