[gl2ps] Translate pixels to points

[Ben Abbott]

> On Sep 13, 2016, at 07:27, Pantxo Diribarne <pantxo.diribarne at gmail.com> wrote:
> 
> 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

I like your suggestion. That would simplify things. Are you working on a patch?

Since "points" has a different meaning for Linux, Windows (96 ppi), and Mac (72 ppi), should the default change with os? Or is the ppi always 72 for pdf?

Ben
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