How to implement rectangle selection?

Question

I don't know if there is another term for it, but I mean this: You hold down a mouse button. A rectangle forms when you move the mouse around. The units inside the box get selected when you let go of the mouse button. I need this for a RTS style game I'm working on, but I don't know how to approach it.

I currently have functionality for selecting a unit, basically if you click within a certain area the unit is added to the list of selected units,

def mouseUp(self, button, pos):    
    adjusted = self.coord_convert(pos)
    if button==1:
        for a in self.agent_list:
            if a.pos.get_distance(self.coord_convert_adjusted(pos)) < 16: 
                self.selected.append(a)
            elif a in self.selected and a.pos.get_distance(self.coord_convert_adjusted(pos)) > 16:
                self.selected.remove(a)

The next step would be to allow for selecting multiple units at once by checking if they are inside a box, can you provide some guidance on how I should proceed?

Edit: To clarify, I'm asking how to create a box which starts at the position of MOUSEDOWN, and changes in size relative to mouse movement while holding the pressed button down, until let go.

Answer 1

For a rectangular selection, it would be a lot like your existing code.

def mouseDown(self, button, pos):
    self.selection_rectangle_start = self.coord_convert(pos)

def mouseMotion(self, pos):
    self.selection_rectangle_end = self.coord_convert(pos)
    self.selection_rectangle = calc_rect(self.selection_rectangle_start, self.selection_rectangle_end)
    for a in self.agent_list:
        if a not in self.selected and a.intersects(selection_rectangle):
            self.selected.append(a)
        if a in self.selected and not a.intersects(selection_rectangle):
            self.selected.remove(a)

intersects is just a simple overlapping rectangle check. Pygame provides Rect.colliderect for such purposes (http://www.pygame.org/docs/ref/rect.html#Rect.colliderect). calc_rect would check both the points to find the top/left/width/height and construct a valid rectangle from them.

Answer 2

I have to admit i am unfamiliar with python, but i think following steps seem appropriate:

1. Once the mouse is pressed down store the position it is currently at.
2. Start checking for a change in the mouse's position whenever the mouse is moved, or every X milliseconds.
3. If the mouse position has changed before the mouse is released then store the new mouse position in another variable.
4. Draw a rectangle from one position to the other.
5. When the mouse is released check each of your units x and y values to see if they are between the 2 positions.
6. Select each unit.

Hope this helps.