What are the hard bounds for drawing coordinates in GDI+?

Question

I am rendering an interpolation curve thusly:

e.Graphics.DrawLines(new Pen(Color.Red), _interpolationPoints.ToArray());

which sometimes throws an OverflowException.

Examination of the _interpolationPoints array shows some very large values in scientific notation e.g. {X = 0.0 Y = -1.985174E+10}

I suspect that Y = -1.985174E+10 is a value that GDI+ cannot handle. That's fine but what are the max/min X and Y values into which I can draw and so constrain the data (and warn the user) rather than catching the overflow exception during paint? Are the limits documented?

For example, I would like to do something like this:

if (yVal < float.MinValue || yval > float.MaxValue) 
      throw new OverflowException("Interpolation value too large to be rendered.");

during the population of the _interpolationPoints array and stop the process. (float mix/max does not work btw. i still get the exception.)

Answer 1

OK, I needed to know so I tested incrementally and came up with these limits:

positive:    1,073,741,951
negative:   -1,073,741,760

The code I used looked something like this:

int lastGoodVal = 0;
for (int i = -1073000000; i > -1073832999; i -= 1)
{
    g.DrawLine(Pens.Blue, new Point(0,0), new Point(0, i));
    lastGoodVal = i;
}

The loop above was the final test, stepping by 1, through a range of negative values established by earlier tests. As you can see, lastGoodVal holds the last successful painting iteration and therefore the real limit which I'll use as a constant.

I tried to correlate these numbers to a value in the .NET primitives but couldn't. Each limit is close to the value of 2^30 but is not exactly on it. Any other insight would be much appreciated.

I also only tested with the DrawLine method. It's possible that different limits exist for other functions in the API but I have not had a chance to explore that yet.

Also, after finishing this experiment and then Googling for the value 1073741951 I came across this article which correlates my findings. I also found this in a Mono code archive of some sort which mentions a near, though not exact correlation to float limits.

Answer 2

FYI - I ran into this situation with a simple implementation of a 2D plot. When I zoomed in too far on the image the corresponding pixel locations were WAY off the display area and caused Graphics.DrawLine to throw an OverflowException. So naturally I added a check to ensure that the values where within the limits defined by Paul above. Interestingly though when the Y value got too large (but less than suggested positive value of 1,073,741,951) the resulting line went from being drawn down as expected (to a Y pixel location greater than my last reasonable point) to being drawn up (to the top of the window).

After further investigation I discovered that a value of 8,388,607 (0x7FFFFF) draws the line correctly and a value of 8,388,608 (0x800000) inverts the line.

Looks like signed 24-bit values are used here.

Answer 3

I've not heard of specific limits for DrawLines() or any other GDI drawing function. Why don't you use e.ClipRectangle as a constraint?

Drawing points outside the visible region is not needed anyway. Just make sure that only lines are discarded that have both points outside.