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OpenGL screen click to world conversion

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thekid    122
hi all, im working on a project that requires me to know what poly on in 3d space was clicked when the user clicks on the screen. In this case it is just a simple grid of cells (minesweeper clone for uni project) i know this is possible but I have had little luck finding good sources of info on the topic. I am using OpenGL for the graphics API and glut for the windowing. thanks in advance.

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nts    968
look up gluProject and gluUnproject (for 2d to 3d coords and back), or take take a look at the Selection buffer. Tutorials on both can be found at Nehe, Selection buffer is there for sure, i think he made a game where u shoot things that has it.


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CosmoKramer    100
in general this is called picking. But I dont think you need anything as complicated as in the general case. If you have a grid of cells, you couldnt ask for anything easier.

position_of_mouse_click_X / size_of_each_cell = some_number

drop the fraction part from some_number, and youve got the cell# in the x direction. repeat for y.

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thekid    122
thanks to nts, i am trying the gluUnproject, i am getting segmentation faults however when trying to get the mode, projection matrices and the viewport using glGetdoublev and glGetIntegerv. any ideas? yes CosmoKramer i realize that the problem is trivial if the board is always facing the user directly but i want to be able to rotate around the gameboard thus the problem of picking.

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nts    968
i would suggest using floats to get the matrix

glGetFloatv(GL_MODELVIEW_MATRIX, mat);
etc, integers for viewport are fine

where are u getting the faults?

EDIT: make sure you have a valid RC, all that i can think of

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alf259    122
You can choose to do this using immage space, if you have a system that is capable of rendering to textures you can render a version of your checker board where each square has a unique integer stored as color. You can then lock the texture and read the color at the mouse position.

This is relatively simple and fast.



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Puppet    181
You just need to spawn a ray though the pixel in screen into the world from the camera position. Then intersect that ray with the potential polygons. If you know the cameras field of view and aspect ratio, it's trivial.

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gowron67    152
It's really no help saying things are 'trivial'. They may be trivial to you but people wouldn't post on here if they could already do what they were asking.

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Puppet    181
Get your camera's position, look, up and right vector. These can be extracted from the view matrix if you don't explicitly know them already. Field of view is the horisontal one in this example. With reservations for coding-on-the-spot-errors, the general priciple:

float screenW2 = (float)screenWidth / 2.0f;
float screenH2 = (float)screenHeight / 2.0f;
float viewWidth2 = tanf(FieldOfView / 2.0f);
float aspectRatio = (float)screenHeight / (float)screenWidth;

Vector3 vScreenX = vCameraRight * ((float)(mouseX - screenW2) / screenW2) * viewWidth2;

Vector3 vScreenY = vCameraUp * -((float)(mouseY - screenH2) / screenH2) * viewWidth2 * aspectRatio;

Vector3 vRayDir = vCameraLook + vScreenX + vScreenY;
Vector3 vRayPos = vCameraPos;

// Intersect ray with world .. ray to triangle or whatever..

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thekid    122
thanks for all the info. I still have a problem however. I am redering 2*2 quads to represent the cells on the board. I do a for loop to render them like this:

for(int i=0; i<game->getBoard()->getWidth(); i++) {

for(int j=0; j<game->getBoard()->getHeight(); j++) {

glTranslatef(((float)(i*2)) - adjustment, ((float)(j*2)) - adjustment, -40.0f);

//--draw cell

glVertex3f(-1.0f, 1.0f, 0.0f);
glVertex3f( 1.0f, 1.0f, 0.0f);
glVertex3f( 1.0f, -1.0f, 0.0f);
glVertex3f(-1.0f, -1.0f, 0.0f);

//--end drawing


so how do i know what cell i am clicking because I translate each one of them indiviually i dont know their final world coordinates. also i output the coordinates i get from gluUnproject and they are really strange.

i have to adjust the board so that it is in the center of the screen perhaps i just dont understand the coordinate system in opengl.

should i draw it at the world origin and just move the camera?

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