QGIS Time manager Changing Color Polygon

Question

I'm using QGIS 3.6.3. Noosa.

I have 10 polygons, each has a starting time and an end time. Getting Time Manager to run was no problem at all. However, there are two things I would like to add in my dynamic visualization.

1. With each ongoing time frame I would like the color of the polygon to change, meaning that when in January 2019 the polygon is dark red, it should be less dark in February 2019 and by the end date in December 2019 be a very light red color.
2. Polygons disappear at the end of the time span, I would like them to be visible (as stated in 1, in a different color than at the beginning of the time span).

Does anyone have any suggestions?

Answer 1

By using this expression :

color_hsv(0, scale_exp( day( age( "END" , animation_datetime() )),0,365,10,100,2),100)

In the data defined override for your polygon fill color you get the effect from red to pale red as the time goes.

• This work like this color_hsv set the color with H set at 0 (for red), S set as a scale_exp fonction to change over time and V set as 100.

  the scale_exp used to modulate the S value of the HSV color take 6 argument:

  scale_exp(val,domain_min,domain_max,range_min,range_max,exponent)

  • val : the value to be scaled is (set as day(age("END",animation_datetime() )) this calculate the number of day between the datetime set in the "END" field and the current
    animation datetime )
  • domain_min : the minimum value in the input domain, set as 0
  • domain_max : Specifies the maximum value in the input domain (set as 365 : difference between the 1 jan 2019 and 31 dec 2019, adjust if your start and end date are different (if each of your polygon have different start and end time make this an expression to)
  • range_min : Specifies the minimum value in the output range, set at 10 (0 will give you white and not a pale red, adjust if needed)
  • range_max : Specifies the maximum value in the output range, set as 100 for a bright red
  • exponent : A positive value (greater than 0), which dictates the way input values are mapped to the output range, set as 2, adjust as you want to have the progressive change you like the most

Result :

For keeping the polygon visible at the end of the time span I think you have to check the "Accumulate features" box in the Time manager setting, if that didn't work another way would be to duplicate the layer and display it below the time managed layer and in another color.

Answer 2

Make a duplicate of each polygon for each month.

Fill the "date" field with the dates for Jan 2019 to Dec 2019, so there's one copy of each polygon for every month.

Style the polygons with a graduated style using the "date" attribute. Use a custom color gradient from dark red for Jan 2019 to light red for Dec 2019.