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Mouse, 3d terrain, coordinates, OpenGL

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I want to translate the mouse coordinates (x,y of the window) to x,y,z coordinates of my world. I have a terrain that can be scrolled and rotated but I haven't been able to select a point on this map with a mouse. The terrain is not flat but there is only one z for each (x,y) pair. I managed to calculate a solution but it is not accurate (it misses the selected spot a bit). Perheaps more information about the viewing frustum would help me. Knowing the parameters of the frustum: glFrustum ( GLdouble left , GLdouble right , GLdouble bottom , GLdouble top , GLdouble zNear , GLdouble zFar ) how can I calculate the angle that frustum walls make with the z axis? (I need it for my algorithm) ___________ far \0000|0000/ 0\___|___/ near 00\00|00/ 000\0|a/ I need this angle. 0000\|/ 00000V Perheaps what I'm doing is not necessary because there exists a special opengl function or a library I do not know about... If so tell me.

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As I suspected. It's a basic problem. I can use gluUnProject. I've found the answer here

In case it dissappears:

Using gluUnProject (OpenGL, gluUnProject, glut)

I have been using OpenGL since a long time and a fundamental problem many would like is how to find out where the user clicked in the 3D-world given the mouse co-ordinates. OpenGL and GLU functions can be used to effectively solve this problem. Since i could not find any good tutorials regarding this i thought i'd put up one by myself.

There is a simple function that glu provides. It's called gluUnProject, this function takes in 6 arguments.. which include the window x,y,z co-ordinates, modelview matrix, projection matrix, viewport and X,Y,Z for storing the world co-ordinates of that particular point. Dont' get worried about window z co-ordinate which is given to this function. This value is obtained by using glReadPixels( x, viewport[3]-y, GL_DEPTH_COMPONENT, GL_FLOAT, &z ); We need to use viewport[3]-y (this is equivalent to windowheight-y where x,y are the mouse location co-ordinates) instead of just y because the glut window has origin at top left corner where as OpenGL (glReadPixels) has the origin at bottom left corner.

Once you get the z value of the window (using glReadPixels), we can pass the window x,y,z values to the gluUnProject and get the world co-ordinates. In the example here a red cube is moved to the location where the user clicks on the green plane.

double objx, objy, objz;
//The drawing function glutDisplayFunc( )
void display( )
//Clearing the screen with black and clearing the depth buffer

//The green polygon
glBegin( GL_POLYGON );
glColor3f( 0, 1, 0 );
glVertex3f( -100, 0, 100 );
glVertex3f( -100, 0, -100 );
glVertex3f( 100, 0, -100 );
glVertex3f( 100, 0, 100 );
glEnd( );

//store the current transformation matrix
//and translate the cube to the world location
glPushMatrix( );
glColor3f( 1, 0, 0 );
glTranslatef( objx, objy, objz );
glutSolidCube( 10 );
glPopMatrix( );

glFlush( );
glutSwapBuffers( );

//The function that handles the mouse clicks glutMouseFunc( )
void mouse( int button, int state, int x, int y )
double modelview[16], projection[16];
int viewport[4];
float z;
//get the projection matrix
glGetDoublev( GL_PROJECTION_MATRIX, projection );
//get the modelview matrix
glGetDoublev( GL_MODELVIEW_MATRIX, modelview );
//get the viewport
glGetIntegerv( GL_VIEWPORT, viewport );

//Read the window z co-ordinate
//(the z value on that point in unit cube)
glReadPixels( x, viewport[3]-y, 1, 1,

//Unproject the window co-ordinates to
//find the world co-ordinates.
gluUnProject( x, viewport[3]-y, z, modelview,
projection, viewport, &objx, &objy, &objz );

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