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repeated calls using immediate mode

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I know that people have said that you shouldn't have a call to glBegin and it's corresponding glEnd inside a loop because it slows things down. Exactly how much?
#define LIMIT 1000000

float temp;
echo::util::CHiResTimer timer;

echo::util::array<float> arr;

for(int j=0; j < 20; ++j){


	// so we get just the time taken to render everything inside the loop

	for(int i=0; i < LIMIT; ++i){
			// first point
			glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
			glVertex3f(-0.5f, 1.0f, -3.0f);

			// second point
			glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
			glVertex3f(0.5f, 1.0f, -3.0f);

	temp = timer.getElapsedSeconds();

	printf("\n Time taken for %d lines: %f seconds", LIMIT, temp);



float temp2 = 0.0f;
// print out the average time taken
for(int i=0; i < arr.getSize(); ++i)
	temp2 += arr;

printf("\n Average time taken: %f\n", temp2 / (float)arr.getSize() );

I ran this code with the glBegin and glEnd, inside (as they are now) and also outside the loop. The time with them inside was an average of 0.919 seconds, and the time with them outside was an average of 0.911 seconds. Is it because I'm just drawing a simple line? Is that the reason why there's not much difference? If I were drawing a quad, would inside be slower than outside? Or am I wrong, and there is just no difference whatsoever?

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You are correct in thinking that including the Immediate Mode (glBegin and glEnd ) specification commands inside that kind of loop will slow things down. There is quite a bit of overhead involved with setting up immediate mode (not really that much involved in ending it, I think) ...so break down the loop of your logic a little bit more. ...You specify that you are drawing a line, you submit the points, then you tell OpenGL that you are done drawing lines. Then you specify that you are drawing a line, you submit the points, then you tell OpenGL that you are done drawing lines.

Yes, it would run slower if you were specifying different polygons. There is a lot less overhead in setting up 'GL_LINES point submission' than in 'GL_POLYGON point submission'

...That procedure is kind of counterintuitive to the natural operation of OpenGL. The standard algorithm for doing something like that is to format the points for OpenGL by telling it a poly format. You then submit all of your points that specify objects of that format, then glEnd. There is no reason to start|stop start|stop start|stop start|stop. The 'starts' are expensive. ...Imagine if you refreshed your whole scene like that, especially with any other bottlenecks that might come from implementing other features and you can quickly run into a program that will be impossible to manage.

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Original post by Joakim_ar
I think it's very implementation specific. Ie. If you run that program with another ogl implementation, it may be slower.

Just to expand on this a bit. It really depends on when the implementation decides to validate state. OGL doesn't actually require the implementation to do anything with the data you submitted until you call flush. It is free to store up the data in a buffer somewhere. The main advantage here is reducing overhead by collecting may vertices before submitting them. The implementation may wait until a predefined number of primitives are available, or it may wait until there is a state change, or even until it sees a flush command.

There will probably be some overhead from calling glBegin/end more often than you need to. At the very least you are paying for the function call. However, the ammount of overhead will be dependant on your vendor, and could potentially be very small.

A bigger problem is the overhead of using immediate mode at all. If you are trying to improve the performance of vertex submission, take a look at vertex arrays or VBOs.

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