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Re: How to implent a rubber band Class in u++ [message #17373 is a reply to message #17298]

Sat, 09 August 2008 18:43

tojocky
Messages: 607
Registered: April 2008
Location: UK

Contributor

mrjt wrote on Wed, 06 August 2008 12:04

I can think of two possible optimisations (besides PolyLine, which is definitely the first one to use)

1- Avoid adding adjacent collinear points. In practice this is a very minor optimisation except for special cases, but it appeals to my sense of correctness.
.....

2- Use a back buffer. Providing you have a static background (one that will not change while the rubber band is being drawn), you can use a back buffer to cache the background + the current rubber band and only draw the last line.

This was a major improvement for me, eliminating flickering entirely.
.........

Very Hard optimization. Thank you!

Bu in this example is an error!

void RubberBand::AddPoint(const Point &newp)
{
	if (points.GetCount() < 2)
		return points.Add(newp);
	// Get line vectors
	const Point &p = points[points.GetCount()-2];
	Point p1 = points.Top() - p;
	Point p2 = newp - p;
	// If gradient is different, add the new point
	if (p1.x*p2.y - p2.x*p1.y)
		points.Add(newp);
	// Otherwise update last points
	else
==>HERE		points.Top() = newp;
}

The bug is when the newpoint is in the line draws from the last 2 points AND the newpoint is in between from the last 2 points!
Corrected code is:

void RubberBandClass::AddPoint(const Point &newp) {
	if (points.GetCount() < 2)
		return points.Add(newp);
	// Get line vectors
	const Point &p_second_last = points[points.GetCount()-2];
	const Point &p_last = points.Top();
	
	Point p1 = p_last - p_second_last;
	Point p2 = newp - p_second_last;
	// If gradient is different, add the new point
	if (p1.x*p2.y - p2.x*p1.y)
		points.Add(newp);
	// Otherwise update last points if the newpoint is continue of the last point from the second last point
	else{
		int pos_from_last = 0;
		
		int pos_1=newp.x, pos_2=p_last.x, pos_3=p_second_last.x;
		
		if (!((p_last.x == p_second_last.x)&&(newp.x == p_last.x)))
			int pos_1=newp.y, pos_2=p_last.y, pos_3=p_second_last.y;
		
		if (pos_2 > pos_3 ){
			if (pos_1 > pos_2) {
				pos_from_last = 1;
			}
			else if ( pos_1 < pos_3) {
				pos_from_last = 2;
			}
		}
		else {
			if (pos_1 < pos_3) {
				pos_from_last = 1;
			}
			else if (pos_1 >pos_2){
				pos_from_last = 2;
			}
		}
		if (pos_from_last) points[points.GetCount()-pos_from_last] = newp;
	}
}

In base of this example I can write paint method of controls more optimized!

Thanks a lot about back buffer (BackDraw)!

Is other ideas?

[Updated on: Sat, 09 August 2008 18:51]

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