Question

For graphics exercises and some self-improvement sorta stuff, i’ve decided to basically just mess around and try and recreate some of the functionality of paint within a winform. I’ve got a lot of the standard ones to work, for example paint can, drawing dots around the cursor, free hand draw and what not, however i’m a little puzzled as to how paint does the mid-drawing animations. For example;

To draw a simple line i can simply get mouse coordinates on MouseUp and MouseDown events, and use the graphics class to draw a line between the two.

However, on MSpaint whilst drawing a line, you get almost a “preview” of the line after you click the first point, and whilst dragging it to the second point the line follows your cursor, but i’m a little stuck as to how this would be done? Does it involve constant redrawing of the line and the graphics device? It’d be great if someone could give me some hints / inner knowledge, i’ve had a search around the internet but can’t REALLY find anything of use..

Answer 1

And very modern via ControlPaint.DrawReversibleLine method 🙂

Point? startPoint;
Point? endPoint;

private void Form_MouseDown(object sender, MouseEventArgs e)
{
    startPoint = PointToScreen(e.Location);
}

private void Form_MouseMove(object sender, MouseEventArgs e)
{
    if (!startPoint.HasValue)
        return;

    if (endPoint.HasValue)
        ControlPaint.DrawReversibleLine(startPoint.Value, endPoint.Value, Color.White);

    endPoint = PointToScreen(e.Location);
    ControlPaint.DrawReversibleLine(startPoint.Value, endPoint.Value, Color.White);
}

private void Form_MouseUp(object sender, MouseEventArgs e)
{
    startPoint = null;
    endPoint = null;
}