On 01/10/2011 06:18 PM, John B. Matthews wrote:
In article<8EMWo.10172$111.3194@newsfe12.iad>,
Knute Johnson<nospam@knutejohnson.com> wrote:
On 01/10/2011 01:59 PM, John B. Matthews wrote:
In article<C2HWo.10145$111.6619@newsfe12.iad>,
Knute Johnson<nospam@knutejohnson.com> wrote:
On 01/08/2011 11:49 PM, John B. Matthews wrote:
In article<gfjii6dionkmboef1jrtr1b9i562h11gs5@4ax.com>,
Roedy Green<see_website@mindprod.com.invalid> wrote:
[...]
Is there any way to get part of the image back to transparent? Or
do I have to carefully arrange things that I never paint any part
of the image that will be transparent?
[...]
You should be able to fill with Clear:
g2d.setComposite(AlphaComposite.Clear);
g2d.fillRect(x, y, w, h);
Here's some examples:
<https://sites.google.com/site/drjohnbmatthews/composite>
<http://stackoverflow.com/questions/2166500>
Doesn't Clear set the alpha to 1.0f?
Yes, the static instance named AlphaComposite.Clear has its alpha
set to 1.0, but that value does not change what the
AlphaComposite.CLEAR rule does. When the graphics context's
composite is set to an instance of Clear, no pixels change. Later,
as fillRect() modifies the destination pixels, the CLEAR rule is
applied: "Both the color and the alpha of the destination are
cleared (Porter-Duff Clear rule). Neither the source nor the
destination is used as input." It's the one (only?) scenario in
which you can "paint with alpha," if I may borrow your phrase.
[...]
I just discovered why this has been causing me so much problem in the
past. Well one of the reasons anyway.
If you Clear an image that is OPAQUE the result is black with an alpha
of 1.0f. If you Clear a TRANSLUCENT image, the result is black with an
alpha of 0.0f. That is not well documented at least for the
semi-literate amongst us.
[...]
I added int[] ia = new int[4] and checked the raster before and after,
getting an alpha of 0 in both cases:
kittens.getRaster().getPixel(w/2, h/2, ia);
System.out.println(Arrays.toString(ia));
g.setComposite(AlphaComposite.Clear);
g.fillOval(w / 2 - 80, h / 2 - 60, 160, 160);
kittens.getRaster().getPixel(w/2, h/2, ia);
System.out.println(Arrays.toString(ia));
Opaque
[174, 160, 131, 0]
[0, 0, 0, 0]
Translucent
[174, 160, 131, 255]
[0, 0, 0, 0]