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Bug 3401 - SDL_GetDisplayDPI returns a wrong value on Linux and FreeBSD (X11)
Summary: SDL_GetDisplayDPI returns a wrong value on Linux and FreeBSD (X11)
Status: ASSIGNED
Alias: None
Product: SDL
Classification: Unclassified
Component: video (show other bugs)
Version: 2.0.4
Hardware: x86_64 Linux
: P2 normal
Assignee: Ryan C. Gordon
QA Contact: Sam Lantinga
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2016-08-01 09:05 UTC by Lena
Modified: 2017-08-11 21:00 UTC (History)
1 user (show)

See Also:

Attachments
X11: a sample code that works and returns the correct DPI (950 bytes, text/x-c++src)
2016-08-01 10:24 UTC, Lena 		Details
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Description Lena 2016-08-01 09:05:02 UTC 
On my system with Linux 4.6.4 and X11 1.18.4 the function SDL_GetDisplayDPI returns a wrong value. The same happens on FreeBSD 10.2 too.

My physical DPI is 96, while the functions always gives me 129.280685 average. Can I even trust this function on X11 to calculate a proper content scale factor?
Comment 1 Lena 2016-08-01 10:24:20 UTC 
Created attachment 2535 [details]
X11: a sample code that works and returns the correct DPI
Comment 2 Philipp Wiesemann 2016-08-01 20:28:25 UTC 
Does the program from "test/testdisplayinfo.c" also report a wrong value?

And is XRandR support activated on the two systems and also compiled into SDL?

There are two implementations to get the DPI in SDL (both in "src/video/x11/SDL_x11modes.c"). The older looks similar to the attached sample. It is used if no XRandR support is available.
Comment 3 Lena 2016-08-02 06:43:15 UTC 
The test program has the same values as my code had.

INFO:     DPI: ddpi=128.876648; hdpi=128.336838; vdpi=130.628571

This are the X11 libraries my SDL2 build is using: xcursor xdbe xinerama xinput2 xinput2_multitouch xrandr xscrnsaver xshape xvidmode


Maybe a compute issue in the XRandR code?