Zoom to mouse position using glOrtho
I am currently using glOrtho for zooming and panning a 2D plot I am presenting.
I have set the viewport to standard width and height. Then I set glOrtho so that my frustrum forces the screen coordinates to match the world coordinates.
//////////////////////////////////////////////// //////////////////
glViewport(0, 0, window_width,window_height); glMatrixMode(GL_PROJECTION); glLoadIdentity(); glOrtho(0, window_width,window_height,0 , 100, -100);
//////////////////////////////////////////////// /////////////////
When I execute the zoom function in the mouse callback, I multiply the frust edges by the zoom factor ....
glOrtho( 0 * zoomOut,
window_width * zoomOut,
window_height * zoomOut,
0 * zoomOut,
100, -100);
My question is, how do I zoom in using the mouse position as the center?
I tried this ... (where mouseStoreX and mouseStoreY are the position saved on the first click)
glOrtho( (0 -mouseStoreX )* zoomOut + mouseStoreX, (window_width - mouseStoreX) * zoomOut + mouseStoreX, (window_height - mouseStoreY) * zoomOut + mouseStoreY, (0 - mouseStoreY) * zoomOut + mouseStoreY, 100, -100);
Seems to work, but frustrum jumps when I make a new click. I think that somewhere I am not taking into account the zoomOut factor when maintaining the mouse position.
EDIT * *********** here is my final code that I am still struggling with ...
void ZoomOrtho(){ //ON MOUSE CLICK.....
if (zooming == false){
keyStore.LMx = keyStore.Mx; //store mouse pos for next comparison
keyStore.LMy = keyStore.My;
//get mouse pos
mouseStoreX = keyStore.Mx;//mouse pos at this moment
mouseStoreY = keyStore.My;
//get current projection matrices
glGetDoublev( GL_MODELVIEW_MATRIX, modelview );
glGetDoublev( GL_PROJECTION_MATRIX, projection );
glGetIntegerv( GL_VIEWPORT, viewport );
//flip Y for opengl reasons
winY = (float)viewport[3] - winY;
//get world mouse coordinate
gluUnProject( mouseStoreX, mouseStoreY , 0.0, modelview, projection, viewport, &posX_,&posY_, &posZ_);
// calc difference between mouse world pos and centre of 'camera'
dx = posX_ - FS.centerX;
dy = posY_ - FS.centerY;
}
//ON DRAG......
zooming = true;
//do mouse movement detection and increment zoomOut
//#################################################
int xDiff = keyStore.Mx - keyStore.LMx; //mouse drag difference in screen space just for incrementing zoom
int yDiff = keyStore.My - keyStore.LMy; //
if (xDiff > 0 && (zoomFactor >= 0.5 ) ) {
zoomFactor -= zoomInc;
if (zoomFactor < 0.5 ) {zoomFactor = 0.5;}
}
else if (xDiff < 0 && (zoomFactor <= 2.0 )) {
zoomFactor += zoomInc;
if (zoomFactor > 2.0){zoomFactor = 2.0;}
}
//#################################################
//fill structure with clipping plane values. zooms ortho projection and keeps mouse pos anchored.
FS.left = ((FS.centerX - dx - (window_width/2.0))*zoomFactor) +dx;
FS.right = ((FS.centerX -dx + (window_width/2.0))*zoomFactor)+dx ;
FS.bottom = ((FS.centerY -dy + (window_width/2.0))*zoomFactor)+dy;
FS.top = ((FS.centerY -dy - (window_width/2.0))*zoomFactor) +dy;
// store last mouse pos for next comparison.
keyStore.LMx = keyStore.Mx;
keyStore.LMy = keyStore.My;
}
void zoomRelease(){
cout << " releasing" << std::endl;
//set zoom to false so we know we are not draggin mouse anymore.
zooming = false;
keyStore.LMx = 0;
keyStore.LMy = 0;
// recenter by taking midpoint between new left and right clipping planes so dx has a reference point
FS.centreX = (FS.right-FS.left)/2.0;
}
void DrawGui(){
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(FS.left, FS.right,FS.bottom, FS.top, 1, -1);
glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
//do drawing
}
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4 answers
My guess is that when you do the second click, you store its value in mouseStoreXY. If so, it triggers jumping. You were drawing with the offset of the old mouseStoreXY, and suddenly you were offset with the new one.
The solution would be to persist in the sets of projection matrices and then modify them in stages at each frame.
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Take a picture:
// g++ main.cpp -o main -lglut -lGL && ./main
#include <GL/glut.h>
double centerX = 0, centerY = 0;
double width = 0, height = 0;
void mouse( int button, int state, int mx, int my )
{
// flip mouse y axis so up is +y
my = glutGet( GLUT_WINDOW_HEIGHT ) - my;
// convert mouse coords to (-1/2,-1/2)-(1/2, 1/2) box
double x = ( mx / (double)glutGet( GLUT_WINDOW_WIDTH ) ) - 0.5;
double y = ( my / (double)glutGet( GLUT_WINDOW_HEIGHT ) ) - 0.5;
if( GLUT_UP == state )
{
double preX = ( x * width );
double preY = ( y * height );
double zoomFactor = 1.5;
if( button == GLUT_LEFT_BUTTON )
{
// zoom in
width /= zoomFactor;
height /= zoomFactor;
}
if( button == GLUT_RIGHT_BUTTON )
{
// zoom out
width *= zoomFactor;
height *= zoomFactor;
}
double postX = ( x * width );
double postY = ( y * height );
// recenter
centerX += ( preX - postX );
centerY += ( preY - postY );
}
glutPostRedisplay();
}
void display()
{
glClear( GL_COLOR_BUFFER_BIT );
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
glOrtho
(
centerX - ( width / 2.0 ),
centerX + ( width / 2.0 ),
centerY - ( height / 2.0 ),
centerY + ( height / 2.0 ),
-1,
1
);
glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
glColor3ub( 255, 0, 0 );
glBegin( GL_TRIANGLES );
glVertex2i( 0, 0 );
glVertex2i( 150, 0 );
glVertex2i( 0, 150 );
glVertex2i( 0, 0 );
glVertex2i( -150, 0 );
glVertex2i( 0, -150 );
glEnd();
glutSwapBuffers();
}
int main( int argc, char **argv )
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
glutInitWindowSize( 600, 600 );
glutCreateWindow( "GLUT" );
glutDisplayFunc( display );
glutMouseFunc( mouse );
width = glutGet( GLUT_WINDOW_WIDTH );
height = glutGet( GLUT_WINDOW_HEIGHT );
glutMainLoop();
return 0;
}
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Take a picture:
// g++ main.cpp -o main -lglut -lGL && ./main
#include <GL/glut.h>
#include <cmath>
void getMouseCoords( int mx, int my, double& x, double& y )
{
// flip mouse y axis so up is +y
my = glutGet( GLUT_WINDOW_HEIGHT ) - my;
// convert mouse coords to (-1/2,-1/2)-(1/2, 1/2) box
x = ( mx / (double)glutGet( GLUT_WINDOW_WIDTH ) ) - 0.5;
y = ( my / (double)glutGet( GLUT_WINDOW_HEIGHT ) ) - 0.5;
}
int btn;
double baseX, baseY;
double baseWidth, baseHeight;
double centerX = 0, centerY = 0;
double width = 0, height = 0;
void mouse( int button, int state, int mx, int my )
{
baseWidth = width;
baseHeight = height;
btn = button;
getMouseCoords( mx, my, baseX, baseY );
}
void motion( int mx, int my )
{
if( btn != GLUT_LEFT_BUTTON )
{
return;
}
double x, y;
getMouseCoords( mx, my, x, y );
double preX = ( baseX * width );
double preY = ( baseY * height );
double zoomFactor = exp( baseY - y );
width = baseWidth * zoomFactor;
height = baseHeight * zoomFactor;
double postX = ( baseX * width );
double postY = ( baseY * height );
// recenter
centerX += ( preX - postX );
centerY += ( preY - postY );
glutPostRedisplay();
}
void display()
{
glClear( GL_COLOR_BUFFER_BIT );
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
glOrtho
(
centerX - ( width / 2.0 ),
centerX + ( width / 2.0 ),
centerY - ( height / 2.0 ),
centerY + ( height / 2.0 ),
-1,
1
);
glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
glColor3ub( 255, 0, 0 );
glBegin( GL_TRIANGLES );
glVertex2i( 0, 0 );
glVertex2i( 150, 0 );
glVertex2i( 0, 150 );
glVertex2i( 0, 0 );
glVertex2i( -150, 0 );
glVertex2i( 0, -150 );
glEnd();
glutSwapBuffers();
}
int main( int argc, char **argv )
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
glutInitWindowSize( 600, 600 );
glutCreateWindow( "GLUT" );
glutDisplayFunc( display );
glutMouseFunc( mouse );
glutMotionFunc( motion );
width = glutGet( GLUT_WINDOW_WIDTH );
height = glutGet( GLUT_WINDOW_HEIGHT );
glutMainLoop();
return 0;
}
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If you want to take a different route and just use glTranslate and gluPerspective, you can achieve the same effect. A scroll wheel mouse event (using PyOpenGL) might look something like this:
def MouseWheelScroll(self, event):
"""Called when the mouse scroll wheel is scrolled up or down. This modifies the zoomFactor
which renders the graphics closer or further away on the screen. It also translates the graphics
slightly based on the position of the mouse. This creates an effect of zooming to the location
of the mouse on the screen.
"""
scrolledUp = event.GetWheelRotation() # Returns positive for up, negative for down
self.x, self.y = event.GetPosition()
viewport = glGetIntegerv(GL_VIEWPORT)
width = viewport[2]
height = viewport[3]
centerX = width / 2.0
centerY = height / 2.0
# Make sure cursor is on the screen
if ((self.x > 0 and self.x < width) and (self.y > 0 and self.y < height)):
if (scrolledUp > 0):
self.zoomFactor -= 2.0
self.translation[0] -= (self.x - centerX)
self.translation[1] += (self.y - centerY)
else:
self.zoomFactor += 2.0
self.translation[0] += (self.x - centerX)
self.translation[1] += (self.y - centerY)
if (self.zoomFactor > 150.0):
self.zoomFactor = 150.0
elif (self.zoomFactor < 0.1):
self.zoomFactor = 0.1
self.Refresh(False)
You would just need to translate the graphics, adjust your perspective and then render the scene.
# Translate graphics
glTranslatef(0.0, 0.0, (self.translation[1]/100.0) * (math.tan(self.cameraPosition[0]/self.cameraPosition[1])))
glTranslatef(0.0, (self.translation[0]/100.0) * (math.tan(self.cameraPosition[0]/self.cameraPosition[1])), 0.0)
# Set Perspective
glMatrixMode(GL_PROJECTION)
glLoadIdentity()
gluPerspective(self.zoomFactor, float(width)/float(height), self.nearPlane, self.farPlane)
# Render Scene
glMatrixMode(GL_MODELVIEW)
...Draw stuff here...
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