Question

I’ve been messing around with the Graphics class to draw some things on a panel. So far to draw, I’ve just been using the Rectangle Structure. On a panel, by clicking a button, it makes a rectangle in a random place and adds it to an array of other rectangles (They’re actually a class called UIElement, which contains a Rectangle member). When this panel is clicked, it runs a test with all the elements to see if the mouse is inside any of them, like this:

void GUIDisplay::checkCollision()
{
    Point cursorLoc = Cursor::Position;
    for(int a = 0; a < MAX_CONTROLS; a++)
    {
        if(elementList[a] != nullptr)
        {
            if(elementList[a]->bounds.Contains(cursorLoc))
            {
                elementList[a]->Select();
                //MessageBox::Show("Click!", "Event");
                continue;
            }
            elementList[a]->Deselect();
        }
    }
    m_pDisplay->Refresh();
}

The problem is, when I click the rectangle, nothing happens.

The UIElement class draws its rectangles in the following bit of code. However, I’ve modified it a bit, because in this example it uses the DrawReversibleFrame method to do the actually drawing, as I was using Graphics.FillRectangle method. When I changed it, I noticed DrawReversibleFrame drew in a different place than FillRectangle. I believe this is because DrawReversibleFrame draws with its positions relative to the window, while FillRectangle does it relative to whatever Paint event its in (Mines in a panel’s Paint method.) So let me just show the code:

void UIElement::render(Graphics^ g)
{
    if(selected)
    {
        Pen^ line = gcnew Pen(Color::Black, 3);
        //g->FillRectangle(gcnew SolidBrush(Color::Red), bounds);
        ControlPaint::DrawReversibleFrame(bounds, SystemColors::Highlight, FrameStyle::Thick);
        g->FillRectangle(gcnew SolidBrush(Color::Black), bounds);
        //g->DrawLine(line, bounds.X, bounds.Y, bounds.Size.Width, bounds.Size.Height);
    }
    else
    {
        ControlPaint::DrawReversibleFrame(bounds, SystemColors::ControlDarkDark, FrameStyle::Thick);
        //g->FillRectangle(gcnew SolidBrush(SystemColors::ControlDarkDark), bounds);
    }
}

I add in both DrawReverisbleFrame and FillRectangle so that way I could see the difference. This is what it looked like when I clicked the frame drawn by DrawReversibleFrame:

The orange frame is where I clicked, the black is where its rendering. This shows me that the Rectangle’s Contains() method is look for the rectangle relevant to the window, and not the panel. That’s what I need fixed 🙂

I’m wondering if this is happening because the collision is tested outside of the panels Paint method. But I don’t see how I could implement this collision testing inside the Paint method.

UPDATE:

Ok, so I just discovered that it appears that what DrawReversibleFrame and FillRectangle draw are always a certain distance apart. I don’t quite understand this, but someone else might.

Answer 1

Both Cursor::Position and DrawReversableFrame operate in screen coordinates. That is for the entire screen, everything on your monitor, and not just your window. FillRectangle on the other hand operates on window coordinates, that is the position within your window.

If you take your example where you were drawing with both and the two boxes are always the same distance apart, and move your window on the screen then click again, you will see that the difference between the two boxes changes. It will be the difference between the top left corner of your window and the top left corner of the screen.

This is also why when you check to see what rectangle you clicked isn’t hitting anything. You are testing the cursor position in screen coordinates against the rectangle coordinates in window space. It is possible that it would hit one of the rectangles, but it probably won’t be the one you actually clicked on.

You have to always know what coordiante systems your variables are in. This is related to the original intention of Hungarian Notation which Joel Spolsky talks about in his entry Making Wrong Code Look Wrong.

Update:
PointToScreen and PointToClient should be used to convert coordinates between screen and window coordinates.