Implementing HiDpi scaling

[Emmanuele Bassi]

The way "high DPI" support is implemented is typically this:

- You retrieve the scaling factor from the windowing system; in the case of
X11 you can deduce the scaling factor from the ratio between the width and
the height of the target monitor: anything below 192 is generally
considered "1" and anything above is generally considered "2". If you want
to go into fractional scaling territory, you'll likely need some other
mechanism, like a user configuration setting.
- Once you have a scaling factor, you create a windowing system surface
that has two units:
  1. device pixels, which are the physical pixels on the monitor
  2. logical pixels, which are the scaled pixels for drawing operations

Your surfaces have to be created with the size in device pixels; then you
call cairo_surface_set_device_scale() with the scaling factor you computed:

https://www.cairographics.org/manual/cairo-cairo-surface-t.html#cairo-surface-set-device-scale

After that, every drawing operation must be in **logical** pixels, and the
Cairo will automatically scale every coordinate and size appropriately.

It is your responsibility to deal with device and logical coordinates; for
instance, windowing system events will be in device coordinates, but if
you're exposing their information to other code you will need to scale them
appropriately.

Ciao,
 Emmanuele.

On Wed, 20 Mar 2024 at 11:47, Federico Angelilli <list at fedang.net> wrote:

> Hello,
> Sorry for the late reply.
>
> I am using cairo for drawing and pango for the text. I am creating a
> window with xcb and using cairo_xcb_surface.
>
> I can easily extract the dpi information from the xrandr api calls. Now
> that I have this ratio, I have to use it somewhere to scale my canvas
> appropriately.
>
> I will try using cairo_scale, and should I also scale the size of the
> xcb_surface by the same ratio?
>
> Anyway my main concern about antialising is that since I am making layered
> shapes (think buttons) the border and the background will not match up
> pixel perfect (which is something that I have seen in other project using
> cairo).
>
> Does what I described about scaling the surface makes sense or am I
> misinterpreting cairo_scale?
>
> Regards
>
> Ps:
> I will soon try and post the results
>
>
> On March 18, 2024 1:46:54 PM GMT+01:00, Dov Grobgeld <
> dov.grobgeld at gmail.com> wrote:
>
>> I think that you should describe in detail what you expect.
>>
>> In pure vector graphics a contour may cut through a pixel and partially
>> obscure it, and it is then the responsibility of the graphics engine to
>> create anti-aliased graphics by mixing the colors of the background and the
>> foreground. As a user you typically don't need to concern yourself with
>> that as it is done automatically.
>>
>> However, e.g. for text this may cause "fuzzy" characters and the engine
>> will then fallback to moving the contours and scale the font so that the
>> glyphs fall on whole pixels.
>>
>> Note that the use of anti-aliased graphics is less needed, and less
>> visible, for an HDPI device.
>>
>> In any case all that you will typically need is cairo_scale() as Uli
>> Shlachter already described.
>>
>> Try it!
>>
>> Regards,
>>
>>
>> On Sun, Mar 17, 2024 at 12:46 PM Federico Angelilli <list at fedang.net>
>> wrote:
>>
>>> Hi
>>>
>>> Will cairo_scale make a pixel perfect image for non integer scales? To
>>> my understanding the scale matrix will be multiplied to all the coordinates
>>> scaling the coordinates appropriately. Should I also scale the whole
>>> surface by that same factor?
>>>
>>> Regards
>>>
>>>
>>> On March 16, 2024 6:27:56 PM GMT+01:00, Uli Schlachter <psychon at znc.in>
>>> wrote:
>>>
>>>> Hi
>>>>
>>>> Am 14.03.24 um 16:34 schrieb fedang:
>>>>
>>>>> Hello all,
>>>>>
>>>>> I am writing a small X11 app using cairo as the drawing backend. I was wondering if and how to implement
>>>>> fractional scaling to accommodate high dpi screens. After searching online I found almost nothing, so here I am.
>>>>>
>>>>> Can someone give me some pointers on what to do? I can get a scale factor for the screens (depending on the size)
>>>>> but I have no idea how to actually use it in the draw code.
>>>>>
>>>>
>>>> I'm not sure if I understand the question. You can use cairo_scale() to set a scale factor for all drawing. So I would say that you can just apply the scale factor with cairo_scale() before drawing.
>>>>
>>>> Of course, this assumes that you don't use cairo_identity_matrix() or cairo_set_matrix() in your drawing. Instead, cairo_save() and cairo_restore() should be used around temporary modifications of the transformations.
>>>>
>>>> Cheers,
>>>> Uli
>>>>
>>>>

-- 
https://www.bassi.io
[@] ebassi [@gmail.com]
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