Bug 386620

Krita 4.0 pre-alpha (git 86851fa) exposes to the user a frame limiting function under "Settings > Configure Krita > Performance > Advanced > Limit Frames per Second while painting".

After fiddling around with it for a while, trying different settings (making sure to restart the program to apply the new values), I came to the conclusion that the frame limiting function might not be working as intended, producing as a result an *effective* canvas update framerate that is roughly (slightly more than) half the user set value.


==How to observe it?==

1) Try setting a low frame limit value, like for example 20 fps.
2) Reboot Krita.
3) Create a new image and proceed to recording a short video capture of the desktop/program at the frame rate of the display frequency rate (typically 60 Hz) using for example the open source OBS (Open Broadcast Studio) software, and a small brush at 100% canvas zoom in order to avoid any GPU bottleneck issue.
4) The framerate of the canvas in Krita can then be easily analyzed using a standard video player (e.g. VLC) advancing the video frame by frame.


==Results==

At a Krita canvas fps of 20 fps, and a video framerate of 60 fps, brush strokes and the brush cursor appear to update roughly every 5 or 6 frames (sometimes 4 or 7). This corresponds to an *effective* canvas frame rate of 10-12 fps, which is significantly lower than the user set value.

At a Krita canvas fps of 60 fps (equivalent to the display refresh rate) and again a recording video frame rate of 60 fps, bursh strokes appear to update most of the time 2 frames, less often every frame. In a test, the canvas updated 44 times over the course of 69 frames, implying an actual framerate of about 38 fps. That the canvas updates at a much lower rate than the screen refresh rate is also subjectively noticeable by eye during ordinary program operation.


==System configuration==

Windows 10 64bit Fall Creators Update
AMD Radeon RX480 with Radeon Software 17.11
Intel i5 3550 4-core GPU.