In this demo we have a cube in the scene. If you click on the cube, the cube changes its color to yellow:
the cube is now selected. Starting from a cube selected, a click on the plane moves the cube in the position where we have clicked. This video shows some examples:

The demo uses the unProject method as in the Picking post . You will find in the code some modification for drawing the cube in a different position on the plane.

In this post I want to share an easy implementation of picking through unProject method. The demo is realized in C, so we have only the main.cpp file and vector3.h file. In the demo we have a cube in the center of the scene, when we click on the cube, the cube changes its color.

If you open the main.cpp source file you can note the following lines of code:

The variables posX,posY and posZ give us the unproject mouse click coordinates.
The variables cubeX,cubeY and cubeZ save the cube position.
The variable cubSize set the size of the cube;
The boolean variable cubePick is true when the cube is clicked;

When we click in the scene, the unProject method is called:

void unProject(){ GLint viewport[4]; // Where The Viewport Values Will Be Stored glGetIntegerv(GL_VIEWPORT, viewport); // Retrieves The Viewport Values (X, Y, Width, Height) GLdouble modelview[16]; // Where The 16 Doubles Of The Modelview Matrix Are To Be Stored glGetDoublev(GL_MODELVIEW_MATRIX, modelview); // Retrieve The Modelview Matrix GLdouble projection[16]; // Where The 16 Doubles Of The Projection Matrix Are To Be Stored glGetDoublev(GL_PROJECTION_MATRIX, projection); // Retrieve The Projection Matrix GLfloat winX, winY, winZ; // Holds Our X, Y and Z Coordinates winX = (float)mouse2D.x; // Holds The Mouse X Coordinate winY = (float)mouse2D.y; // Holds The Mouse Y Coordinate glReadPixels(winX, winY, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &winZ); gluUnProject( winX, winY, winZ, modelview, projection, viewport, &posX, &posY, &posZ); //ROUND 2 DECIMAL posX= ((int)(posX * 10 + .5) / 10.0); posY= ((int)(posY * 10+ .5) / 10.0); posZ= ((int)(posZ * 10 + .5) / 10.0); }

the method saves posX,posY and posZ variables. After the pickingCheck method is called: