Implementing HiDpi scaling

[Federico Angelilli]

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
>>>
>>>
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