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Drag Example

There are two ways to handle dragging with the mouse, "polling" and "callbacks". Polling is the older way of handling mouse events, and it still works. Starting with VPython 6, there is another scheme involving callbacks. For more information about these two methods, see general mouse documentation. We show here two programs that achieve the same result by the two different methods. You can use either method, but you cannot mix polling and callback methods in the same program.

Both the polling and callback versions of the program achieve the same goal: dragging a sphere across the screen.

 

Handling dragging with callbacks

Here is the sequence of mouse events involved in dragging something:

1) On a 'mousedown' event, see what object (if any) has been "picked" (lies under the mouse).

2) On a 'mousemove' event, update the position of the object based on the new mouse position.

3) On a 'mouseup' event, stop dragging the object.

Here is a callback version of a program in which you can drag a sphere across the screen. Copy this into an edit window and try it!

from visual import *
scene.range = 5 # fixed size, no autoscaling
ball = sphere(pos=(-3,0,0), color=color.cyan)
cube = box(pos=(+3,0,0), size=(2,2,2), color=color.red)
drag_pos = None # no object picked yet

def grab(evt):
    global drag_pos
    if evt.pick == ball:
        drag_pos = evt.pickpos
        scene.bind('mousemove', move, ball)
        scene.bind('mouseup', drop)

def move(evt, obj):
    global drag_pos
    # project onto xy plane, even if scene rotated:
    new_pos = scene.mouse.project(normal=(0,0,1))
    if new_pos != drag_pos: # if mouse has moved
        # offset for where the ball was touched:
        obj.pos += new_pos - drag_pos
        drag_pos = new_pos # update drag position

def drop(evt):
    scene.unbind('mousemove', move)
    scene.unbind('mouseup', drop)

scene.bind('mousedown', grab)

First, we bind 'mousedown' events to the function named grab, which checks to see whether the sphere has been touched. If so, it binds future 'mousemove' and 'mouseup' events to the move and drop functions.

If you do a lot of processing of each mouse movement, or you are leaving a trail behind the moving object, you may need to check whether the "new" mouse position is in fact different from the previous position before processing the "move", as is done in the example above. For example, a trail drawn with a curve object that contains a huge number of points all at the same location may not display properly.

Most VPython objects can be "picked" by touching them. Here is a more general routine which lets you drag either the tail or the tip of an arrow. Copy this into an edit window and try it!

from visual import *
scene.range = 8 # fixed size, no autoscaling
arr = arrow(pos=(2,0,0),axis=(0,5,0))
by = 1 # touch this close to tail or tip
drag_pos = None

def grab(evt):
    global drag_pos
    drag = None
    if mag(arr.pos-evt.pos) <= by:
        drag = 'tail' # near tail of arrow
    elif mag((arr.pos+arr.axis)-evt.pos) <= by:
        drag = 'tip' # near tip of arrow
    if drag is not None:
        drag_pos = evt.pos # save mousedown location
        scene.bind('mousemove', move, drag)
        scene.bind('mouseup', drop)

def move(evt, drag):
    global drag_pos
    new_pos = evt.pos
    if new_pos != drag_pos: # if mouse has moved
        displace = new_pos - drag_pos # how far
        drag_pos = new_pos # update drag position
        if drag == 'tail':
            arr.pos += displace # displace the tail
        else:
            arr.axis += displace # displace the tip

def drop(evt):
    scene.unbind('mousemove', move)
    scene.unbind('mouseup', drop)

scene.bind('mousedown', grab)

 

 

Handling dragging with polling

Here is the sequence of mouse events involved in dragging something using polling, assuming that m1 = scene.mouse.getevent():

1) m1.press is true when you depress the mouse button (it is 'left' if left button; any quantity that is nonzero is considered true in Python).

2) m1.drag is true when the mouse coordinates change from what they were at the time of m1.press.

At the time of the drag event, the mouse position is reported to be what it was at the time of the press event, so that the dragging can start at the place where the user first pressed the mouse button. If the mouse is in motion at the time of the press event, it is quite possible that the next position seen by the computer, at the time of the drag event, could be quite far from the initial mouse down position. This is why the position of the drag event is reported as though it occurred at the press location.

3) No events occur while dragging; you continually use scene.mouse.pos to update what you're dragging.

4) m1.drop is true when you release the mouse button.

You can program dragging with the mouse simply by continually reading the current value of scene.mouse.pos. Here is a complete polling routine for dragging a sphere with the left button down. Copy this into an edit window and try it!

from visual import *
scene.range = 5 # fixed size, no autoscaling
ball = sphere(pos=(-3,0,0), color=color.cyan)
cube = box(pos=(+3,0,0), size=(2,2,2), color=color.red)
pick = None # no object picked out of the scene yet
while True:
    rate(30)
    if scene.mouse.events:
        m1 = scene.mouse.getevent() # get event
        if m1.drag and m1.pick == ball: # if touched
            drag_pos = m1.pickpos # where on the ball
            pick = m1.pick # pick now true (not None)
        elif m1.drop: # released at end of drag
            pick = None # end dragging (None is false)
    if pick:
        # project onto xy plane, even if scene rotated:
        new_pos = scene.mouse.project(normal=(0,0,1))
        if new_pos != drag_pos: # if mouse has moved
            # offset for where the ball was touched:
            pick.pos += new_pos - drag_pos
            drag_pos = new_pos # update drag position

If you do a lot of processing of each mouse movement, or you are leaving a trail behind the moving object, you may need to check whether the "new" mouse position is in fact different from the previous position before processing the "move", as is done in the example above. For example, a trail drawn with a curve object that contains a huge number of points all at the same location may not display properly.

Most VPython objects can be "picked" by touching them. Here is a more general routine which lets you drag either the tail or the tip of an arrow. Copy this into an edit window and try it!

from visual import *
scene.range = 8 # fixed size, no autoscaling
arr = arrow(pos=(2,0,0),axis=(0,5,0))
by = 1 # touch this close to tail or tip
drag = None # have not selected tail or tip of arrow
while True:
    rate(30)
    if scene.mouse.events:
        m1 = scene.mouse.getevent() # obtain event
        if m1.press:
            if mag(arr.pos-m1.pos) <= by:
                drag = 'tail' # near tail of arrow
            elif mag((arr.pos+arr.axis)-m1.pos) <= by:
                drag = 'tip' # near tip of arrow
            drag_pos = m1.pos # save press location
        elif m1.drop: # released at end of drag
            drag = None # end dragging (None is False)
    if drag:
        new_pos = scene.mouse.pos
        if new_pos != drag_pos: # if mouse has moved
            displace = new_pos - drag_pos # how far
            drag_pos = new_pos # update drag position
            if drag == 'tail':
                arr.pos += displace # displace the tail
            else:
                arr.axis += displace # displace the tip

Here is general mouse documentation.