Re: Histograms with fill solid and monochrome Postscript

[Ethan A M. <merritt@u.washington.edu>]

On Wednesday 30 July 2008, Philipp K. Janert wrote:
> 
> Ok, apparently we really misunderstood each other.
> Let me try again.
> 
> If I say:
> 	set style fill pattern
> 	set style histogram clustered
> 	set style data histogram
> 	plot "data" u 1, "" u 2, "" u 3
> 
> gnuplot will iterate through the available patterns
> for data in column 1, 2, and 3.
> 
> If instead I say
> 	set style fill solid
> 	...
> gnuplot will iterate through the available colors
> (red, green, blue, ...)
> 
> But what if I want a monochrome output? I can't 
> use red, green, blue, .... Instead I must use shades
> of gray

But isn't that exactly what you are saying you want?

> What I therefore want is for gnuplot to iterate through 
> densities:  not patterns, not colors. Just a single color, 
> but different densities of that color.

Color, or grey?  I guess I'm still confused about whether you
really care about density, or if that's just a convenient stand-in
for a monochrome greyscale.  The example I showed used grey partly
because there are pre-defined names for the incremental grey values.
It would work just as well for arbitrary color densities, but you'd
have to give them in the form "lc rgb '#RRGGBB'"

Unfortunately, not that many terminals support a uniform density
setting or a uniform transparency setting.  And even if they did,
it would end up with the same grayscale values as in the example
I gave.  So I don't think this would actually add anything to the
set of capabilities.

It happens that there are a few terminals that implement pattern
fill as a cycle of greyscale settings because they don't really 
offer patterns.  You could experiment with the emf terminal, say,
and see what you think of it.  I suppose we could offer this as
an optional alternative to pattern-fill on other terminal types. 

 
> I wonder whether there is currently support for that.
> 
> Best,
> 
> 		Ph.
> 
> 
> 
> On Wednesday 30 July 2008 22:03, you wrote:
> > On Wednesday 30 July 2008, Philipp K. Janert wrote:
> > > What I hear you say here is this, I think:
> > >
> > > Gnuplot has a way to automatically select
> > > different fill patterns (which will work in B&W),
> > > and different fill colors (which require a color
> > > terminal).
> >
> > Both, actually. It's your choice whether to use colors
> > or fill-patterns or both.
> >
> > > But gnuplot does NOT have a way to automatically
> > > select different monochrome fill densities. To
> > > achieve such an effect, I have to start hacking
> > > Postscript.
> >
> > No, I didn't intend to say that at all.
> > I may have misunderstood your question.
> > I thought you were asking about how you might print a
> > previously-created PostScript file on a monochrome printer
> > even though it was originally created using color.  I explained
> > one wasy to do this.
> >
> > If you are asking instead how you could create halftone output in
> > the first place, that's a different question.  What exactly are
> > you trying to do?  There may well be several alternatives available.
> >
> > > What we really would want is something that cycles
> > > through fill densities, in the same way that gnuplot
> > > cycles through line types.
> >
> > I think that is possible now, although perhaps not as conveniently
> > as you would like.
> >
> >   # Define a set of sequential gray values
> >   set style line 101 lc rgb "grey10"
> >   set style line 102 lc rgb "grey70"
> >   set style line 103 lc rgb "grey20"
> >   set style line 104 lc rgb "grey80"
> >   ...
> >   set style increment user # horrible syntax, but it's what we've got
> >
> >   set style histogram cluster
> >   set style fill solid 1 noborder
> >
> >   plot newhistogram lt 101, for [i=1:10] 'datafile' using i with histogram
> >
> > > Alternatively, we could have user-defined "fill styles",
> > > and then gnuplot could cycle through those. This way,
> > > users can define either a set of color fill styles, or
> > > monochrome density fill styles (or any combination
> > > thereof), and gnuplot would cycle through those. (I
> > > have no idea how hard that would be, and I would not
> > > think it is high priority, though.)
> >
> > Adding a mechanism to re-map the order of existing fill patterns would be
> > easy. What seems well-nigh impossible is to provide a terminal-independent
> > way of defining new fill styles.
> >
> > 	Ethan
> >
> > > On Wednesday 30 July 2008 19:05, you wrote:
> > > > On Wednesday 30 July 2008, Philipp K. Janert wrote:
> > > > > Let's say I am trying to generate histograms
> > > > > like this:
> > > > >
> > > > > set style fill solid
> > > > > set style histogram clustered
> > > > > set style data histogram
> > > > > plot "data" u 1, "" u 2, "" u 3
> > > > >
> > > > > I get a plot of histograms, with each "bin"
> > > > > in a different color (red, green, blue).
> > > > >
> > > > > Now, I would like to export this to a monochrome
> > > > > Postscript terminal. I would expect that the colors
> > > > > are mapped to different levels of grayscale.
> > > > >
> > > > > Instead, all colors seem to be mapped to black
> > > > > (or very nearly black).
> > > > >
> > > > > Is there a way to achieve the kind of automatic
> > > > > color-to-grayscale mapping I envision, or is this
> > > > > currently not supported?
> > > >
> > > > The issue is that there are two uses of color.
> > > >
> > > > One is to create a palette that captures a continuous quantity.
> > > > To convert this to a monochrome plot you want a greyscale mapping,
> > > > and that's what the postscript terminals do if you select "mono".
> > > >
> > > > The other use of color is simply to differentiate object A from
> > > > object B.  Here a greyscale is irrelevant, and you instead want
> > > > some mapping that makes the common colors maximally distinct from
> > > > each other.
> > > >
> > > > I suggest that a clever way to achieve the latter goal is to
> > > > in this case redefine the PostScript  setrgbcolor operator so
> > > > that you get an NTSC (black-and-white TV) mapping of the colors.
> > > > One line in the PostScript prolog suffices for this, so local
> > > > modification without touching the gnuplot executable is very
> > > > easy.  I've posted complete instructions several times, but
> > > > I don't seem to have saved a copy on my local disk.
> 
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-- 
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle 98195-7742