The extrap control allows the users to select a "target" object (like a precip
cell or a clearing line in a satellite image), track it for a few time steps,
and the extrapolate that target....or (more useful,
in my opinion) use the movement obtained from the representative target to see the
arrival time of associated objects (like other precip cells or a line or...) at a selected
point.
This idea was derived a decade ago in the McIDAS-X command "when" -- as in "when will it get
here"? I use this extensively when biking home in the summertime! The implementation in
FlAniS is similar; and we've now added an alternative "target selection" mode. Two examples
follow.
In order to use this, you must be able to specify the time of each image. Presently, there
are 3 ways to do this:
You may use the times = parameter in the configuration file; this is useful if
you are generating the file whenever there is new data, or it never changes.
You may use the times = parameter in the file_of_filenames file; this is useful if
you are generating the file whenever there is new data, or can otherwise just modify this
file.
Note that you may add the time information into the file_of_filenames, using a new
curly-brackets clause (e.g., {1245} ). For example:
background.gif {1245} "label" overlay=file1.gif file11.gif....
Finally, you must also specify the "extrap" control, so the user can selectively enable this
mode.
Here is the complete config file for the following, simple example:
controls = startstop,speed,step,zoom,toggle,extrap
filenames=rad0.jpg, rad1.jpg, rad2.jpg, rad3.jpg, rad4.jpg
times= 1200,1230,1300,1330,1400
times_label_style = 0xffff00, -5, CDT, -1, 12, bold
The "times_label_style" sets the foreground color to yellow, specifies a 5 hour offset from UTC, and a
timezone label of "CDT". The background is transparent (the "-1"), 12 point type is
used, and the weight is "bold".
Here is a made-up example for practice -- using the original selector methods (see below for
a more user-friendly approach!):
The extrapolation mode:
Click on the "extrap" mode control button
Clicking the mouse button moves through the frames
Shift+click on a frame, selects the point as the target for that frame
When you have tracked your target on all frames, then
Move the mouse pointer to your location
Alt+click will then show you what stuff is moving toward you, and at what time it is
expected to arrive
(For you more traditional thinkers, you can do a Shift+Alt+click on a cell and
the direction and times "away" from that point will be shown)
NOTE (of course) -- this is linear (first order) interpolation. Stuff changes
-- there are accelerations, things grow and shrink, stuff moves irratically....but for the
short period, there tends to be a lot of continuity....
Below is an example using a slightly different, but easier method to select targets.
To change the mode, we used:
extrap_mode = key
to_from_lock = to
Which modifies the target selection method (see below), and spcifies that that ALT+click
should always only be "to" the selected point ("from" is also an option; otherwise, the
ALT+SHIFT+click switches between the two).
See the modified user instructions below.
The label style is also changed to:
times_label_style = 0xffffff,-7,PDT,0x808080
The "key" extrapolation mode:
Click on the "extrap" mode control button
Use the "A" key or the Right Arrow to step through the frames if you need to
Put the mouse over the target and Click. This selects the point as the target for that frame
and after a short delay, steps to the next frame
When you have tracked your target on all frames, then
Move the mouse pointer to your location
Alt+click will then show you what stuff is moving toward you, and at what time it is
expected to arrive
NOTE (of course) -- this is linear (first order) interpolation. Stuff changes
-- there are accelerations, things grow and shrink, stuff moves irratically....but for the
short period, there tends to be a lot of continuity....