Re: font size problem with pango

[e98cuenc free fr]

Selon Gürer Özen <madcat e-kolay net>:

> I'm working on a small application which takes a data file and renders
> it into a GtkDrawingArea with the help of pango.
> 
> Data format is something like:
> This is a 28cm x 21cm page.
> There is a "hello world" text at 7cm,1.25cm with 12pt size.
> There is a 5cm x 5cm gfx at 0,0
> ...
> 
> All objects are scaled to the widget size, so you can zoom the image via
> changing the window size. That is important for application.
> 
> My problem is with font size. If font size is 12pt, then
> 
> font_size_cm = 12 * 2.54 / 72;
> font_size_pix = font_size_cm * widget_height_pix / page_height_cm;
> 
> gives the correct size for font height in pixels. i can use this method
> for drawing lines, curves, and pixbufs but pango api uses a special
> pango unit and i couldn't find a way to make that convertion.
> 
> If i use (font_size_pix * PANGO_SCALE * 0.8) in <span size="..."> I get
> a mostly correct rendering on my 75 dpi laptop, but render is broken on
> a 91 dpi monitor with the same data and same widget size. Changing 0.8
> to 0.6 fixes rendering on the monitor.
> 
> Since this factor changes with display device, i guess pango is changing
> its unit size depending on the dpi?

If I understand your problem, you want to draw 12pt text with a resolution of 
widget_height_pix / page_height_cm.  So you calculated correctly the number of 
pixels of such a size with that resolution.

Pango is expecting the size on "points divided by PANGO_SCALE".  And for pango 
the resolution for converting from points (a 1/72 inch) to pixels is the 
fontconfig dpi (usually 96 these days, I guess).

So, to convert from your pixels to points at fontconfig dpi you should do:

pango_size = font_size_pix * PANGO_SCALE * 72 / fontconfig_dpi;

If your fontconfig dpi is 75, the last 72 / fontconfig_dpi will be almost 1, 
so you were getting an almost right resolt with this resolution.  With a dpi 
of 91, the factor is 0.79.  And so on.

I think that there is a GtkSetting or something to get the fontconfig dpi, but 
I'm not sure.  You can also get it using directly the fontconfig API.

Hope it helps!

Cheers,