Question

I’m working on a little example, where I have loaded an object from a wavefront file – and am trying to get my picking right, I’ve gone over this and a few tutorials about 10 times… but must be missing something. Was wondering if anyone could provide an extra set of eyes.

I’ve used a saved list to draw the object, which appears fine on the screen… At the moment, when gl_select(x, y) runs, I get a hit no matter what, and if I enable the translate/rotate code (which is currently commented out) – I get no hits what-so-ever.

Relevant code blocks:

// gl_select, is called when the mouse is clicked, with its x and y coords
void gl_select(int x, int y)
{
    GLuint buff[256];
    GLint hits;
    GLint view[4];

    //Buffer to store selection data
    glSelectBuffer(256, buff);

    //Viewport information
    glGetIntegerv(GL_VIEWPORT, view);

    //Switch to select mode
    glRenderMode(GL_SELECT);

    //Clear the name stack!
    glInitNames();

    //Fill the stack with one element
    glPushName(0);

    //Restric viewing volume
    glMatrixMode(GL_PROJECTION);
    glPushMatrix();
    glLoadIdentity();

    //Restrict draw area
    gluPickMatrix(x, y, 1.0, 1.0, view);
    gluPerspective(60, 1, 0.0001, 1000.0);

    //Draw the objects onto the screen
    glMatrixMode(GL_MODELVIEW);

    //Draw only the names in the stack
    glutSwapBuffers();
    DrawSavedObject();

    //Back into projection mode to push the matrix
    glMatrixMode(GL_PROJECTION);
    glPopMatrix();

    hits = glRenderMode(GL_RENDER);

    cout << hits;

    //Back to modelview mode
    glMatrixMode(GL_MODELVIEW);
}

And the draw functions:

void DrawSavedObject()
{

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glColor3f(1.0,0.0,0.0);

    //translate and rotate
    //glRotated(rotation,0.0,0.0,1.0);
    //glTranslated(7.0, 7.0, 0.0);

    //Draw the saved object
    glLoadName(7);
    glCallList(list_object);

    glutSwapBuffers();
}

And where the list is saved:

void SaveDisplayList(){     
    glNewList(list_object, GL_COMPILE);

    glVertexPointer(3, GL_DOUBLE, 3*sizeof(GLdouble), vertices);
    glEnableClientState(GL_VERTEX_ARRAY);
    glDrawElements(GL_TRIANGLES, verticesSize ,GL_UNSIGNED_INT, triangles);

    glDisableClientState(GL_VERTEX_ARRAY);

    glEndList();
}

Sorry again for the chunkiness of the code blocks.

Answer 1

A few things to consider here:

• OpenGL selection mode is deprecated and never was HW accelerated, except on a few SGI boxes and 3DLabs GPUs.

• DisplayLists don’t mix with Vertex Arrays.

• Why do you call glutSwapBuffers right before drawing your list of saved objects? Makes absolutely no sense at all.