Jeff Molofee - NeHe's OpenGL Tutorials

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 out 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.