Re: Plot failure: "Visible pixel grid has a scan line longer than previous scan lines"

[Ethan A M. <me...@uw...>]

On Monday, 29 June 2020 20:55:26 PDT Dima Kogan wrote:
> Ethan A Merritt <me...@uw...> writes:
> 
> > On Monday, 29 June 2020 19:51:04 PDT Dima Kogan wrote:
> >> Alright, I debugged this (rr was invaluable), and I know mostly what's
> >> going on. The flow is roughly this:
> >>
> >> 1. The data is read here:
> >>
> >>    https://sourceforge.net/p/gnuplot/gnuplot-main/ci/branch-5-4-stable/tree/src/plot3d.c#l1037
> >>
> >>    At the end of the first line, x is 3999.0000000000005, which is just outside the
> >>    xrange [0:3999]. This isn't unexpected due to the "using" expression
> >
> > But the x coordinates should come out identically the same in the "good" and "bad"
> > cases, right?  So how does this explain a different between them?
> 
> In the "good" case we're getting lucky in step 5. The corners (which
> were out-of-bounds in step 2, and were thus ignored for computing
> view_port_z[]) still end up in-bounds in step 5. There's an extra step
> that I didn't mention in the previous email:

Very strange.  The z range should be totally irrelevant to "set view map".
z as a coordinate doesn't matter because it's in projection.
z as an image pixel value doesn't matter because the image grid doesn't
care about the pixel value, only the number of pixels.

But you're right, it does seem to make a difference in this case
For instance:
    gnuplot> splot $MATRIX matrix  using ($1*266.6):($2*199.9090909090909):3 title "" with image
         warning: Visible pixel grid has a scan line longer than previous scan lines.
    gnuplot> splot $MATRIX matrix  using ($1*266.6):($2*199.9090909090909):(0):3 title "" with image
         works with no problem

But opening up the z range explicitly does not fix the problem.
So I still don't understand exactly what's going on here. 

	I'll look harder tomorrow,

		Ethan


> 3.5. We expand the autoranged axis extents to nice, round numbers:
> 
>      https://sourceforge.net/p/gnuplot/gnuplot-main/ci/branch-5-4-stable/tree/src/axis.c#l960
> 
> This provides some margin so that getting lucky in this way is possible.