Hi I followed the tutorial on Swiftless. The last post I worked through was about Keyboard input and it does work. Basically what he covers is how to detect whether a key was pressed or realeased.

So I tried to experiment around. To do so I wanted to make the application to terminate when 'e' is pressed.

#include "glew.h"// Include the GLUT header file  
#include "glut.h" // Include the GLEW header file  

bool* keyStates = new bool[256](); // Create an array of boolean values of length 256 (0-255)
bool* keySpecialStates = new bool[256](); // Create an array of boolean values of length 256 (0-255)
bool* keyPreviousStates = new bool[256]();


void renderPrimitive(void)
{
	glBegin(GL_QUADS); // Start drawing a quad primitive
	glVertex3f(-1.0f,-1.0f,0.0f); //Bottom Left
	glVertex3f(-1.0f,1.0f,0.0f); //Top Left
	glVertex3f(1.0f,1.0f,0.0f); //Top Right
	glVertex3f(1.0f,-1.0f,0.0f); //Bottom Right
	glEnd();
}

void keyOperations(void)
{
	if ((!keyStates['e']) && keyPreviousStates['e']) // If the 'a' key has been pressed 
	{
		// Perform 'e' key operations
		exit(0);
	}
}

void keySpecialOperations(void)
{
	if (keySpecialStates[GLUT_KEY_LEFT]) // If the left arrow key has been pressed
	{
		// Perform left arrow key operations
	}
}

void display(void) {
	keyOperations();
	keySpecialOperations();
	glClearColor(0.706f, 0.706f, 0.706f, 2.0f); // Clear the background of our window to red  
	glClear(GL_COLOR_BUFFER_BIT); //Clear the colour buffer (more buffers later on)  
	glLoadIdentity(); // Load the Identity Matrix to reset our drawing locations  

	glTranslatef(0.0f,0.0f,-5.0f); // Push eveything 5 units back into the scene, otherwise we won't see the primitive
	renderPrimitive(); //render the primitive
	glFlush(); // Flush the OpenGL buffers to the window  
	keyPreviousStates = keyStates;
}

void reshape(int width, int height)
{
	glViewport(0, 0, (GLsizei)width, (GLsizei)height); // Set our viewport to the size of our window. (0,0) being bottom left in the window.
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity(); // Reset the projection matrix to the identity matrix so that we don't get any artifacts (cleaning up)
	gluPerspective(60,(GLfloat)width/(GLfloat)height,1.0,100.0); // Set the Field of view angle (in degrees), the aspect ratio of our window, and the new and far planes
	glMatrixMode(GL_MODELVIEW);
}

void keyPressed(unsigned char key,int x, int y)
{
	keyStates[key] = true; //Set the state of the current key to pressed	
}

void keyUp(unsigned char key,int x, int y)
{
	keyStates[key] = false; //Set the state of the current key to not pressed
}

void keySpecial(int key, int x,int y)
{
	keySpecialStates[key] = true;
}

void keySpecialUp(int key,int x,int y)
{
	keySpecialStates[key] = false;
}

int main(int argc, char **argv) {
	glutInit(&argc, argv); // Initialize GLUT  
	glutInitDisplayMode(GLUT_SINGLE); // Set up a basic display buffer (only single buffered for now)  
	glutInitWindowSize(500, 500); // Set the width and height of the window  
	glutInitWindowPosition(100, 100); // Set the position of the window  
	glutCreateWindow("Your first OpenGL Window"); // Set the title for the window  

	glutDisplayFunc(display); // Tell GLUT to use the method "display" for rendering  
	glutReshapeFunc(reshape); // Tell GLUT to use the method "reshape" for reshaping
	glutKeyboardFunc(keyPressed); // Tell GLUT to use the method "keyPressed" for key presses 
	glutKeyboardUpFunc(keyUp); // Tell GLUT to use the method "keyUp" for key up events  
	glutSpecialFunc(keySpecial); // Tell GLUT to use the method "keySpecial" for special key presses
	glutSpecialUpFunc(keySpecialUp); // Tell GLUT to use the method "keySpecialUp" for special up key events

	glutMainLoop(); // Enter GLUT's main loop  
}

So I changed the keyOperations() code a little. I tried to add another bool-array which stores the state of the keys from the previous display()-iteration. Then I try to check if 'e' is currently released and if its previous state was pressed. But nothing happens

I guess this a very basic thing but I am very new to OpenGL and C++ so I could only think of the way I would implement this in XNA. Btw I am assigning the previous key states at the end of the display function.

Thanks in advance.

After the first run through display() both keyStates and keyPreviousState point to the same memory area. You probably want to use memcpy to copy the memory area pointed to by keyStates to the memory area pointed to by keyPreviousStates, but actually you copy the pointer keyStates to keyPreviousStates.

EDIT: To avoid such problems and clearly denote the situation, you should use const to make the variables read-only, in this case

bool* const keyStates = new bool[256]();
bool* const keyPreviousStates = new bool[256]();

Notice that this does not make the array elements read-only but just the pointers themselves.

After the first run through display() both keyStates and keyPreviousState point to the same memory area. You probably want to use memcpy to copy the memory area pointed to by keyStates to the memory area pointed to by keyPreviousStates, but actually you copy the pointer keyStates to keyPreviousStates.

EDIT: To avoid such problems and clearly denote the situation, you should use const to make the variables read-only, in this case

bool* const keyStates = new bool[256]();
bool* const keyPreviousStates = new bool[256]();

Notice that this does not make the array elements read-only but just the pointers themselves.

So now I declared the pointers as constants and tried the following in order to cpoy the actual array:

memcpy(keyPreviousStates,keyStates, sizeof(keyStates));

Also tried:

memcpy(keyPreviousStates,keyStates, 256 * sizeof(bool));

It still does not work. Any suggestions?

Is the display routine invoked regularly at all? Set a breakpoint into the display() routine and look what happens.

Is the display routine invoked regularly at all? Set a breakpoint into the display() routine and look what happens.

Yes it is, otherwise it would not draw anything I guess. But while stepping through the method something strange happened. When the debugger hit the memcpy() line it VS opened a OpenFileDialog and said that it was missing a memcpy.asm file.

What does that mean it didn't throw any errors when compiling without breakpoints.

Is the display routine invoked regularly at all? Set a breakpoint into the display() routine and look what happens.

Yes it is, otherwise it would not draw anything I guess.

There's between being called once and being called regularly. GLUT will only issue a call to the display function when the contents is invalidated which typically happens when for example, the window is created, the window is resized, or when the window is moved behind other windows. Otherwise, the display callback is not called again unless you explicitly force a redisplay.

So, your display function is called once which explains why something is rendered, but not regularly which explains why nothing more is happening after that. You need to use the idle callback to keep the program busy all the time instead of idling when there's nothing new to display.

Quick solution:

void idle()
{
    glutPostRedisplay();
}
 
int main()
{
    ...
    glutIdleCallback(idle);
    glutMainLoop();
}
As soon as there's nothing else for GLUT to do, it calls the idle callback which then forces GLUT to redraw the window.

But while stepping through the method something strange happened. When the debugger hit the memcpy() line it VS opened a OpenFileDialog and said that it was missing a memcpy.asm file.

What does that mean it didn't throw any errors when compiling without breakpoints.

You're stepping into the memcpy call but the debugger can't find the source for the memcpy function. Could be that the source directories are not set up properly or you haven't installed the runtime library source. Quick solution is to step over the call, and into it.

There's between being called once and being called regularly. GLUT will only issue a call to the display function when the contents is invalidated which typically happens when for example, the window is created, the window is resized, or when the window is moved behind other windows. Otherwise, the display callback is not called again unless you explicitly force a redisplay.



So, your display function is called once which explains why something is rendered, but not regularly which explains why nothing more is happening after that. You need to use the idle callback to keep the program busy all the time instead of idling when there's nothing new to display.



Quick solution:

void idle()
{
glutPostRedisplay();
}

int main()
{
...
glutIdleCallback(idle);
glutMainLoop();
}
As soon as there's nothing else for GLUT to do, it calls the idle callback which then forces GLUT to redraw the window.

Hey this advice did the job! Maybe two more things I noticed here:

1. I implemented a concole application which copied one array to another under the very same circumstances and it did work.
2. I had to assign the idle()-function using glutIdleFunc() not glutIdleCallback() (this one wasn't even suggested by VS).

But now it works thanks a lot. You were right coming from XNA I assumed that display here would be somehow similar to the Draw() function which is called in regular intervals.

I had to assign the idle()-function using glutIdleFunc() not glutIdleCallback() (this one wasn't even suggested by VS).

My mistake, glutIdleFunc is of course correct.