[gl2ps] Translate pixels to points

[Pantxo Diribarne]

2015-08-30 19:51 GMT+02:00 Ben Abbott <bpabbott at mac.com>:

> On Aug 30, 2015, at 7:43 AM, Pantxo Diribarne <pantxo.diribarne at gmail.com>
> wrote:
>
> Le 29/08/2015 19:55, Ben Abbott a écrit :
>
> On Aug 28, 2015, at 6:21 AM, Pantxo Diribarne <pantxo.diribarne at gmail.com>
> wrote:
>
> Hi,
>
> In Octave we are using the default GL2PS_USE_CURRENT_VIEWPORT argument in
> the page initialization. This leads to a mismatch between the original
> figure size (for which the viewport is returned in pixels) and the ouput
> eps/pdf that has the same size in points. To sum up: a 400x400 *pixels*
> opengl window will be printed as a 400x400 *points* eps file.
>
> This would not really matter if we had only vectorial objects in our
> figures ... but we have text that are supposed to have a fixed size. For
> this we use (intentionally???) a nasty trick which leads to have a font
> size on-screen that is lower than it should (a 10 pts font is displayed as
> 10 pixels one...)
>
> How should we go if we want to draw an eps that has the same *physical*
> size has our on-screen figure, e.g. is there a way to specify the screen
> resolution so that gl2ps is able to translate pixels to points?
>
> Pantxo
>
>
> The root object (handle = 0) includes the property “ScreenPixelsPerInch”.
> This can be used to determine the physical figure size. Then it *should* be
> as simple as setting PaperPosition property to the physical size.
>
> I had worked on the displayed fontsize, and recall some confusion (on my
> part) when reading the Matlab docs and examining the rendered results. My
> confusion was compounded by my display’s resolution being 72 pixels/inch.
>
> My present display has 129 pixels/inch. As a test, I tried ...
>
> clf ()
> text (0.5, 0.5, {‘FOO’,’BAR'}, 'fontsize', 72, 'horizontalalignment',
> 'center')
>
> … in both Octave (default branch) and Matlab.
>
> The results are below. The lines are separated by 110 pixels high for
> Matlab and 84 for Octave.
>
> I think the intention was that the on screen size should be correct and
> that the print() function would handle the scaling between pixels and
> points.
>
> Ben
>
>
> <Mail Attachment.png><Mail Attachment.png>
>
>
> My first concern was initially the size of on-screen characters that I
> found to be much smaller than e.g. in LibreOffice (see [1]). The size of
> font in printed images is right though.
>
> AFAIU Octave uses a fontsize expressed in points (10 by default) to call
> fontconfig which expects pixels.
> Now in gl2psSimple.c, glut is used to render text bitmaps of size 24: this
> is a comparable approach as in Octave. If you compare the on-screen string
> from this example with the same string in libreoffice you'll also notice
> that the font is smaller. The same remark can be done about the screen
> shots you provide.
>
> Does it make sense to have, in gl2ps, a scaling factor between pixels and
> points that external programs such as Octave can provide or am I completely
> misunderstanding the printing process?
>
> Pantxo
>
> [1] https://savannah.gnu.org/bugs/?45600
>
>
> I think that could work, but some care needs to be taken to ensure the
> fontsizes are correct when specified as an option to the print comment.
>
>      '-FFONTNAME'
>      '-FFONTNAME:SIZE'
>      '-F:SIZE'
>           Use FONTNAME and/or FONTSIZE for all text.  FONTNAME is
>           ignored for some devices: dxf, fig, hpgl, etc.
>
> Since print.m already scales the fontsizes, I think the easiest solution
> is to set the proper default for opt.scalefontsize in print.m.
>
> opt.scalefontsize = 72 / get (0, ‘screenpixelsperinch’);
>
> Ben
>
>
>
>
I'd like to come back to this discussion in regard to the discrepancy
between SVG and EPS output size: from a M-by-N *pixels* opengl viewport,
gl2ps draws a M-by-N *points* figure for EPS format and a M-by-N *pixels*
figure for SVG format.
I think one of those should be fixed.

As I said in the beginning of the discussion, I'd vote for introducing a
new "resolution" (default 72 pixels per inch) argument for gl2psBeginPage.
This would provide a way to translate screen coordinates into whatever
units we like in gl2ps.

Pantxo
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