[cairo] cairo / xlib not updating window content

Theo Veenker theo.veenker at beexy.nl
Sat Jul 28 17:31:49 UTC 2018

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,

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

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