The code was modified once again to address problems that some video cards had with glDepthMask(). It seems this command would not effectively enable and disable depth buffer testing on some cards, so I've changed back to the old fashioned glEnable and Disable of Depth Testing.
The alpha value that is used for transparency can be read from a texture map just like color, to do this, you will need to get alpha into the image you want to load, and then use GL_RGBA for the color format in calls to glTexImage2D().
If you have any questions, feel free to contact me at stanis@cs.wisc.edu.
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Lesson 09
Welcome to Tutorial 9. By now you should have a very good understanding of OpenGL. You've learned everything from setting up an OpenGL Window, to texture mapping a spinning object while using lighting and blending. This will be the first semi-advanced tutorial. You'll learn the following: Moving bitmaps around the screen in 3D, removing the black pixels around the bitmap (using blending), adding color to a black & white texture and finally you'll learn how to create fancy colors and simple animation by mixing different colored textures together.
We'll be modifying the code from lesson one for this tutorial. We'll start off by adding a few new variables to the beginning of the program. I'll rewrite the entire section of code so it's easier to see where the changes are being made.
#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
The following lines are new. twinkle and tp are BOOLean variables meaning they can be TRUE or FALSE. twinkle will keep track of whether or not the twinkle effect has been enabled. tp is used to check if the 'T' key has been pressed or released. (pressed tp=TRUE, relased tp=FALSE).
BOOL twinkle; // Twinkling Stars
BOOL tp; // 'T' Key Pressed?
num will keep track of how many stars we draw to the screen. It's defined as a CONSTant. This means it can never change within the code. The reason we define it as a constant is because you can not redefine an array. So if we've set up an array of only 50 stars and we decided to increase num to 51 somewhere in the code, the array can not grow to 51, so an error would occur. You can change this value to whatever you want it to be in this line only. Don't try to change the value of num later on in the code unless you want disaster to occur.
const num=50; // Number Of Stars To Draw
Now we create a structure. The word structure sounds intimidating, but it's not really. A structure is a group simple data (variables, etc) representing a larger similar group. In english :) We know that we're keeping track of stars. You'll see that the 7th line below is stars;. We know each star will have 3 values for color, and all these values will be integer values. The 3rd line int r,g,b sets up 3 integer values. One for red (r), one for green (g), and one for blue (b). We know each star will be a different distance from the center of the screen, and can be place at one of 360 different angles from the center. If you look at the 4th line below, we make a floating point value called dist. This will keep track of the distance. The 5th line creates a floating point value called angle. This will keep track of the stars angle.