[cairo] Scaled fixed size fonts

Uli Schlachter psychon at znc.in
Sat Jan 18 08:51:03 PST 2014


On 18.01.2014 13:17, Fred Kiefer wrote:
> It took me a bit longer to find the time to write up a simple example. I
> started of with one of the example files I found on this list. It simply
> creates an SVG surface and writes two lines of text in two different
> point sizes. Run the code as it is now, copy the result to somewhere
> save and then exchange the two writing blocks and run it again. In the
> first case the second line will look horrible in the second case that
> line is OK, but the first one is unreadable.
> This proves that for fixed sized fonts the font selection is order
> dependent and that the first selected font will be scaled for other sizes.
> Does this explain the issue in enough detail?

Not really. Attached are two SVGs. The file temp-orig.svg was created by your
program, the file temp.svg by swapping lines 31-33 with lines 35-37. To my
untrained eyes, the SVGs look the same.

> As I explained in my original mail, this has nothing to do with the SVG
> backend, I only use that to show the issue. It is caused by the way the
> FT fonts get created. Please take the time to scroll down to the
> original mail.
> Fred
> On 14.01.2014 09:25, Fred Kiefer wrote:
>> Hi Behdad,
>> I hope to find time to write a full example later today. Here a
>> simple way to reproduce the issue. Create a font face for one of the
>> Adobe fixed fonts. Create a scaled font with 12 pixel size. Next
>> create another scaled font from the font face with 24 pixel size use
>> that for drawing. The 12 pixel font will get used, but scaled to 24
>> pixel.
>> Fred
>> On the road
>> Am 14.01.2014 um 09:12 schrieb Behdad Esfahbod <behdad at behdad.org>:
>>> Hi,
>>> Your message is a bit vague in details.  Do you have a simple test
>>> case?
>>> behdad
>>>> On 14-01-06 03:48 AM, Fred Kiefer wrote: In the GNUstep project
>>>> we use cairo as our default drawing backend. One user complained
>>>> that with cairo some fonts get displayed worse than with the
>>>> direct xlib based backend. It turns out that on this computer
>>>> only the Adobe fonts are installed and this are fixed size
>>>> fonts. I investigated a bit in our code and later in the cairo
>>>> code and this burns down to the function
>>>> cairo_scaled_font_create() returning not the best suited fixed
>>>> size font, but a scaled version of the standard 12 point font.

I'll assume that I cannot reproduce your issue because I do not have the right
fonts. Do you happen to know which packages I would have to install on debian
testing to get different images?

>>>> The reason is that _cairo_ft_font_face_get_implementation() in 
>>>> cairo-ft-font.c reuses the same resolved font even when the font
>>>> isn't scalable and the matrix did change. My suggestion now is to
>>>> extend the tests in this function to first inspect the pattern
>>>> whether it is scalable. If it isn't, compare the pixel size of
>>>> the cached resolved font with the new pixel size (This code needs
>>>> to be extracted from _cairo_ft_resolve_pattern) and if that
>>>> doesn't match within certain limits (which might be the hard bit
>>>> to define) the old resolved font gets thrown away and replaced by
>>>> a newly created one. Or we leave the cached one alone and just
>>>> create a new one?
>>>> This solution will use up more resource than the current one and
>>>> if there is no better matching fixed pixel font it will result in
>>>> the same display with a lot more overhead. Still for some cases
>>>> it will result in better drawn fonts. And for the most common
>>>> case where scalable fonts get used it will be almost no
>>>> overhead.
>>>> Would cairo be willing to accept a patch in this direction? I am
>>>> willing to write one, but only if this effort isn't completely
>>>> wasted.

Who needs a ~/.signature anyway?
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