moving an image accross a transparent background

Java 2D
#1

How do I move an image across a transparent background ?

// create a transparent background

BufferedImage drawCanvas = gc.createCompatibleImage(screenW,screenH,
Transparency.TRANSLUCENT);
.
.
.

//loop

// draw an Image on the background

g.drawImage(image, x,y,w,h);

// save previous position of the image
x0=x;
y0=y;

//clip off the background
clipImage=drawCanvas.getSubimage(x0,y0,w,h);

//redraw the part of the background where image was
//This does not work before the transparency of clipImage
.
.
g.drawImage(clipImage x0,y0,w,h);

//end loop

Thanks

Jay

#2

you say your background is transparent, what’s behind it?

if the background never actually changes (perhaps aside from scrolling or something) it might be better to paint it to a non transparent image and use that for the background, update it only whenever you have to. this way you wouldnt be repainting sections of a translucent image.

if there’s something behind the background that moves however, or it’s an animated background, then you can’t expect much performance

#3

The background color for the component is white. Yes, I need to have the transparent background. I can somehow get around the issue by creating another non transparent image with white color :

// create a transparent background

BufferedImage drawCanvas = gc.createCompatibleImage(screenW,screenH,
Transparency.TRANSLUCENT);

BufferedImage whiteImage = gc.createCompatibleImage(screenW,screenH, Transparency.OPAQUE);
whiteImage_g2=whiteImage.createGraphics();
// set the image color white
fillWhite(whiteImage_g2,0,0,screenW,screenH);

.
.

//loop

// draw an Image on the background

g.drawImage(image, x,y,w,h);

// save previous position of the image
x0=x;
y0=y;

//clip off the white color image
clipImage=whiteImage.getSubimage(x0,y0,w,h);
// clear the background
g.drawImage(clipIamge,x0,y0,w,h);

//clip off the transparent background with whatever being drawn onto it
clipImage=drawCanvas.getSubimage(x0,y0,w,h);

//redraw the part of the background where image was
g.drawImage(clipImage, x0,y0,w,h);

//end loop

This works fine except that I have to create a non transparent buffered image with white color and waste some memory. I thought that I could just clip off the transparent background and fill it with white color. But it does not seem to work with transparent image. When I tried with the transparent image, the foreground image appears with a gray color clipped region when the foreground image is being dragged across the screen.

Thanks

Jay

#4

just curious. do you mean transparent or translucent?

is it like a gif file with some parts you can see thru and all the rest opaque or is there semi-transparency in it as well?

i use transparent gifs all the time and they work just as fast as anything else, but if you try using png alpha trnaspatent images it kills your performance.

#5

Alpha blending is not hardware accelerated yet, bitmasks are.

There is a property you can set for 1.4.2 beta that will enable some hardware acceleration for alpha transparency. trembovetski mentioned it in an earlier thread…

#6

[quote]Alpha blending is not hardware accelerated yet, bitmasks are.

There is a property you can set for 1.4.2 beta that will enable some hardware acceleration for alpha transparency. trembovetski mentioned it in an earlier thread…
[/quote]
1.4.1_02 has the flags for accelerated alpha as well.