[Gimp-user] Brush dynamics when stroking a path

[Gary Aitken <gimp dreamchaser org>]

Using gimp 2.8.10

I'm trying to paint a whirlpool stream for a scientific diagram,
similar to what you would see out the back of a boat from the 
propeller wash.

I've drawn a path for the stream to follow, sort of a modified sine 
wave, starting at the propeller and ending a ways downstream.

What I'd like to do is paint from one end of the path to the other,
with the width of the stroke gradually getting wider, and the ink
deposited fading, the way a FG => BG gradient does.

I was finally able to get what I wanted using the dynamic options 
for painting using the paintbrush tool and defining a new one, but 
it is not at all clear to me how it works.

Is there a tutorial around for how the brush dynamics work with 
stroking a path?

When one brings up the paint dynamics editor and selects one of
the painting properties such as "Size", a graph is displayed.  The 
vertical axis is labelled with the property selected ("Size" in 
this case) and the horizontal axis is initially labelled with an
input device attribute, "Pressure", the top attribute in the list 
below the graph.  The check box for "Pressure" is not checked.  

The explanations I've seen of this graph describe it as a mapping 
between a property of the brush (e.g. Size or Color) and a property
of the input device (e.g. Pressure on a tablet). 

However, one of the attributes selectable for the graph, "Fade", 
is not a property of an input device (I don't think; I don't have
a tablet).

In any case, when stroking a path there is no input device.  So what
determines the value of the input device attributes on the other
axis of the graph?  

I'm guessing the vertical axis goes from zero to max, where max is 
the set "Size" (in this case) of the brush.  Correct?

The label on the horizontal axis changes depending on which of the
attributes below the graph is selected, and the check box determines
whether or not that attribute affects the property being edited.
Initially the "Pressure" attribute is selected (grey background), but
not activated (no check).  If I want the "Size" of the brush to change
while stroking a path, what input device attribute should I select? 
By trial and error, I was able to get what I wanted using "Fade", 
but it is not at all clear to me why.  Are there preset curves for
these attributes over the course of a stroked path?  If so, what are
they?   

After selection, a line/curve is displayed showing how the property
of the brush ("Size") will be varied according to the attribute 
("Fade") value.  If I want the brush to start 25% of its total size 
and grow to 100% over the full length of the path, I adjust the curve 
to start 25% up the axis on the far left and end at the upper right 
corner.

What does the length of the horizontal axis represent?  From the
behavior I am seeing with "Size" and "Fade", it appears to represent 
the path from beginning, at the left end, to end, at the right end.  
So when painting, the brush property (in this case "Size") will have 
its attribute(s) (in this case "Fade") varied when painting along the 
path as shown by the curve going from left to right.  However, it's 
not clear to me what "Fade" means in relation to "Size" (see next
paragraph).

If I uncheck the "Fade" box and instead check "Pressure", but establish
the same curve I had for "Fade", I get a different behavior.  The 
stroked path starts and ends small, with the maximum size in the middle.
Since the stroking is being done by the path traversal tool and not a
human manipulated instrument, there is no change in pressure, so what
is happening?  Using "Velocity" causes the brush "Size" to vary in the 
opposite way from when "Fade" is used.  Again, the velocity should not
be changing, so what is really going on?  "Direction" seems to vary 
"Size" based on some weighted average notion of the direction of travel, 
with horizontal yielding large sizes and vertical yielding small.  "Tilt", 
"Wheel", and "Random" don't seem to do anything.  

If I want the "Color" to "Fade" over the course of the curve, I have to
start the curve at the lower left (No fading?, i.e. 100% color) and have it 
end somewhere higher (some fading) on the right.  What does fading mean in
this case?  changing opacity?

The above two brush properties, "Size" and "Color", seem to behave in
opposite ways when the "Fade" attribute is selected.  For "Size", the
"Fade" axis seems to have nothing to do with "Fading" the size, but simply
represents the overall length of the path; but for "Color", it seems to
indicate the inverse opacity (amount of fading) of the brush.

Thanks for any tips / explanations,

Gary