Creating a transparent image with MIDP 1.0

Java on Mobile Devices J2ME
#1

Does anyone know how to create a transparent image with MIDP 1.0?

I have a problem when creating the sprites for my game. I will try to explain it.

I have a transparent PNG file with all my tiles and sprites. What I want to do is to create subimages of this file in order to have independent images for each sprite or tile.

MIDP 2.0 API has a method to perform this operation, but MIDP 1.0 hasn’t it, and I need to use MIDP 1.0.

So I have created this method on my own:

public Image createImage(Image source, int x, int y, int width, int height)

And the code inside the method is as follows:

Image subimage = Image.createImage(width, height);
Graphics g = subimage.getGraphics();
g.drawImage(source, -x, -y, 0);
return subimage;

It works fine if we don’t use transparent images… But when I want to create a transparent subimage it doesn’t work, because the method “Image.createImage” creates an image with a white background (and this means that the transparencies are transformed to white).

How can I create a transparent image? ???

Is it possible?

If not, I think the solution is to use an “empty” transparent PNG file… (and create all tiles and sprites with the same size, the size of the transparent PNG image)

Thanks for your help.

#2

You’re right, you can’t create a mutable (modifiable) transparent Image in MIDP 1.0. (Though if you’re just targeting Nokia devices, the proprietary Nokia UI API has this capability).

A solution is to use clipping: draw the whole Image, with a clip window for the region you really want to draw, and the Image offset left and/or up so that the correct region overlaps the clip window. For instance, one of my games has a PNG image per character with all the animation frames for that character in a horizontal row; my Sprite class’s ‘draw’ method then looks like this:


  void draw(Graphics g) {
    g.setClip(x, y, width, height);
    g.drawImage(framesImg,
                x - width * frameIndex[state][frame], y
                Graphics.TOP | Graphics.LEFT);
  }

You see that the state (i.e. walk-left, walk-right) and frame indices select the correct frame from the image, and the image is drawn offset left by that many times its width, so that the correct frame overlaps the clip region.