user interface - Writing a paint program à la MS Paint - how to interpolate between mouse move events?

Question

I want to write a paint program in the style of MS Paint.

For painting things on screen when the user moves the mouse, I have to wait for mouse move events and draw on the screen whenever I receive one. Apparently, mose move events are not sent very often, so I have to interpolate the mouse movement by drawing a line between the current mouse position and the previous one. In pseudocode, this looks something like this:

var positionOld = null

def handleMouseMove(positionNew):
    if mouse.button.down:
        if positionOld == null:
            positionOld = positionNew
        screen.draw.line(positionOld,positionNew)
        positionOld = positionNew

Now my question: interpolating with straight line segments looks too jagged for my taste, can you recommend a better interpolation method? What method do GIMP or Adobe Photoshop implement?

Alternatively, is there a way to increase the frequency of the mouse move events that I receive? The GUI framework I'm using is wxWidgets.

GUI framework: wxWidgets.
(Programming language: Haskell, but that's irrelevant here)

EDIT: Clarification: I want something that looks smoother than straight line segments, see the picture (original size):

EDIT2: The code I'm using looks like this:

-- create bitmap and derive drawing context
im      <- imageCreateSized (sy 800 600)
bitmap  <- bitmapCreateFromImage im (-1)    -- wxBitmap
dc      <- memoryDCCreate                   -- wxMemoryDC
memoryDCSelectObject dc bitmap

...
-- handle mouse move
onMouse ... sw (MouseLeftDrag posNew _) = do
    ...
    line dc posOld posNew [color     := white
                          , penJoin  := JoinRound
                          , penWidth := 2]
    repaint sw                              -- a wxScrolledWindow

-- handle paint event
onPaint ... = do
    ...
    -- draw bitmap on the wxScrolledWindow
    drawBitmap dc_sw bitmap pointZero False []

which might make a difference. Maybe my choices of wx-classes is why I'm getting a rather low frequency of mouse move events.

Answer 1

Live demos

• version 1 - more smooth, but more changing while you draw: http://jsfiddle.net/Ub7RV/1/
• version 2 - less smooth but more stable: http://jsfiddle.net/Ub7RV/2/

The way to go is 

Spline interpolation of the points

The solution is to store coordinates of the points and then perform spline interpolation.

I took the solution demonstrated here and modified it. They computed the spline after you stop drawing. I modified the code so that it draws immediately. You might see though that the spline is changing during the drawing. For real application, you probably will need two canvases - one with the old drawings and the other with just the current drawing, that will change constantly until your mouse stops.

Version 1 uses spline simplification - deletes points that are close to the line - which results in smoother splines but produce less "stable" result. Version 2 uses all points on the line and produces much more stable solution though (and computationally less expensive).