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radioact1ve

OpenGL Points jumping...

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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]

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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.

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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 matrix
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();

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So your code is:

glMatrixMode(GL_PROJECTION); // set projection matrix current matrix
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();


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();

// Or

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();




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 lighting
glDisable(GL_CULL_FACE); // No face culling
glBegin(GL_TRIANGLES)
glColor3f(1.0, 0.0, 0.0); // Red
glVertex3f(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);

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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

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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.

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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.

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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).

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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?)

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