Re: [Plplot-devel] alpha channel/transparency for our background colour

[Alban R. <a.r...@im...>]

Dear Alan and PLplot community,

Please find attached an update for the Qt driver, allowing changes to
the background colors (supporting transparency), based on svn revision
10819.

I should submit quite soon another patch for the driver, making the code
a bit cleaner (not changing the general architecture of the driver
though) and making the driver faster.

Alban

Alan W. Irwin wrote:
> I have just (revision 10188) enabled alpha channel/transparency for our
> background colour for the cairo and svg device drivers.  These drivers thus
> join the gd device driver in handling this correctly, and I hope later this
> week, Alban will be able to enable the same functionality for the qt devices
> (along with his planned changes to enable rgb background colour for the qt
> devices.)
>
> For now you can see what semi-transparent backgrounds look like
> by locally changing the second line of cmap0_black_on_white.pal from
>
> #ffffff 1.0
>
> to, e.g.,
>
> #ff0000 0.3
>
> which gives a red mostly transparent background for the gd, cairo, and svg
> devices (and an opaque red background for our devices which are not
> transparency aware).
>
> I would also like to specify the transparency of the background conveniently
> using the -bg command-line option. I was thinking along the lines of
>
> -bg ff0000_0.3
>
> to specify the same semitransparent background that I gave as an example
> above. The implementation would simply check the length of the option string
> and if greater than 6, parse the first 6 as hex, check for the underscore,
> and parse the remainder as a floating point value.  Please comment _soon_ if
> you forsee any trouble with that implementation idea.
>
> Of course, our current semitransparent background results look rather dull
> because linux applications still are not as transparency aware as they
> should be, and therefore, they currently tend to impose opaque backgrounds
> of their own upon which the PLplot background is superimposed.  To sort out
> which background is which, you can sometimes specify a background colour for
> the application.  For example, the ImageMagick "display" application has the
> -background option to specify an opaque colour for their background, and I
> was able to use that "display" option to prove that _our_ background
> transparency was working correctly for the gd, cairo, and svg devices. (Of
> course, it would be nice if the "display" background could be
> semi-transparent as well so I hope some ImageMagick developer is working on
> that.)
>
> In the future, as Linux and other OS's become completely transparency aware,
> I can forsee our semi-transparent background capability allowing for some
> really cool effects such as superimposing our plots on top of other images
> or just on top of the general desktop behind where one of our plots is
> displayed. Anyhow, that was the motivation for the latest commit and will
> also be the motivation for the planned change to the -bg command-line option.
>
> Alan
> __________________________
> Alan W. Irwin
>
> Astronomical research affiliation with Department of Physics and Astronomy,
> University of Victoria (astrowww.phys.uvic.ca).
>
> Programming affiliations with the FreeEOS equation-of-state implementation
> for stellar interiors (freeeos.sf.net); PLplot scientific plotting software
> package (plplot.org); the libLASi project (unifont.org/lasi); the Loads of
> Linux Links project (loll.sf.net); and the Linux Brochure Project
> (lbproject.sf.net).
> __________________________
>
> Linux-powered Science
> __________________________
>
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