Render...Function Plotter analytical derivatives incorrect

[Sean McGivern]

In the Function Plotter, entering an analytical derivative in "First derivative" and unchecking "Calculate first derivative numerically" leads to an incorrect function in 0.46. This can be seen by entering x**2 as the function and 2*x as the first derivative with x-range -5.0 to 5.0 and y-range -0.1 to 30, any number of samples (at least up to 32). With "Calculate first derivative numerically" checked, beautiful parabola; without it, not a beautiful parabola. I also confirmed this with exp(-x*x) as the function and -2*x*exp(-x*x) as the derivative.

[Alvin Penner (apenner)]

attached is a modified version of the file C:\Program Files\Inkscape\share\extensions\funcplot.py, which I believe should fix this problem. I modified the usage of the analytical first derivative.

hth...

[sas (sas-sas)]

The problem is just that there are a couple of zeros that should be ones. I've attached a patch.

(This patch is essentially equivalent to Alvin Penner's, because

        dx0 = (x1 - x0)/ds # Only works for rectangular coordinates
        dy0 = fp(xstart)*dx0

in his version can be simplified to

        dx0 = 1 # Only works for rectangular coordinates
        dy0 = fp(xstart)

as (x1 - x0)/ds == 1, and similarly for dx1 and dy1.)

[Alvin Penner (apenner)]

sounds good to me ...

[Rygle (rygle)]

I am unable to test this on either 0.46 Win32 or 0.46+dev Win32 (built from SVN two days ago), as neither of them are giving any visible results. Didn't patch and rebuild, but manually edited the .py file in the existing build.

Sas, is there any possibility that this could be related the the problem you picked up in the Polyhedron 3D extension? I'm doubtful, as that one worked on 0.46 and not on the dev version, while this doesn't seem to be working on either for me.

[Rygle (rygle)]

Using Inkcl.bat to catch the stderr output, I get the following;

====
** (inkscape.exe:3580): CRITICAL **: void Inkscape::GC::Anchored::release() cons
t: assertion `_anchor' failed
====

Can't get anything from gdb, as when I run it and try doing running function plotter, it keeps displaying the message 'Function Plotter' working, please wait..., and starts hogging system resources (99% processor). Have to quit the DOS window (and hence gdb and Inkscape) to get control back.

[Rygle (rygle)]

Actually, forget that. The same message comes up even when I don't run Function Plotter.

[Alvin Penner (apenner)]

not sure if you've run this before or not, but when you use Function Plotter it is essential to first draw a rectangle and the rectangle must be selected, otherwise there is no output

[Rygle (rygle)]

OK Thanks Alvin. Did that and it works well.

I've patched this in SVN at revision 18663

[Rygle (rygle)]

Since this is also a problem with an existing function, I'll nominate this for 0.46.1