Variable time between videoFrames


I was looking to use the WebPagetest API to test and compare a few websites.
After playing around with it a little bit, I noticed that the videoFrames you get back don’t always have the same time between them.

Sometimes you don’t get a frame for 4000ms, sometimes you get 2 in the span of 2000ms. First, I thought this had to due with the visualComplete progess (ie. don’t show a frame if there hasn’t been any progress). But this isn’t the case, because there were multiple frames with the same visual progress.

Is there a logic behind the frame times I’m not seeing?
Also, it’s not possible to ask for a frame every x ms right?

Thanks in advance,


The video frames are recorded every 100ms (on desktop - every 16ms on Mobile) but only unique frames are kept (i.e. at least one pixel changed). The visual progress can be the same but still have a change in the actual frame (either small enough to not change the progress or a layout change that wouldn’t change how the progress is measured).

No, there is no way to ask for explicit spacing and frames for everything. The way the filmstrip code works is it just creates a list for every 100ms and looks for the frame that would have been on screen at that time and uses it (repeating a given frame as necessary).