Game Loop involving GLUT

[camelCase]

So I tried to use GLUT and played around with it and I'm rather satisfied with the experience.
(Yeah, I only took action now, for those who know or remember that I was going to try it)

However, I was wondering how my game loop would look like.
I've got a rough idea but wanna' see if I've got it right before I use it.

int main (int arg, char *argv[]) {
	//Initialize GLUT

	//Set some callbacks
	glutDisplayFunc(render);
	glutReshapeFunc(reshape);
	glutKeyboardFunc(key);
	glutSpecialFunc(sKey);

	//Enable 2D Texture, Blend..

	glutMainLoop();
}

void render () {
	//clear
		//Umm..
		//Thinking of looping through a vector of 'Sprite' classes and displaying them.
		//Sprites are just primitives (GL_QUADS) with a texture on them.
	//swap buffer
	//Timer to call display thingy
	//AI, Movement, controls, etc.
}

Did I get it right? =/
Also, I didn't know how to load images so I found a very old snippet off the internet and updated it for C++.

Anyways, I have a question about textures, image loading and the drawing process.
This is what I *think* I should be doing:

01) Load image only once (for each sprite that uses a different image); should get an array of pixels
02) For each call to function render()
	a) Loop through a vector of 'Sprite' classes
	b) For each 'Sprite' instance do the following:
		glBindTexture(GL_TEXTURE_2D, someID);
		glColor4f(1.0f, 1.0f, 1.0f, someAlpha);
		glBegin(GL_QUADS);
			glTexCoord2f(0.0f, 0.0f);
			glVertex2f(/*Insert co-ords here*/);
			glTexCoord2f(1.0f, 0.0f);
			glVertex2f(/*Insert co-ords here*/);
			glTexCoord2f(1.0f, 1.0f);
			glVertex2f(/*Insert co-ords here*/);
			glTexCoord2f(0.0f, 1.0f);
			glVertex2f(/*Insert co-ords here*/);
		glEnd();

I'm having doubts because in my test (of only one sprite; there's no 'Sprite' class yet),
I did this:
01) load bmp file in main()
02) glBindTexture(GL_TEXTURE_2D, someID); in main()
03) draw one GL_QUADS primitive in render loop

The result is that the texture is still on the primitive despite me only calling 'glBindTexture' once in the main function.
Like, the GL_QUADS primitive is drawn multiple times (even rotated and moved about for fun) but the glBindTexture was only called once.

 

[saw792]

The binding of glBindTexture is persistent for the life of the program. That is, if you call it once then every primitive with texture coordinates that you draw will use that texture until you change it.

The OpenGL environment is a big state machine. Most/all gl calls you make change the state of the underlying machine, making the change apply to every operation you call henceforth. This means that if you bind a texture, do a translation/rotation/whatever operation, or do any number of other things they will apply to every object you draw afterwards. This is useful because you can, say, decide that you want to draw a cube up in the air and only call glTranslate once before drawing all the quads.

As far as your drawing goes, look into drawing everything using triangles/triangle strips instead of quads and store all the drawing functions in display lists. This reduces your drawing code to glCallList(your_display_list) for every object/set of objects you want to draw. This is useful because you can call translation and rotation operations before you draw the list and it will apply those transformations to the drawn objects. In the future you will want to investigate vertex arrays and vertex buffer objects.

Anyway I've probably given you too much information for now so let me know how you go.

 

[camelCase]

I looked into display lists but they couldn't, like, handle values that changed constantly.
So I looked into vertex arrays and they work well.
(Can't say I read all the theory and stuff and understood it)

//qArr's values change constantly; just testing stuff
GLfloat qArr[][2] = {
	{-320, -240},
	{-320, 240},
	{320, 240},
	{320, -240}
};

void render () {
	glVertexPointer(2, GL_FLOAT, 0, qArr);
	glDrawArrays(GL_QUADS, 0, 4);
}
int main () {
	//Other bits of code
	glutDisplayFunc(render);
	glEnableClientState(GL_VERTEX_ARRAY);
	return 0;
}

I didn't use triangle strips yet =x
Anyways, if I want to have, like, a lot of quads (Or quads drawn with triangle strips) that can move independant of each other,
will I have to repeat the glVertexPointer(), glDrawArrays() code for each quad?

And it works and all but it seems to have broken the method I use to apply textures to primitives =/
glBindTexture(GL_TEXTURE_2D, id); doesn't work anymore.

[EDIT]
Woops, a little bit of googling helped.
glEnableClientState (GL_TEXTURE_COORD_ARRAY_EXT); <-- Had to call that

[EDIT]
Is it possible to change the color diffusion of a texture?
Calling glColor4f() and adjusting the alpha argument adjusts the texture's transparency but changing the RGB values doesn't affect how the texture's colors are affected.

 

[saw792]

The whole point of display lists is to conserve the shape of the objects you are drawing. Unless you are actually distorting the shape you can use them in conjunction with glScale/glTranslate/glRotate operations to actually put them where you want them and move them. To take advantage of this you really need to learn about the available OpenGL matrices, specifically the ModelView matrix. These matrices store the current state of the environment including things like the current rotation amount, translation amount, scaling amount etc. to be applied to every object. Do some research on OpenGL transformations and you will find information about all of these things.

Vertex Buffer Objects are another step beyond vertex arrays which can boost your rendering speeds dramatically beyond vertex arrays/display lists by storing the vertex data in the graphics card memory. Once again these things can/should be manipulated with the transformation operations so you don't have to change coordinates every time you want to draw.

I'll get back to you about the color diffusion issue.

 

[camelCase]

So..
Since I'm creating a 2D game where every thing is just a rectangle-plane with a texture on it..
I just need to create a display list for a triangle-strip, right?

Then, when I need to draw, say, 80 or more different things on the screen, I just do this:

	//for each 'sprite'..
		glPushMatrix();
		glTranslatef(/*coords*/);
		glRotatef(/*angle, coords*/);
		glScalef(/*vector*/);
		glBindTexture(GL_TEXTURE_2D, /*some unsigned int*/);
		glCallList(1); //Since I only have one list
		glPopMatrix();
	//endloop

Right?

[EDIT]
This is what I'm using for textures now (I took it off the web):

/*	
*/
void Image::genTexture () {
	glGenTextures(1, &id);
	glBindTexture(GL_TEXTURE_2D, id);
	glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
	glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);
	glTexImage2D(GL_TEXTURE_2D, 0, type, w, h, 0, type, GL_UNSIGNED_BYTE, i.get());
	i.reset();
}

So, every time I load an Image, that genTexture() method is called.
Anyways..
How does glTexImage2D() know that when I call glBindTexture() again, I want to load whatever was copied with glTexImage2D() ?
I didn't pass an 'id' of any sort =/

Or has this got to do with OpenGL being a state machine? o.0

 

[saw792]

Pretty much right about the basic drawing display lists/transformations part.

You're right about the state machine being the explanation for your second question. When you call glBindTexture() it binds the current GL_TEXTURE_2D name to your id. This means any operations involving GL_TEXTURE_2D affect the id/memory you specified in the glBindTexture() call. So yes, in future when you call glBindTexture() to prepare to draw something it will contain the information you saved earlier using glTexImage2D() for that particular id.

 

[camelCase]

Thanks, saw =)