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Perspective

http://elysianshadows.com/phpBB3/viewtopic.php?t=6659

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Perspective

Posted: Fri Jan 13, 2012 4:17 pm
by Benjamin100
OK, So I'm trying to use perspective so that things further back look smaller.

I use glFrustum and set up some locations
but when I when I move a square back it just stays the same size until it disappears.

Is there some way to enable perspective?

Re: Perspective

Posted: Fri Jan 13, 2012 4:59 pm
by dandymcgee
Benjamin100 wrote:OK, So I'm trying to use perspective so that things further back look smaller.

I use glFrustum and set up some locations
but when I when I move a square back it just stays the same size until it disappears.

Is there some way to enable perspective?
Have you tried reading a tutorial? I know I've already linked you there once, but you should really check out http://nehe.gamedev.net/.

Re: Perspective

Posted: Fri Jan 13, 2012 11:23 pm
by Ginto8
glFrustrum, and perspective in general, have a tendency to be bitches. Between the two, however, I would recommend using gluPerspective over glFrustrum because it makes at least setting up the perspective less of a pain in the ass.

Also, always post relevant code if possible. We are programmers, not mind readers, so when you have a problem and say "I did x and some stuff and it doesn't work", the only clues we have are that either x or the "some stuff" caused a problem. Unfortunately, calling glFrustrum can be done multiple ways, and if you didn't call it correctly it won't work how you want it to; also, "setting up some locations" is so vague that it could mean any number of things, from simply drawing some vertices to actually doing translations, or who knows what else.

Perspective, and 3D in general, is a big beast, and you have to be careful. You have to have depth testing (unless you're going to sort all your faces before rendering), you need to make sure your perspective is set up correctly, and you may even have to deal with the pain in the ass of alpha blending. You need to have an idea of what you're doing going in, or at least you need to follow the example of someone who does. For that, I defer to dandy's suggestion of Nehe, because those tutorials are extremely useful.

Re: Perspective

Posted: Sat Jan 14, 2012 7:46 am
by Pornomag
Umm use Depth Testing?

Code: Select all

glEnable(GL_DEPTH_TEST);
and you have to clear that buffer too so:

Code: Select all

glClear(GL_DEPTH_BUFFER_BIT);

// you can use it in one function like so:
glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
If it's still the same, see if you're transformations and such are ok? S:?

Just realised that the above post already wrote this -.-

Re: Perspective

Posted: Sun Jan 15, 2012 7:22 pm
by Benjamin100
OK.
Here is what the function I want to render and move the blue triangle back...

Code: Select all

void rect_display(void)   
{
	glClearColor(0, 0, 0, 0);    
	glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);  
	glMatrixMode(GL_MODELVIEW);

    
	glPushMatrix();
	glTranslatef(0, 0,translationAmount);
	translationAmount+=0.0001;
	glBegin(GL_TRIANGLES);
	     glColor3f(0.0, 0.0, 1.0);  //front.
		     glVertex3f(0.3, 0.8, 0.2);
			 glVertex3f(0.3, 0.2, 0.2);
			 glVertex3f(0.8, 0.8, 0.2);
	
    glEnd();
	glPopMatrix();


	
	glFlush();
	glutSwapBuffers();
	glutPostRedisplay();
	
}
gluPerspective is set at "0.0, 2.0, 0.1, 100.0"

Apparently the first parameter is supposed to be an angle. But when I set it above 0 it seems the triangle will not appear on the screen. "GL_DEPTH_TEST" is enabled, and so is the buffer. Yet it doesn't appear to move back.
Do I HAVE to use perspective AND frustum?

EDIT:Excuse the function name, it was originally a rectangle.

Re: Perspective

Posted: Sun Jan 15, 2012 7:27 pm
by Benjamin100
It seems to disappear at a certain point now, so I think it IS moving back, but it doesn't change size.

Re: Perspective

Posted: Sun Jan 15, 2012 7:43 pm
by Ginto8
Post the code where you set up your projection matrix, because that seems to be the issue. You shouldn't be using both glFrustrum and gluPerspective, and it seems weird to me that calling gluPerspective with a fov angle of 0 would even work, although if you are calling both it might just be that it does nothing and yields to your glFrustrum call.

For future reference, the fov angle is the field-of-view angle, and higher values create a "fisheye lense" effect. Usually 45 is a decent setting for fov, but tweaking can yield more natural-seeming results

Re: Perspective

Posted: Sun Jan 15, 2012 8:01 pm
by Benjamin100
OK. I've been fooling around with it some more.
I added in glOrtho, which I didn't have before because I didn't know if it could work with gluPerspective. Apparently it can.
Still, I've been having problems...

Code: Select all

void initialize_it(void)   //sets up graphic stuff.
{
	glClearColor(0.0, 0.0, 0.0, 0.0);
	glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
	glutInitWindowSize(400,200);
	glutInitWindowPosition(100,50);
	glutCreateWindow("Moving Square");
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	glOrtho(0.0,1.0,0.1,1.0,-1.0,1.0);
	gluPerspective(0.0f,2.0f,0.1f,100.0f);
	glEnable(GL_DEPTH_TEST);
	glDepthFunc(GL_LEQUAL);
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();

}

Still not understanding what "load identity" does, even after looking at tutorials.

Re: Perspective

Posted: Sun Jan 15, 2012 8:03 pm
by Benjamin100
If it helps, I could clarify that putting the fov at almost ANYTHING above 0 seems to make it disappear.
Even 0.0001.

Thanks,

-Benjamin

Re: Perspective

Posted: Sun Jan 15, 2012 8:31 pm
by Ginto8
Well, I see a couple things:
a) glClearColor should be after glutCreateWindow(), because until the window is created, you have no clue if there's a valid context to use.
b) glOrtho should not be there. Using orthographic and perspective projection at the same time will not behave like you think it does.

and regarding glLoadIdentity(), it sets the currently selected matrix to the "identity matrix":
Image

Re: Perspective

Posted: Mon Jan 16, 2012 12:01 am
by Benjamin100
OK, Thanks.

Still not moving the triangle, not sure why. That's basically it along with the main function, which initializes, displays, and then the glut main loop.

Re: Perspective

Posted: Mon Jan 16, 2012 12:08 pm
by Ginto8
Well it's gotten to the point that the only way I can help you is to have the whole program. I can't see anything majorly wrong with what's been posted, as long as you fixed what I pointed out.

Re: Perspective

Posted: Mon Jan 16, 2012 12:55 pm
by Benjamin100
header

Code: Select all

#include <GL/glut.h>
#include <GL/GLU.h>
#include <GL/GL.h>


void initialize_it(void)  
{
	glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
	glutInitWindowSize(400,200);
	glutInitWindowPosition(100,50);
	glutCreateWindow("Moving Square");
	glClearColor(0.0, 0.0, 0.0, 0.0);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluPerspective(0.0f,2.0f,0.1f,100.0f);
	glEnable(GL_DEPTH_TEST);
	glDepthFunc(GL_LEQUAL);
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();

}
cpp

Code: Select all

(include files)

float translationAmount=0.0;


void rect_display(void)   
{
	glClearColor(0, 0, 0, 0);    
	glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);  
	glMatrixMode(GL_MODELVIEW);


	glPushMatrix();
	glTranslatef(0, 0,translationAmount);
	translationAmount+=0.0001;
	glBegin(GL_TRIANGLES);
	     glColor3f(0.0, 0.0, 1.0);  
		     glVertex3f(0.3, 0.8, 0.2);
			 glVertex3f(0.3, 0.2, 0.2);
			 glVertex3f(0.8, 0.8, 0.2);
	
    glEnd();
	glPopMatrix();


	
	glFlush();
	glutSwapBuffers();
	glutPostRedisplay();
	
}





int main(int argc,char** argv)     
{
	glutInit(&argc,argv);    
	initialize_it();     
	glutDisplayFunc(rect_display); 
	glutMainLoop();   

return 0;
}

Re: Perspective

Posted: Mon Jan 16, 2012 5:52 pm
by szdarkhack
Your frustum's near plane is 0.1 unit away from the camera. Since you do NOT change the view matrix, the camera is positioned at 0 and looking down the NEGATIVE z axis. Thus, the vertices you have defined are *behind* your view. Set a normal FoV and either flip the vertices' z coordinate to negative or translate the camera back along the positive z axis.

When i started learning OpenGL i also had a similar confusing moment. Remember 2 things:
1) By default (with the identity modelview matrix) you are looking towards the NEGATIVE half of the z axis.
2) The near and far values in gluPerspective() are *distances*, not locations. No matter how you rotate your view, these are always positive and signify how far away
from the camera you start and stop rendering objects, NOT positions on the axes.


EDIT: By the way, why is there a function implementation in your header? :shock:

Re: Perspective

Posted: Mon Jan 16, 2012 5:57 pm
by Benjamin100
Well, when I set the angle to something other than 0 it doesn't show anything.
This way it shows something that doesn't appear to get smaller.
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