[cairo] cairo / xlib not updating window content

Pavel Petrovic pavel.petrovic.level2 at gmail.com
Sun Jul 29 12:22:42 UTC 2018

Hi Theo,

Many thanks for the hints!
I forgot to attach a link to a new version in my second e-mail
with those three findings, it's here:

( I have saved the version from Cedrix here:
   https://kempelen.dai.fmph.uniba.sk/test_cairo/test_cairo_Cedric_Roux.c )

Also now modified my second version according to your hints
- i.e. dropping the windows[] array and always created a new
cairo_t when drawing something

Is there, please, any reason for doing so?
Why is it better to always create a new context?
Just because of the possibility something could have changed
with the surface meanwhile, so a fresh context will be up-to-date,
or are there any other reasons? How about efficiency?
Is creating a new context a cheap operation?

I am also responding to resizing of the window now
with resizing back to this fixed size, although I did not go
into preventing maximizing, which seems more tricky.


On Sat, Jul 28, 2018 at 7:31 PM, Theo Veenker <theo.veenker at beexy.nl> wrote:
> Hi Pavel,
> I think you should just each time when you want to draw something:
> - create cairo context
> - draw stuff
> - destroy context
> So basically get rid of your windows[] array. You shouldn't need flush in
> your case.
> Since you keep a surface for each window, make sure to update its size when
> the target window has changed size.
> Have fun,
> Theo
> On 28/07/18 12:34, Pavel Petrovic wrote:
>> Hello,
>> I tried to investigate in more depth, here are some findings:
>> 1. Filled rectangles not showing up:
>>    - changing them for circles solves it, so it really is about the
>> rectangles
>>    - workaround: not only fill, but draw again and then stroke, this
>> solves the issue
>>    pending question: why drawing filled rectangles without borders is
>> not possible/flushed?
>> 2. cairo_paint() effect lost due to window not shown yet:
>>    - adding ExposureMask to XSelectInput() and then painting only after
>>      the Expose event arrives, solves the problem
>>    pending question: why drawing to a window that has been properly
>> constructed is lost if it was not exposed yet?
>> 3. the red line drawn from keyboard events not being updated:
>>     - adding extra cairo_surface_flush() after each piece of line helps
>> somewhat,
>>       it seems that all parts of line are drawn eventually, but
>> sometimes it takes
>>       even several seconds, while the machine cpu/memory is pretty idle
>>    follow-up question:
>>      Does cairo keep all the shapes that have been drawn in some kind of
>> list
>>      which slows it down so much when there are so many shapes?
>>      (sorry for such a beginner/disturbing question)
>> Thanks.
>> Pavel
>> On Sat, Jul 28, 2018 at 9:50 AM, Pavel Petrovic
>> <pavel.petrovic.level2 at gmail.com> wrote:
>>> I am trying to learn how to use Cairo 2D drawing library with xlib
>>> surfaces.
>>> I wrote a little test program that allows creating multiple windows.
>>> Each function may have a custom paint() function that is called
>>> regularly to add some graphics content to the window, or redraw it
>>> completely if desired. There is also an option to define mouse and key
>>> listener. The main routine checks for X events (to delegate them to
>>> mouse and key listener) and for timeout for periodic call of those
>>> paint() functions.
>>> I tried with the 1.14.6 version of Cairo (that is currently available
>>> as package in Ubuntu 16.04), and the latest 1.15.12, but the results
>>> are the same.
>>> The expected behavior of this demo is to open 3 windows. One will have
>>> random rectangles being added, another one random texts, and the third
>>> random circles.
>>> In addition, clicking into windows should produce lines (connecting to
>>> mouse click, or randomly), and using arrow keys should draw a red line
>>> in the window with circles.
>>> The circles and text seem to show up regularly as expected. All three
>>> windows should have white background, but two of them are black. And
>>> the worst, the window with rectangles does not get updated much (and
>>> it does not matter if it is the first window created or not, it is
>>> always the rectangles that do not show up properly).
>>> They are only shown when the focus changes to or from that window -
>>> then the remaining rectangles that should have been drawn meanwhile
>>> suddenly appear.
>>> I am calling cairo_surface_flush() on the surface of each window after
>>> adding any content, but that does not help. I also tried posting
>>> XEvents to that window of various kind (such as focus), they arrive,
>>> but rectangles do not show up.
>>> Furthermore, even though drawing lines with mouse works fine, drawing
>>> line with key arrows suffers from the same problem - it is drawn, but
>>> not shown properly.
>>> I am obviously wrong in some of my assumptions about what this library
>>> can do, but I am not sure where.
>>> It seems that there are some two competing versions of drawing being
>>> shown, since it happens sometimes that one or two rectangles, or
>>> pieces of the red line are flashing. Some kind of strange buffering,
>>> caching? It may just be some bug in my program, I do not know.
>>> Another observation - the black background is because drawing white
>>> background happens before the window is shown, and thus those
>>> cairo_paint calls are somehow discarded. I do not know how to make the
>>> window appear earlier, it seems it appears only after some later
>>> changes on the screen.
>>> I am stuck on this after a couple of desperate days, could you help me
>>> out at least in part, please?
>>> The program is here:
>>> https://kempelen.dai.fmph.uniba.sk/test_cairo/test_cairo.c
>>> An example screenshot (with a broken red line drawn by keys, and
>>> rectangles not showing up properly):
>>> https://kempelen.dai.fmph.uniba.sk/test_cairo/test_cairo.png
>>> To compile (on Ubuntu 16.04 or similar system):
>>> gcc -o test_cairo test_cairo.c -I/usr/include/cairo -lX11 -lcairo
> --
> Theo Veenker  |  Beexy - Behavioral Experiment Software
> +31(0)524-541531  |  +31(0)6-42525777 mobile
> theo.veenker at beexy.nl  |  www.beexy.nl
> --
> cairo mailing list
> cairo at cairographics.org
> https://lists.cairographics.org/mailman/listinfo/cairo

More information about the cairo mailing list