0075: Nodes-n-noodles II – Mouse Noodling

We’re still working our way through Phase One of this nodes-n-noodles concept and this time around, we’ll leave static noodles behind and get some animation happening.

Controlling the Noodle with the Mouse

Results of this example:
Current example output
Current example output
Current example terminal output
Current example terminal output (click for enlarged view)

The next step toward drawing a noodle connection between nodes entails two things:

  • using the mouse position as the curve’s end point, and
  • taking control of window redraw (animation).

To that end, we’re going to borrow from an earlier Cairo animation example Blog Post #0064 as well as the mouse tracker example Blog Post #0015 we looked at back in March.

From these earlier examples, we’re reminded that:

  • the mouse pointer’s position is supplied by the onMotionNotify signal, and
  • gaining control over window redraw means working with a Timeout object.

Draw and Redraw

The Timeout object, as we saw before, decides when we’ll call a function similar to this example’s onFrameElapsed() function (which is taken, unchanged, from the mouse tracker example):

bool onFrameElapsed()
	GtkAllocation size;
	queueDrawArea(size.x, size.y, size.width, size.height);
} // onFrameElapsed()

What it does is:

  • instantiate a GtkAllocation object from which we…
  • extract the size of the DrawingArea, and
  • call for a redraw.

The Cubic Bezier’s Starting Point

For now, we’ll use a hard-coded starting point and turn our attention to another problem…

Grabbing the Mouse Pointer Position

We can use the onMotion() function from the mouse tracker example to grab the mouse pointer position and use those x/y coordinates, along with the hard-coded starting point coordinates, to set our control points. From the previous post in this series, here’s a reminder of what those control point coordinates will be:

  • the coordinates of the first control point are:
    • x is the same as the end point’s x,
    • y is the same as the start point’s y,
  • the second control point’s coordinates:
    • x is the same as the start point’s x, and
    • y is the same as the end point’s y.
public bool onMotion(Event event, Widget widget)
	// make sure we're not reacting to the wrong event
	if(event.type == EventType.MOTION_NOTIFY)
		// get the curve's end point
		xEnd = event.motion.x;
		yEnd = event.motion.y;
		// Recalculate the control points so we always have
		// a nice-looking double curve.
		controlPointX1 = xEnd;
		controlPointY1 = yStart;
		controlPointX2 = xStart;
		controlPointY2 = yEnd;

} // onMotion()

And indeed, in the onMotion() function, we’re doing just that. After grabbing the mouse pointer position to use as the curve’s end point (xEnd/yEnd), we set the controlPoints and we’re ready to draw the curve. And the drawing itself is handled by the onDraw() function.

How the onDraw() Function Changes

Two things change in onDraw():

  • a Timeout object is harnessed to set the redraw rate in frames per second, and
  • instantiation of the control point coordinates has moved up to the preamble section of the MyDrawingArea class.

And that leaves the onDraw() function looking like this:

bool onDraw(Scoped!Context context, Widget w)
	if(_timeout is null)
		_timeout = new Timeout(fps, &onFrameElapsed, false);

	// set up and draw a cubic Bezier
	context.setSourceRgb(0.3, 0.2, 0.1);
	context.moveTo(xStart, yStart);
	context.curveTo(controlPointX1, controlPointY1, controlPointX2, controlPointY2, xEnd, yEnd);

} // onDraw()

And the MyDrawingArea preamble now looks like:

Timeout _timeout;
int fps = 1000 / 24; // 24 frames per second
double xStart = 25, yStart = 128;
double xEnd, yEnd;
double controlPointY1 = 128, controlPointX2 = 25;
double controlPointX1, controlPointY2;

The Constructor

As with the previous example, the constructor remains lean:

} // this()

Hook up the signals and we’re done.


So there you have it, run the example and move the mouse around within the window. The curve redraws as the mouse moves and always describes a cubic Bezier curve.

Next time, we’ll take another step wherein a mouse button is clicked to start the drawing of the curve.

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© Copyright 2022 Ron Tarrant