# OpenGL GLUT, OpenGL & mouse coordinates

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How the hell (sorry I thought of evrything but I couldn't think of a way) can you make it so that an object, say a triangle allways lies under the mouse pointer? Please explain clearly, because I still haven't got any advanced math lessons.

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Use glutPassiveMotionFunc to keep track of mouse movements.
Then you have a couple of choices.
You could just use the callback to set a couple of global variables for mouse position (and call glutPostRedisplay().)
Then in your display function, you'd switch over to orthographic mode, such that each pixel corresponds to one OpenGL unit (gluOrtho2D(0,width,heigh,0).) Then just draw a triangle at the mouse position.
If you want to move an object with perspective projection, you could use gluUnProject (in the passive motion callback) to transform the mouse coordinates to a world coordinate. Then, in your display function, you'd just translate and draw.

Well, that's it. Check out the callback, and think it over. If you need more help, just post here [smile]

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Hey, I'm not that dumb that I can't understand global variables and mouse positions... just to say that, because others will spoil their energy on telling simple things.
OK, I got a question, can you change to orthographic view, draw the thing and then change back to perspective view?
And if not, how does that gluUnProject function exactly work?

Thanks for the replies

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Quote:
 Original post by the_cyberlordOK, I got a question, can you change to orthographic view, draw the thing and then change back to perspective view?

Yes.
  glMatrixMode(GL_PROJECTION);  glPushMatrix();  glLoadIdentity();  gluOrtho2D(0,width-1,height-1,0);  // Here we use orthographic projection  glPopMatrix();  // Back to the projection we were using before  glMatrixMode(GL_MODELVIEW);  // Best switch back to modelview

Quote:
 And if not, how does that gluUnProject function exactly work?

It is used to go from window coordinates to world coordinates.
       GLint gluUnProject( GLdouble winX,                           GLdouble winY,                           GLdouble winZ,                           const GLdouble *model,                           const GLdouble *proj,                           const GLint *view,                           GLdouble* objX,                           GLdouble* objY,                           GLdouble* objZ )

(winX,winY,winZ) are the window coordinates, where winX=mouseX, winY=1-mouseY, winZ=depth value in the range [0,1] where 0 is the near plane and one the far one.
model and proj should be the viewing and projection transformations used to setup your scene. view is your viewport.
(objX,objY,objZ) will contain the unprojected coordinates.
Here's an example of its use: link

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Quote:
 OK, I got a question, can you change to orthographic view, draw the thing and then change back to perspective view?

Yes, very simply. I do this when drawing my GUI system. Use glPushMatrix() with the projection matrix active, call glIdentity() and glOrtho() to reset and create an Orthogonal view, then glPopMatrix() to restory your original view point.

Generally, it's best to do this at the end of your main render loop.

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Are there examples on the perspective >> ortho >> perspective manner?

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Why doesn't this work?

double w_mouse_x,w_mouse_y,w_mouse_z;
void MousePassive(int x, int y) {
GLint viewport[4];
GLdouble modelview[16],projection[16];
glGetIntegerv(GL_VIEWPORT,viewport);
glGetDoublev(GL_MODELVIEW_MATRIX,modelview);
glGetDoublev(GL_PROJECTION_MATRIX,projection);
gluUnProject(mouse_x,mouse_y,-5,modelview,projection,viewport,&w_mouse_x,&w_mouse_y,&w_mouse_z); }

object triangle;

GLvoid DrawGLScene()
{

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glPushMatrix();
glTranslatef(0.0,0.0,-5);
triangle.x = w_mouse_x; triangle.y = w_mouse_y;

triangle.draw();

glPopMatrix();

glFlush();
glutSwapBuffers();
}

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gluUnProject(mouse_x,mouse_y,-5

Change the above to
gluUnProject(x,viewport[3]-y,0

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it doesn't do anything
Are there tutorials on this?

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Try posting the whole source (within [ source ] tags) and I'll take a look. Don't know about any tutorials on this...

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http://users.skynet.be/cyber-lord/engine.cpp

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  gluPerspective(310,Width/Height,0,1000);

Do you really want a fov of 310 degrees? I'm not even sure it's possible. Try setting it to something like 45 or 60. Also, a near plane of zero is not something you should have.. Try changing to
  gluPerspective(45,Width/Height,1,1000);

I also don't see you giving triangle.z a value anywhere..

Finally, not that it matters much, but you can remove glFlush() since glutSwapBuffers() takes care of it.

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And it should work if I do all that?
triangle.z exists because I may use it later, and I'm actually trying to make a little basic game engine.

And if you don't make your FoV that large (it works BTW) how can you do so that objects appear smaller? Dividing the view ratio(width/height)?
Really sorry if I'm annoying...
But thanks to you all!
--------------------
EDIT: Where making progress here!
OK, now it shows a reaction that acts a bit like it should act, but when I move the cursor farther away from the center of my window, the triangle allways gets farther and farther from it. so there must be somewhere a division or something... but I don't have a clue where... (if you leave the cursor in the middle, the triangle is under it)
http://users.skynet.be/cyber-lord/engine.cpp >> updated source code.
>> If you solve this, you're my HERO!

[Edited by - the_cyberlord on August 27, 2004 11:59:47 AM]

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I found what the problem is:
gluUnProject(x,viewport[3]-y,0
change to
gluUnProject(x,viewport[3]-y,8
The triangle gets a closer match to the mouse pointer, but still not close enough.
Now the problem is: how do you find that value that should give the perfect match...
Again:
Who solves this is my HERO!

No one???

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Quote:
 Original post by the_cyberlordAnd it should work if I do all that?

I don't know - I just looked at your code and saw a few things that looked strange [disturbed]
Quote:
 And if you don't make your FoV that large (it works BTW) how can you do so that objects appear smaller? Dividing the view ratio(width/height)?

Well, you can use FoV to zoom, but I think it should be kept under 180 degrees. (Think about what happens at 180 degrees [smile])

Quote:
 OK, now it shows a reaction that acts a bit like it should act, but when I move the cursor farther away from the center of my window, the triangle allways gets farther and farther from it. so there must be somewhere a division or something... but I don't have a clue where... (if you leave the cursor in the middle, the triangle is under it)

Ahem. glTranslate(0.0,0.0,-5). It moves the triangle further away from the viewer. And what happens with objects that move away when you have a perspective projection? They look smaller. Of course, movements look smaller too. There's your division.
Start by removing the translation.
The third argument to gluUnProject should be left between 0 and 1. You could set it to 0.1 or something but 0 should work - I don't think there's much risk your triangle gets clipped by the near plane.
You should now have a triangle that moves just inside the view frustum and follows your mouse pointer. If you want it further away, increase winZ (third argument to gluUnProject) a bit.

Hope that works for you [smile]

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Thanks! You're really my hero!
(You see: triangle.z wasn't useless)

Now the fun stuff can begin!

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hey your like me wanting to program for fun and not have to do all the technical crap how awesome atleast there are other people out there who like to program for idea rather then the technical crap that comes with it ! woot inspires me to make tools that can get rid of technicality as much as possible and focus more on creativity and fun programming experiences but still keep technical around for those who like it and are passionate about it as well!

user atmosphere experience should be a choice not a enforcement no matter what your doing! rule number 1 in good api design

technical
creative
or any other genre

should be a choice not an enforcement Edited by ThunderFart

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This topic is almost 8 years old. The original poster is either dead or an old man (or worse—banned) by now.

/Me attempts to close but fails to conjure moderator powers with error message, “Not yet level 60”.

L. Spiro

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