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Zoom in openGL

OpenGL
Started by Srinidhi 8 replies 38.1k views
Original Post
Srinidhi
Srinidhi
Hi can anyone help me with zoom in openGL. I m using orthographic projection. I just need simple zoom in and zoom out.. I m very new to openGL so askign too many questions.
Zakwayda
Zakwayda
Although I've never actually written a zoom feature in ortho mode, I'm guessing you'll want to adjust the parameters to glOrtho() so as to change the width and height of the view volume; smaller will zoom in, larger will zoom out. If you need more details, you might post the code where you call glOrtho() so we can give you a more concrete example.

Again though, I'm kind of guessing here; perhaps someone else can give a more authorititive answer.
_gl_coder_one_
_gl_coder_one_
Yes u can use glOrtho and acheive zooming . That will be true zooming and not scaling .

To zoom in
you reduce the distance between the left and right params and top and bottom params to glOrtho
In effect you are reducing the viewing volume and and the size of the near plane , which is still going to be mapped onto your fixed size VIEWPORT , your screen. So u magnify the view!

To zoom out increase the the params mentioned above , you should get the desired effect.

post back if you have a problem understanding what i have said.
Srinidhi
Srinidhi
My glortho command is as follows:
glOrtho(-1.5, 1.0, -2.0, 0.5, -1.0, 3.5);

and i m doing the following
 case 'o' :	   zoom = zoom  - 0.5;           glMatrixMode(GL_MODELVIEW);	   glOrtho(-1.5 + zoom, 1.0 + zoom, -2.0 + zoom, 0.5 + zoom, -1.0, 3.5);            break;


but im not getting desired result.
Zakwayda
Zakwayda
Try:
zoom = zoom  - 0.5; // Or whatever you want the step to beglMatrixMode(GL_PROJECTION); // You had GL_MODELVIEWglOrtho(-1.5 + zoom, 1.0 - zoom, -2.0 + zoom, 0.5 - zoom, -1.0, 3.5); // Changed some of the signs here
The results of these particular values (off-center view volume and relatively large zoom step) may or may not work so well, but you can adjust them if necessary.
Srinidhi
Srinidhi
Ya thanks very much its workign perfectly.....I was wondering like can I adda slide bar to do this instead of pressing keys u know...
Zakwayda
Zakwayda
Quote:
Original post by Srinidhi
I was wondering like can I adda slide bar to do this instead of pressing keys u know...
OpenGL doesn't offer any built-in GUI support, but there are some free libraries available around the net. For something as simple as a slider though, another option would be to just implement it yourself.
_gl_coder_one_
_gl_coder_one_
This is how i do it 

void	CCamera::zoomIn(){	m_d_left_plane	*=	0.75f;	m_d_right_plane	*=	0.75f;	m_d_top_plane	*=	0.75f;	m_d_bottom_plane	*= 0.75f;        setViewingVolume();}void	CCamera::zoomOut(){	m_d_left_plane	*=	1.33f;	m_d_right_plane	*=	1.33f;	m_d_top_plane	*=	1.33f;	m_d_bottom_plane	*= 1.33f;		setViewingVolume();}



The setViewingVolume function is as below
It does the glOrtho call 
void	CCamera::setViewingVolume(){	double	aspect_ratio	=	(double)canvas_width/(double)canvas_height;		glViewport( 0,0,(int)canvas_width , (int )canvas_height );	glMatrixMode( GL_PROJECTION );	glLoadIdentity();	if( canvas_width > canvas_height ){		glOrtho(m_d_left_plane*aspect_ratio,m_d_right_plane*aspect_ratio,m_d_bottom_plane,m_d_top_plane,m_d_near_plane,m_d_far_plane);	}else{		glOrtho(m_d_left_plane,m_d_right_plane,m_d_bottom_plane*aspect_ratio,m_d_top_plane*aspect_ratio,m_d_near_plane,m_d_far_plane);	}	updateZoomFactors();	glMatrixMode( GL_MODELVIEW );		}
Srinidhi
Srinidhi
thanks GL_coder for the help!!!
ViperG
ViperG
you can also just use glRotatef(Zoom, 0, 0, 0); when in ortho mode. However the last translate must be center screen or it will zoom from last tranlate position.
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