Jeff Molofee - NeHe's OpenGL Tutorials

Calling gluPerspective() multiplies the perspective matrix by the pick matrix which restricts the drawing to the area requested by gluPickMatrix().

We then switch to the modelview matrix and draw our targets by calling DrawTargets(). We draw the targets in DrawTargets() and not in Draw() because we only want selection to check for hits with objects (targets) and not the sky, ground or crosshair.

After drawing our targets, we switch back to the projection matrix and pop the stored matrix off the stack. We then switch back to the modelview matrix.

The last line of code below switches back to render mode so that objects we draw actually appear on the screen. hits will hold the number of objects that were rendered in the viewing area requested by gluPickMatrix().

// Apply The Perspective Matrix

gluPerspective(45.0f, (GLfloat) (viewport[2]-viewport[0])/(GLfloat) (viewport[3]-viewport[1]), 0.1f, 100.0f);

glMatrixMode(GL_MODELVIEW); // Select The Modelview Matrix

DrawTargets(); // Render The Targets To The Selection Buffer

glMatrixMode(GL_PROJECTION); // Select The Projection Matrix

glPopMatrix(); // Pop The Projection Matrix

glMatrixMode(GL_MODELVIEW); // Select The Modelview Matrix

hits=glRenderMode(GL_RENDER); // Switch To Render Mode, Find Out How Many

Now we check to see if there were more than 0 hits recorded. if so, we set choose to equal the name of the first object drawn into the picking area. depth holds how deep into the screen, the object is.

Each hit takes 4 items in the buffer. The first item is the number of names on the name stack when the hit occured. The second item is the minimum z value of all the verticies that intersected the viewing area at the time of the hit. The third item is the maximum z value of all the vertices that intersected the viewing area at the time of the hit and the last item is the content of the name stack at the time of the hit (name of the object). We are only interested in the minimum z value and the object name in this tutorial.

if (hits > 0) // If There Were More Than 0 Hits

{

int choose = buffer[3]; // Make Our Selection The First Object

int depth = buffer[1]; // Store How Far Away It Is

We then loop through all of the hits to make sure none of the objects are closer than the first object hit. If we didn't do this, and two objects were overlapping, the first object hit might behind another object, and clicking the mouse would take away the first object, even though it was behind another object. When you shoot at something, the closest object should be the object that gets hit.

So, we check through all of the hits. Remember that each object takes 4 items in the buffer, so to search through each hit we have to multiply the current loop value by 4. We add 1 to get the depth of each object hit. If the depth is less than the the current selected objects depth, we store the name of the closer object in choose and we store the depth of the closer object in depth. After we have looped through all of our hits, choose will hold the name of the closest object hit, and depth will hold the depth of the closest object hit.

for (int loop = 1; loop < hits; loop++) // Loop Through All The Detected Hits

{

// If This Object Is Closer To Us Than The One We Have Selected

if (buffer[loop*4+1] < GLuint(depth)) {

choose = buffer[loop*4+3]; // Select The Closer Object

depth = buffer[loop*4+1]; // Store How Far Away It Is

}

}

All we have to do is mark the object as being hit. We check to make sure the object has not already been hit. If it has not been hit, we mark it as being hit by setting hit to TRUE. We increase the players score by 1 point, and we increase the kills counter by 1.

if (!object[choose].hit) // If The Object Hasn't Already Been Hit

{

object[choose].hit=TRUE; // Mark The Object As Being Hit

score+=1; // Increase Score

kills+=1; // Increase Level Kills

I use kills to keep track of how many objects have been destroyed on each level. I wanted each level to have more objects (making it harder to get through the level). So I check to see if the players kills is greater than the current level multiplied by 5. On level 1, the player only has to kill 5 objects (1*5). On level 2 the player has to kill 10 objects (2*5), progressively getting harder each level.

So, the first line of code checks to see if kills is higher than the level multiplied by 5. If so, we set miss to 0. This sets the player morale back to 10 out of 10 (the morale is 10-miss). We then set kills to 0 (which starts the counting process over again).

Finally, we increase the value of level by 1 and check to see if we've hit the last level. I have set the maximum level to 30 for the following two reasons… Level 30 is insanely difficult. I am pretty sure no one will ever have that good of a game. The second reason… At the top of the code, we only set up 30 objects. If you want more objects, you have to increase the value accordingly.

It is VERY important to note that you can have a maximum of 64 objects on the screen (0-63). If you try to render 65 or more objects, picking becomes confused, and odd things start to happen. Everything from objects randomly exploding to your computer crashing. It's a physical limit in OpenGL (just like the 8 lights limit).

If by some chance you are a god, and you finish level 30, the level will no longer increase, but your score will. Your morale will also reset to 10 every time you finish the 30th level.

if (kills>level*5) // New Level Yet?

{

miss=0; // Misses Reset Back To Zero

kills=0; // Reset Level Kills

level+=1; // Increase Level

if (level>30) // Higher Than 30?

level=30; // Set Level To 30 (Are You A God?)

}

}

}

}

Update() is where I check for key presses, and update object movement. One of the nice things about Update() is the milliseconds timer. You can use the milliseconds timer to move objects based on the amount of time that has passed since Update() was last called. It's important to note that moving object based on time keeps the objects moving at the same speed on any processor… BUT there are drawbacks! Lets say you have an object moving 5 units in 10 seconds. On a fast system, the computer will move the object half a unit every second. On a slow system, it could be 2 seconds before the update procedure is even called. So when the object moves, it will appear to skip a spot. The animation will not be as smooth on a slower system. (Note: this is just an exaggerated example… computers update ALOT faster than once every two seconds).

Anyways… with that out of the way… on to the code. The code below checks to see if the escape key is being pressed. If it is, we quit the application by calling TerminateApplication(). g_window holds the information about our window.

void Update(DWORD milliseconds) // Perform Motion Updates Here

{

if (g_keys->keyDown[VK_ESCAPE]) // Is ESC Being Pressed?

{

TerminateApplication (g_window); // Terminate The Program

}

The code below checks to see if the space bar is pressed and the game is over. If both conditions are true, we initialize all 30 object (give them new directions, textures, etc). We set game to FALSE, telling the program the game is no longer over. We set the score back to 0, the level back to 1, the player kills to 0 and finally we set the miss variable back to zero. This restarts the game on the first level with full morale and a score of 0.