converting window coordinates to world coordinates

OpenGL OpenGL: Basic Coding
1

Hello,

When the mouse clicks the opengl generated window this passes the x,y coordinates of the screen to glutMouseFunc()… anyone know how to convert these into the coordinates of the opengl window, say to draw something where the user has clicked?

thx!

2

… I forgot to add, this is in 2D. And was wondering if there is a way to do this without using gluUnProject

3

Mouse coords are returned in screen coords. If you know window position everything is simple.

4

Yeah i know the screen coordinates… i am trying to figure out a math function to convert them to the world coordinates… but not having much luck.

like if my window is (800,600) at the bottom right corner this maps to (1,0) in world coordinates… right?

so ‘x’ would be divided by 800.
for ‘y’ i am not sure.

5

Say you have a window with grid on your desktop & you want to know which cell you clicked?
1)get the window coords from desktop coords
2)get the world coords from window coords

For the first substitute upper left corner from your mouse pos and check wheter the coords are not bigger than wsize.
For the second depends on app, but I guess it’s some sort of ortho, isn’t it?

6

there is a way to do this without UnProject: use the same method that UnProject uses in your own code :wink:

http://oss.sgi.com/cgi-bin/cvsweb.cgi/projects/ogl-sample/main/gfx/lib/glu/libutil/project.c

it’s just a bunch of matrix algebra … maybe you can make it faster :slight_smile:

7

Hey com’on guys, this is a beginners group - to many the use of gluunproject is complex - the poster has asked a simple question that I have struggled with too - if you want to help, post a very simple example - otherwise don’t post.

pwonder - I think gluunproject is the way you want to go - try it out with a simple case using some printf statements to see if you are getting the right answer. I’m working on it too and if I meet success will post back.

8

I got it to work… here is what my function that handles mouse click looks like:
Thanks for all of your help

 
void mouseClick( int button, int buttonState, int x, int y ){

  //if any mouse button was pressed 
  if (buttonState == GLUT_DOWN) {


	GLint viewport[4]; //var to hold the viewport info
        GLdouble modelview[16]; //var to hold the modelview info
        GLdouble projection[16]; //var to hold the projection matrix info
        GLfloat winX, winY, winZ; //variables to hold screen x,y,z coordinates
        GLdouble worldX, worldY, worldZ; //variables to hold world x,y,z coordinates

        glGetDoublev( GL_MODELVIEW_MATRIX, modelview ); //get the modelview info
        glGetDoublev( GL_PROJECTION_MATRIX, projection ); //get the projection matrix info
        glGetIntegerv( GL_VIEWPORT, viewport ); //get the viewport info

	winX = (float)x;
        winY = (float)viewport[3] - (float)y;
	winZ = 0;
	
	//get the world coordinates from the screen coordinates
        gluUnProject( winX, winY, winZ, modelview, projection, viewport, &worldX, &worldY, &worldZ);

btw: i use winZ=0; since i am only doing 2d stuff.
and fyi. at this point in the program worldX, worldY have the WORLD x,y coordinates!

9

But if your drawing is scaled?
The procedure fails, how can resolve?