Different rendering speed Qt widgets

Question

I'm building an app (in Qt) that includes a few graphs in it which are dynamic (meaning refreshes to new values rapidly), and gets there values from a background thread.

I want the first graph, whose details are important refreshing at one speed (100 Hz) and 4 other graphs refreshing in lower speed (10Hz).

The problem is, that when I'm refreshing them all at the same rate (100 Hz) the app can't handle it and the computer stucks, but when the refresh rate is different the first signal gets artifacts on it (comparing to for example running them all an 10Hz).

The artifacts are in the form of waves (instead of straight line for example I get a "snake").

Any suggestions regarding why it has artifacts (rendering limits I guess) and what can be done about it?

Answer 1

I'm writing this as an answer even if this doesn't quite answer your question, because this is too long for a comment.

When the goal is to draw smooth moving graphics, the basic unit of time is frame. At 60 Hz drawing rate, the frame is 16.67 ms. The drawing rate needs to match the monitor drawing rate. Drawing faster than the monitor is totally unnecessary.

When drawing graphs, the movement speed of graph must be kept constant. If you wonder why, walk 1 second fast, then 1 seconds slow, 1 second fast and so on. That doesn't look smooth.

Lets say the data sample rate is 60 Hz and each sample is represented as a one pixel. In each frame all new samples (in this case 1 sample) is drawn and the graph moves one pixel. The movement speed is one pixel per frame, in each frame. The speed is constant, and the graph looks very smooth.

But if the data sample rate is 100 Hz, during one second in 40 frames 2 pixels are drawn and in 20 frames 1 pixel is drawn. Now the graph movement speed is not constant anymore, it varies like this: 2,2,1,2,2,1,... pixels per frame. That looks bad. You might think that frame time is so small (16.67 ms) that you can't see this kind of small variation. But it is very clearly seen. Even single varying speed frames can be seen.

So how is this data of 100 Hz sample rate is drawn smoothly? By keeping the speed constant, in this case it would be 1.67 (100/60) pixels per frame. That of course will require subpixel drawing. So in every frame the graph moves by 1.67 pixels. If some samples are missing at the time of drawing, they are simply not drawn. In practice, that will happen quite often, for example USB data acquisition cards can give the data samples in bursts.

What if the graph drawing is so slow that it cannot be done at 60 Hz? Then the next best option is to draw at 30 Hz. Then you are drawing one frame for every 2 images the monitor draws. The 3rd best option is 20 Hz (one frame for every 3 images the monitor draws), then 15 Hz (one frame for every 4 images) and so on. Drawing at 30 Hz does not look as smooth as drawing at 60 Hz, but the speed can still be kept constant and it looks better than drawing faster with varying speed.

In your case, the drawing rate of 20 Hz would probably be quite good. In each frame there would be 5 new data samples (if you can get the samples at a constant 100 Hz).