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Bug 3401

Summary: SDL_GetDisplayDPI returns a wrong value on Linux and FreeBSD (X11)
Product: SDL Reporter: Lena <magiruuvelvet>
Component: videoAssignee: Ryan C. Gordon <icculus>
Status: ASSIGNED --- QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: philipp.wiesemann
Version: 2.0.4   
Hardware: x86_64   
OS: Linux   
Attachments: X11: a sample code that works and returns the correct DPI

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?