Re: 'nohidden3d' option behavior

[Ethan A M. <sf...@us...>]

On Monday, 23 May, 2016 18:24:39 Daniel J Sebald wrote:
> I've read through the documentation for the option "with ... 
> nohidden3d".  My initial understanding of the nohidden3d option is that 
> it could be a means to individually select what items are included in 
> hidden line removal:
> 
> "
>   As of gnuplot version 4.6, hidden3d also affects 3D plotting styles 
> `points`,
>   `labels`, `vectors`, and `impulses` even if no surface is present in 
> the graph.
>   Unobscured portions of each vector are drawn as line segments (no 
> arrowheads).
>   Individual plots within the graph may be explicitly excluded from this
>   processing by appending the extra option `nohidden3d` to the `with` 
> specifier.
> "
> 
> I'm not sure how "even if no surface is present in the graph" is to be 
> interpreted.  I take it to mean there is something in plots, currently, 
> other than surfaces which can have a 2D subspace that obscures the 
> visual field.
> 
> More than that though is this phrase "excluded from this processing". 
> It's ambiguous as to what "this processing" applies to. 

I'll reply in more detail later, but for now the key thing is that
"this processing" means "sorted together on depth (distance from viewer)".
I.e. even if there are no surfaces to obscure pieces of a vector,
it may still be useful to sort the vectors so that the close ones are
drawn on top of the further ones.  

If you attach the "nohidden3d" attribute to something then it isn't sorted
on depth.  All elements of that unsorted plot will appear in the same layer
(could be in front of; could be behind) the objects that were sorted together.

	Ethan



> There are two 
> ways to take this:
> 
> 1) "nohidden3d" applies to any object because "individual plots" would 
> include anything.
> 
> 2) "nohidden3d" only applies to points, labels, vectors and impulses.
> 
> My initial thought was that 1 is the case, but the current version of 
> gnuplot does nothing to "with lines" objects when 'with ... nohidden3d' 
> is specified.  The "with lines nohidden3d" still has hidden3d effects. 
> So, 2 seems more the case and to confirm that, here is the code from 
> plot3d.c:
> 
> 	    /* Rule out incompatible line/point/style options */
> 	    if (this_plot->plot_type == FUNC3D) {
> 		if ((this_plot->plot_style & PLOT_STYLE_HAS_POINT)
> 		&&  (this_plot->lp_properties.p_size == PTSZ_VARIABLE))
> 		    this_plot->lp_properties.p_size = 1;
> 	    }
> 	    if (this_plot->plot_style == LINES) {
> 		this_plot->opt_out_of_hidden3d = FALSE;
> 	    }
> 
> My thought on this is, Why exclude lines from the "nohidden3d" option? 
> If no hidden 3d processing is done on the surface lines, would that pose 
> a problem?  Also, "LINES" is excluded, but why then not "LINESPOINTS"?
> 
> Just to illustrate, try these plots:
> 
> gnuplot> set samples 20
> gnuplot> set hidden3d
> gnuplot> splot 20*sin(x*y)/(x*y) with lines, x+y with lines nohidden3d
> 
> The above just illustrates that "lines" is unaffected by "nohidden3d". 
> Now try
> 
> gnuplot> splot 20*sin(x*y)/(x*y) with lines, x+y with lp nohidden3d
> 
> So, linespoints is affected.  However, although the points behave as 
> expected, the lines have disappeared.  Why should the lines disappear? 
> I would think the second plot should look very similar to this result:
> 
> gnuplot> unset hidden3d
> gnuplot> splot 20*sin(x*y)/(x*y) with lines, x+y with lp
> 
> Again to illustrate the disappearing lines, try:
> 
> gnuplot> set hidden3d
> gnuplot> splot 20*sin(x*y)/(x*y) with linespoints nohidden3d
>           *All* edges undefined or out of range, thus no plot.
> 
> Maybe the current behavior isn't the intended behavior, but I'm not 
> fully comprehending the nohidden3d option.
> 
> Dan
> 
> 
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