QGIS Temporal controller: getting the current animation time (percentage elapsed of total animation duration) with expressions / variables

Question

The question

How can I get the current animation time of a Temporal Controller animation using expressions with QGIS 3.24? I guess I'm almost there (see below), but have trouble dividing a time formatted as interval by a number.

The background (what I need it for)

I finally want to get a value that shows what percentage of the whole animation has elapsed at a certain frame. I want to use this value to style a line layer with geometry generator, using line_substring() function to draw the same percentage of the line's total length as the percentage of time elapsed: so at the beginning, 0% of the line, after 30% of the whole animation the first 30% of the line's lenght etc. Like this, I could animate the line without creating interpolated points with timestamps (what would be a workaround) by activating Dynamic Temporal Contral in the layer properties with Configuration Redraw Layer Only.

The expression to create this would look like: line_substring( $geometry, 0, $length * [percentage] ) - where [percentage] is the value I'm trying to create.

What I tried

My idea was to the get the total length of the whole anmimation with @animation_end_time - @animation_start_time. This return a time interval (like <interval: 10.375 minutes>). Unfortunately, there is no variable like @animation_current_time that returns the time of the current frame.

There is, however, the variable @frame_number that returns the current frame number during animation playback. The variables @frame_timestep and @frame_duration return the temporal duration of each animation frame.

Example

Let's say I have the following values. A total time of 24 frames * 8 seconds = 192 seconds have elapsed, that is 30.8% of the total animation time of 10.375 minutes:

• Total length of animation: <interval: 10.375 minutes> ( = @animation_end_time - @animation_start_time)
• Current frame number: 24 ( = @frame_number)
• Temporal duration of each animation frame: 8 (using @frame_timestep) or <interval: 8 seconds> (with @frame_duration)
• What I want to get (pseudocode): <current-time> / <total-duration> -> 0.308 (30.8%)

So how to divide <interval: 10.375 minutes> by the time elapsed : 192 (@frame_number * @frame_timestep)?

Answer 1

You can get the percentage by using an expression such as:

second(datetime_from_epoch(epoch(to_datetime(to_string("mytime")+'Z')) - epoch(@animation_start_time)) - make_datetime(1970,1,1,0,0,0)) / second(@animation_interval)

Be aware of the timezones! Seems like QGIS temporal-controller-variables always work with UTC, while your datetime may be in another timezone. You can correct this by adding or subtracting the hour difference in seconds. Or a little more elegant change the 0-time of make_datetime(1970,1,1,0,0,0) to e.g. make_datetime(1970,1,1,1,0,0) In my example above I had to manually set UTC as timezone, thats why I use to_string("mytime")+'Z'. Remove that part if you dont need it!

Example result: