Using c# Graphics.IsVisible seems to be off

Question

0.00/5 (No votes)

See more:

Hello, hopefully this is super simple for someone experienced with graphics and can explain the problem here (I'm guessing it has to do with Rounding.

I'm drawing a triangle here and I'm using IsVisible and doing a hit test on a point (to see if the point is inside the triangle. For testing purposes I'm scanning a few points in a couple of lines. The triangle is (0,0),(10,0),(10,10). I would've expected the point 5,5 to be right on the line (i.e. unsure if it hits), but certainly nothing earlier in X (e.g. 4.9,5) to be a hit. However if you look at my results below, this starts to evaluate true far earlier at 4.5,5. Is this just a rounding error? (sort of weird since it's so far).

Is there a better way to do a hit test? I can't just do equations as I want my code to be fairly generic.

C#

private static GraphicsPath RunHitTestsWithGP()
{
    var gp = new GraphicsPath();
    gp.StartFigure();
    gp.AddLines(new PointF[]
        {
            new PointF(0, 0),
            new PointF(10, 0),
            new PointF(10, 10),
            new PointF(0, 0)
        });
    gp.CloseFigure();

    for (int i = 0; i < 20; i++)
    {
        var t1 = new PointF(4f + i / 10f, 5);
        System.Diagnostics.Debug.WriteLine("Testing: {0} Yields: {1}", t1, gp.IsVisible(t1));
    }

    for (int i = 0; i < 20; i++)
    {
        var t1 = new PointF(4f + i / 10f, 6);
        System.Diagnostics.Debug.WriteLine("Testing: {0} Yields: {1}", t1, gp.IsVisible(t1));
    }
    return gp;
}

Testing: {X=4, Y=5} Yields: False
Testing: {X=4.1, Y=5} Yields: False
Testing: {X=4.2, Y=5} Yields: False
Testing: {X=4.3, Y=5} Yields: False
Testing: {X=4.4, Y=5} Yields: False
Testing: {X=4.5, Y=5} Yields: True
Testing: {X=4.6, Y=5} Yields: True
Testing: {X=4.7, Y=5} Yields: True
Testing: {X=4.8, Y=5} Yields: True
Testing: {X=4.9, Y=5} Yields: True
Testing: {X=5, Y=5} Yields: True
Testing: {X=5.1, Y=5} Yields: True
Testing: {X=5.2, Y=5} Yields: True
Testing: {X=5.3, Y=5} Yields: True
Testing: {X=5.4, Y=5} Yields: True
Testing: {X=5.5, Y=5} Yields: True
Testing: {X=5.6, Y=5} Yields: True
Testing: {X=5.7, Y=5} Yields: True
Testing: {X=5.8, Y=5} Yields: True
Testing: {X=5.9, Y=5} Yields: True


Testing: {X=4, Y=6} Yields: False
Testing: {X=4.1, Y=6} Yields: False
Testing: {X=4.2, Y=6} Yields: False
Testing: {X=4.3, Y=6} Yields: False
Testing: {X=4.4, Y=6} Yields: False
Testing: {X=4.5, Y=6} Yields: False
Testing: {X=4.6, Y=6} Yields: False
Testing: {X=4.7, Y=6} Yields: False
Testing: {X=4.8, Y=6} Yields: False
Testing: {X=4.9, Y=6} Yields: False
Testing: {X=5, Y=6} Yields: False
Testing: {X=5.1, Y=6} Yields: False
Testing: {X=5.2, Y=6} Yields: False
Testing: {X=5.3, Y=6} Yields: False
Testing: {X=5.4, Y=6} Yields: False
Testing: {X=5.5, Y=6} Yields: True
Testing: {X=5.6, Y=6} Yields: True
Testing: {X=5.7, Y=6} Yields: True
Testing: {X=5.8, Y=6} Yields: True
Testing: {X=5.9, Y=6} Yields: True

Posted 10-Dec-14 17:06pm

rld1971

Add a Solution

2 solutions

Solution 2

Ah, I did a little further reading based on your input, thanks. Kind of annoying that they seem to advertise floating point graphics when in fact it is not well supported.

Posted 11-Dec-14 0:54am

rld1971

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Member 9992980 · Accepted Answer · 2014-12-10T18:30:00

Known issue. Floating point values are rounded to integers when calling graphicspath.isvisible.

unbelievable and sad, but true. I am working on another way. I will update you shortly.

Update:

After thinking about it, it makes sense, like double.equals you would need to define a difference measure, or precision. Otherwise it would be infinitesimally precise. So a simple mathematical workaround could be to scale up and then do the hit test. In other words determine at what decimal place do you want to round(i.e. 4,9, 4.99, 4.999). So the solution below works, but maybe you could refine it a bit, because it will only work to a point and then it overflows. I checked it for 5 decimal places and it works. You also have to consider the precision of the entries, like for graphics in real world scenarios with pixels, for example, you can't detect a fractional pixel. Not sure what the purpose of the app is, but this should point you in the right direction.

C#

private static GraphicsPath RunHitTestsWithGP(int DecimalPlaces)
{
	dynamic gp = new GraphicsPath();
	int Scale = Math.Pow(10, DecimalPlaces);
	gp.StartFigure();
	gp.AddLines(new PointF[] {
		ScalePointF(new PointF(0, 0), DecimalPlaces),
		ScalePointF(new PointF(10, 0), DecimalPlaces),
		ScalePointF(new PointF(10, 10), DecimalPlaces),
		ScalePointF(new PointF(0, 0), DecimalPlaces)
	});
	gp.CloseFigure();
	for (int i = 0; i <= 19; i++) {
		dynamic t1 = new PointF(4f + i / 10f, 5);
		Console.WriteLine("Testing: {0} Yields: {1}", t1, gp.IsVisible(ScalePointF(t1, DecimalPlaces)));
	}

	for (int i = 0; i <= 19; i++) {
		dynamic t1 = new PointF(4f + i / 10f, 6);
		Console.WriteLine("Testing: {0} Yields: {1}", t1, gp.IsVisible(ScalePointF(t1, DecimalPlaces)));
	}
	return gp;
}

private static PointF ScalePointF(PointF PointFToScale, int DecimalPlaces)
{
	return new PointF(PointFToScale.X * (Math.Pow(10, DecimalPlaces)), PointFToScale.Y * (Math.Pow(10, DecimalPlaces)));
}