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Points jumping...

OpenGL
Started by radioact1ve Nov 27, 2005 at 1:25 AM 11 replies 1.1k views
Original Post
radioact1ve
radioact1ve
Hey everyone! I'm getting some weird results with very basic ogl code. It's all samples from "Beginning OpenGL GP" book. I'm just testing the water, VERY basic stuff. Basically: glBegin(GL_POINTS) glVertex3f(0.0, 0.0, 0.0); glVertex3f(1.0, 0.0, 0.0); glVertex3f(2.0, 0.0, 0.0); glEnd(); This does what I expect, three points: . . . But when I do this, change a 'y' value: glBegin(GL_POINTS) glVertex3f(0.0, 0.0, 0.0); glVertex3f(1.0, 3.0, 0.0); glVertex3f(2.0, 0.0, 0.0); glEnd(); results: (Ignore the '-' I couldn't get the spacing to work so I just put those in to show what the points are doing.) -. . . Something like that. For some reason, it jumps away from the previous 'x' spot (the middle). Shouldn't it be doing this: --. .--- . As I said, I'm totally new to ogl, so I'm going over the code from the book like crazy (just chapter 3 so I might be missing something), looking for some state/setting that could be causing this. Can anyone think of any? Or would it be better if I post the code? Thanks a lot everyone!! Greatly appreciate it. Just in case for those that have the book. I'm messing with the code from Chapter 3 pg. 46. [Edited by - radioact1ve on November 27, 2005 1:42:02 AM]
Seroja
Seroja
Just a guess, but how have you setup your modelview and prjection matrices?
(e.g. ortho or frustum/perspective? Any glRotate/glTranslate?)
Although quite unlikely, you could get such results after a couple of transformations.
radioact1ve
radioact1ve
Hey Seroja,

Thanks for the reply. In regard to those functions, this all I could find in the code. Of course I'm not really familiar with this, so I have no idea whats going on. 8)

glMatrixMode(GL_PROJECTION);	// set projection matrix current matrixglLoadIdentity();		// reset projection matrix// calculate aspect ratio of windowgluPerspective(52.0f,(GLfloat)width/(GLfloat)height,1.0f,1000.0f);glMatrixMode(GL_MODELVIEW);	// set modelview matrixglLoadIdentity();
Seroja
Seroja
So your code is:
glMatrixMode(GL_PROJECTION);	// set projection matrix current matrixglLoadIdentity();		// reset projection matrix// calculate aspect ratio of windowgluPerspective(52.0f,(GLfloat)width/(GLfloat)height,1.0f,1000.0f);glMatrixMode(GL_MODELVIEW);	// set modelview matrixglLoadIdentity();glBegin(GL_POINTS)glVertex3f(0.0, 0.0, 0.0);glVertex3f(1.0, 0.0, 0.0);glVertex3f(2.0, 0.0, 0.0);glEnd();// OrglBegin(GL_POINTS)glVertex3f(0.0, 0.0, 0.0);glVertex3f(1.0, 3.0, 0.0);glVertex3f(2.0, 0.0, 0.0);glEnd();


It is strange that you see anything at all. Do you have any more setup code?
You draw the points at 0 Z. But your near plane is 1. You should not see anything. (Never tried points though, always used polygons)

How about trying to to make it a triangle?

glDisable(GL_LIGHTING); // No lightingglDisable(GL_CULL_FACE); // No face cullingglBegin(GL_TRIANGLES)glColor3f(1.0, 0.0, 0.0); // RedglVertex3f(0.0, 0.0, 0.0);glVertex3f(1.0, 3.0, 0.0);glVertex3f(2.0, 0.0, 0.0);glEnd();


I wonder whether you'll see your triangle at all.

If not, return to points, and add before the glBegin:
glTranslatef(0.0, 0.0, -5.0);
radioact1ve
radioact1ve
Oh no no... sorry! :) That's not how the code is. 

Render() // My Render function{	float pointSize = 0.5;	//float yPos = -15.0;        // clear screen and depth buffer	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);   	glLoadIdentity();	gluLookAt(0.0, 6.0, 0.1, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);	// draw a line of points of increasing size	for (float point = -4.0; point < 5.0; point+=0.5)	{		// set the point size		glPointSize(pointSize);				// draw the point		glBegin(GL_POINTS);			//glVertex3f(point, 5.0, 0.0);			glVertex3f(point, 0.0, 0.0);		glEnd();		// increase the point size for the next point		// yPos += 1.0;		pointSize += 1.0;	}}


And then a SetupProjection()
SetupProjection(int width, int height){	if (height == 0)	// don't want a divide by zero	{		height = 1;						}         // reset the viewport to new dimensions	glViewport(0, 0, width, height);          	// set projection matrix current matrix 	glMatrixMode(GL_PROJECTION);			glLoadIdentity();		// reset projection matrix	// calculate aspect ratio of window	gluPerspective(52.0f,(GLfloat)width/(GLfloat)height,1.0f,1000.0f);	glMatrixMode(GL_MODELVIEW);	// set modelview matrix	glLoadIdentity();		// reset modelview matrix	m_windowWidth = width;	m_windowHeight = height;}


As for that triangle, I got a really small/weird red right triangle. But I did see something
Seroja
Seroja
Quote:
Original post by radioact1ve
Oh no no... sorry! :) That's not how the code is.

*** Source Snippet Removed ***

And then a SetupProjection()
*** Source Snippet Removed ***

As for that triangle, I got a really small/weird red right triangle. But I did see something


Well now that you say you got gluLookAt it starts making sense.
I never used it so I can't tell you what to do, but experimenting a bit I think it is that call that is causing you the trouble. After transformations, seems like the Y axis that you are trying to change isn't Y axis anymore.

Try to remove that call, and instead translate back a bit to see what you're drawing.
CRACK123
CRACK123
Hi,

Can you post the whole code itself ?
The more applications I write, more I find out how less I know
Seroja
Seroja
You managed to get me confused about transformations for a whole 15 minutes :P
OK, so you are trying to render this:
-O-
O-O
Right?

Then don't do gluLookAt. Think of it as a drawing canvas. By calling gluLookAt you move the canvas. But instead of moving it away from you a bit just to see what is drawn, you also rotate it. Thus, Y axis as you think of it is now not really Y, but a different one. I don't want to get into gluLookAt to see exactly what transformations it does, but I'm 99% certain that you are drawing on a canvas that is not facing you (!) and thus get unusual results.

Use the following:

Render() // My Render function{	float pointSize = 0.5;	//float yPos = -15.0;        // clear screen and depth buffer	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);   	glLoadIdentity();	//gluLookAt(0.0, 6.0, 0.1, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);	glTranslatef(0.0, 0.0, -6.0);	// draw a line of points of increasing size	for (float point = -4.0; point < 5.0; point+=0.5)	{		// set the point size		glPointSize(pointSize);				// draw the point		glBegin(GL_POINTS);			//glVertex3f(point, 5.0, 0.0);			glVertex3f(point, point/2, 0.0);		glEnd();		// increase the point size for the next point		// yPos += 1.0;		pointSize += 1.0;	}}


This will draw the points in diagonal-up. You can later change the code to draw only one of the points a bit above.
CRACK123
CRACK123
I would advise you to read up gluLookAt and understand it. However I think what you want is gluLookAt(0, 0, 6, 0, 0, 0, 0, 1, 0); This should give the same effect as glTranslatef(0, 0, -6).
The more applications I write, more I find out how less I know
Seroja
Seroja
Quote:
Original post by CRACK123
I would advise you to read up gluLookAt and understand it. However I think what you want is gluLookAt(0, 0, 6, 0, 0, 0, 0, 1, 0); This should give the same effect as glTranslatef(0, 0, -6).


Were you talking to me, saying to read about gluLookAt? If so, I don't need it. I prefer Rotate & Translate, that way I'm sure what I'm doing.

And if I understand correctly, gluLookAt(0, 0, 6, 0, 0, 0, 0, 1, 0) will actually move the camera forward 6 and reverse it. glTranslatef(0, 0, -6) moves the camera backward.
I could be wrong, but I think the correct one would be gluLookAt(0, 0, -6, 0, 0, 0, 0, 1, 0). (Why not use Translate?)
CRACK123
CRACK123
Quote:
Original post by Seroja
Quote:
Original post by CRACK123
I would advise you to read up gluLookAt and understand it. However I think what you want is gluLookAt(0, 0, 6, 0, 0, 0, 0, 1, 0); This should give the same effect as glTranslatef(0, 0, -6).


Were you talking to me, saying to read about gluLookAt? If so, I don't need it. I prefer Rotate & Translate, that way I'm sure what I'm doing.

And if I understand correctly, gluLookAt(0, 0, 6, 0, 0, 0, 0, 1, 0) will actually move the camera forward 6 and reverse it. glTranslatef(0, 0, -6) moves the camera backward.
I could be wrong, but I think the correct one would be gluLookAt(0, 0, -6, 0, 0, 0, 0, 1, 0). (Why not use Translate?)



I was talking to the actual poster. And gluLookAt does it the opposite way of how you described it. Nothing wrong in using Translate. But eventually you will have a look at code to set up your camera wether you use gluLookAt or roll your own version of it. So its a good idea to understand it as well. Even better to see what it does under the hood but that can be done later.

To be technically correct glTranslate moves the axis and not a camera. For something as simple as this gluLookAt is not necessary but it has its usefulness.
The more applications I write, more I find out how less I know
deavik
deavik
radioact1ve, your gluLookAt problem should be easy to explain:

Your original code was: 
gluLookAt(0.0, 6.0, 0.1, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);

The first three params are the (x, y, z) position of the camera. The next 3 are the (x, y, z) of the point you are looking at. Hence in your code the camera is roughly 6 units on the positive y-axis looking down at the origin (round-about where you are drawing your funky stuff). So far so good.

(The next 3 params denote the up-vector which is quite what it is named, by the way)

Now think about what is happenning--your camera is positioned on the y-axis, and your points are all on the x-y plane. You will perceive the difference between points at diferent x positions, but points on the same y-coordinates will look almost superimposed (at (0, 0, 0) they will be superimposed but not quite at other x-cordinates because the projection is non-orthographic).

So, the simple solution (as has been posted already) is to position your camera on the z-axis.
gluLookAt(0.0, 0.0, 6.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);


And that's it! Read the related chapter of the red book [here] to get a clearer picture.
radioact1ve
radioact1ve
Quote:
Original post by Seroja
You managed to get me confused about transformations for a whole 15 minutes :P


lol my bad!!

Your code worked great! That's exactly what I wanted to do. Not sure what I did or how, but I can take care of that by doing a little more reading. Definitely look into all the suggestions put in, especially LookAt and Translate.

Thxs everyone for your help!! Totally kick ass! Got to start reading.



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