transparent images using visad?

[Stuart Maclean]

I am trying to use visad in a web app (Java servlets) to produce 
transparent png responses. I build the DisplayImplJ2D in OFFSCREEN mode. 
  I serialize to png using the javax.imageio.ImageIO routines.  For now, 
the only data is a UnionSet of points, representing a 'route'.

  I cannot get the call to DisplayImplJ2D.getImage to produce a 
'transparent' image.  I have replaced all calls in VisADCanvas2D which 
build BufferedImage of type TYPE_INT_RGB with TYPE_INT_ARGB.  Maybe this 
could be an options in future releases (??)

When I get the BufferedImage back from the call to 
DisplayImplJ2D.getImage(true), it claims to have given me an ARGB type 
of BufferedImage (using getType()) but when I view it I get a filled in 
background color.

If  I replace the visad code in my servlet with this, I get a nice 
transparent image (via the ImageIO calls):

	BufferedImage br = new BufferedImage( 500, 500, 
BufferedImage.TYPE_INT_ARGB );
		Graphics2D g = br.createGraphics();
		g.setBackground( new Color( 0, 0, 0, 0 ) );
		g.setColor( Color.RED );
		g.clearRect( 0, 0, 500, 500 );
		g.drawLine( 100, 100, 200, 200 );


So some Visad code must be doing some other operation(s) to 'ruin' the 
transparency of the composed BufferedImage ?? I have looked in 
VisADCanvasJ2D somewhat but nothing is jumping out at me.

I confess I am no expert in graphics nor graphics files.

BTW: The underlying approach here is to use Visad as a 'data/map server' 
which will produce one 'layer' of data in each image, e.g. a basemap or 
a route or some other plot.  I am then using a web browser/Javascript's 
z ordering to compose the whole 'picture'.  I have an app where the UI 
must be a browser, so i cannot go with a jnlp solution.  If all goes 
well, I will write up the architecture and post here.  As you can 
imagine, I am stuck w/out transparent images.

Any help gratefully appreciated

Stuart

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