Hello everyone,

I started a small Java program just as something fun to do. It simulates
a sonar, like on a submarine.  Right now I'm at the point where I have
multiple objects moving independently around the sonar screen with a
rotating needle.  I keep track of the speed/location/etc. of each object
which extends Thread.  The needle rotates around the screen in a way
that lights up the part it just touched and the glow gradually fades
away using a gradient that I created.  The objects on the screen only
change location when the needle intersects them (using the intersects()
method of java.awt.geom.Line2D.Double).

The problem is that at this point the objects "jump" to the new location
because their last displayed location is still on the display.  I
believe a real sonar works in such a way that all the objects that are
detected gradually fade away and just before totally disappearing the
needle lights them up again in their newest location.  I'm having
trouble figuring out a way to get the object icon (just an oval right
now drawn using fillOval()) and its label (drawn using drawString()) to
gradually fade away.

The first problem I ran into was that the object stayed drawn on the
sonar screen even when it had actually moved out of "range" of the
sonar.  Later I realized that if it faded away like it is supposed to I
wouldn't have to worry about drawing over top of the old location when
it moved out of range because it would go away on its own and come back
when it was back in range on the sonar.

Currently I'm saving the old location but painting the icon and text
again using a transparent color obviously doesn't erase the painted
object and text from its last location. I also can't actually paint over
the existing icon/text with background colors because it is over top of
the gradient from the needle glow fading away and I wouldn't be able to
come up with the perfect pattern to paint over it to match the gradient
at that location at a particular point in time.  I tried to paint all
objects and labels before the sonar needle was painted with its gradient
but in order to still see them through the gradient requires
transparency on the gradient and that slows down the rendering
tremendously, too much for it to be worth it. I have the gradient
stretch the entire 360 degrees of the circle so there is no point on the
sonar where there isn't a gradient. (It goes from yellow to green).

I've attempted to get the icon/text to fade away like I do the needle
glow by setting up a loop and gradually increasing the transparency
level but that doesn't seem to work even with just 1 object. I have a
feeling I wouldn't be able to get it to work with multiple objects whose
fade has to be independent anyway because the sonar needle would go over
them at different times.

I'm running out of ideas on how to get this to work so I'm just fishing
around for ideas at this point.

I tried to explain my program and my problem the best I could especially
for those not familiar with a sonar device but if needed I can probably
post the whole program this time (unlike my other posts about my other
program) since the full program is just under 500 lines. Then anyone who
wants to can test it to see what I'm talking about.

thanks for any ideas.

Brandon