Resolution of recording surface

[Bernhard R. Fischer]

Please find attached a small demo which shows the problem.

I'm on cairo 1.18.0.

Increase the scale (in main()) to eliminate the problem.

Best regards,
Bernhard


On 14.12.2024 10:34, Bernhard R. Fischer wrote:
> Dear list,
>
> It has been a long time since I posted here for the last time. Sorry 
> for the long text but it's to so easy to explain.
>
> I'm using Cairographics extensively for an OSM data rendering software 
> since many years (https://github.com/rahra/smrender), so I'm not new 
> to it.
>
> What I regularly do is to use a recording surface internally and 
> finally paint it to a PDF, SVG, or whatever surface is required.
>
> In respect to the units I always assumed that 1 unit within the 
> recording surface is to be 1 millimeter in the final result. To 
> accomplish this I create the recording surface with the size of the 
> final page (e.g. create_recording_surface(210, 297) for an A4 page 
> have 210x297mm), then I draw on it and finally at the cairo_paint to 
> the PDF surface I scale the paint operation to meet the points unit of 
> the PDF.
>
> This concept works fine but unfortunately recently within a different 
> project (where I use the same drawing concept) I made the discovery 
> that the resolution of the recording surface seems to be limited at 
> least with the toy text API. I use the get_text_extents and similar 
> functions to place some text at specific positions and with some 
> debugging I found out that it does not use any fractional part of the 
> extents member variables.
>
> E.g. the text_extents.width always returns e.g. 3.0, 4.0, 8.0, but 
> never something like 3.17, 4.28 and so on. The final result is that 
> the text (obviously) is not placed on exact positions (where it should 
> be).
>
> With further experiments I found out that if I make the recording 
> surface width/height e.g. 100 times bigger (21000x29700) and then 
> scale it by factor 100 the text_extents will work with higher 
> resolution as expected. But if I further increase the recording 
> surface to a factor of e.g. 1000, it will internally hang somewhere 
> within an endless loop (within libcairo). So there seems to be some 
> upper limit.
>
> So my question is: is there a limit in the resolution of the recording 
> surface? Or is this a limitation of the toy text API? Or did I stumble 
> on something else? Or is my drawing concept somehow incorrect?
>
> Best regards,
> Bernhard
>
>
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