Move object with mouse

OpenGL OpenGL: Basic Coding
1

I´ve been searching the forum but with no luck I am afraid. I have a fairly simple question I would guess. I use the library “Shapelib” to load a shape-file and all I want to do is to be able to move the shapefile with the mouse. I click and hold the left-button and then when i move the mouse the shape should follow.
Is there someone who can give me a point in the right direction on this one?
My idea was to use glTranslatef(x,y,0.0) and then calculate how many pixels the mouse moved. But it didn’t work that well.

/Robert

2

Could u post what u did?
Nobody will code it from scratch for u.

3

Absolutely. Mostly I was just looking to get some theoretical tips or hints on how to approach the problem and I´m not sure if this helps in any way but maybe you get the idea.

I thought I could just translate the coords here. It kind of works but it´s not very nice to look at. It´s flickering and most of the time the shape-file just dissapears.


void Widget::draw()
{
    glMatrixMode(GL_MODELVIEW);
    glTranslatef(offsetX,offsetY,0.0);

    glPushMatrix();
    //Loading shape-file
    .
    .
    .
    glEnd();

    glPopMatrix();
    glFlush();
}

Hopefully I got all the code that matters below.


void Widget::initializeGL()
{
   glClearColor(1.0f, 1.0f, 1.0f, 1.0f); //White
   glShadeModel(GL_FLAT);
   glEnable (GL_LINE_SMOOTH);
   glHint (GL_LINE_SMOOTH_HINT, GL_NICEST);
   glEnable (GL_BLEND);
   glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
   QGLWidget::setAutoBufferSwap(true);
   glDisable(GL_DEPTH_TEST);
   glPointSize(3);
}


void Widget::paintGL()
{
     glClear(GL_COLOR_BUFFER_BIT);
     glClearColor(1.0f, 1.0f, 1.0f, 1.0f); //White
     glLoadIdentity(); //Clear the matrix.
     draw();
}

void Widget::resizeGL(int width, int height)
{
	if(height<=0) height=1;
	   glViewport (0, 0, (GLsizei) width, (GLsizei) height);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity ();
	glOrtho(sBoundingBox.minX, sBoundingBox.maxX,
                sBoundingBox.minY,sBoundingBox.maxY,-1,1);
	glMatrixMode(GL_MODELVIEW);
}

void Widget::mouseMoveEvent(QMouseEvent *event)
{
   //1st position
   startX = Mouse.x;
   startY = Mouse.y;

   // I convert the coordinates from mouse here 
   line2 = convCoords(event->x(),event->y());

   //2nd position
   Mouse.x = line2[0]; //Longitude
   Mouse.y = line2[1]; //Latitude

   offsetX = startX - Mouse.x;
   offsetY = startY - Mouse.y;

   update();
}