Fill complex image with color

Question

Let's say I have a PNG image with transparency, like so:

I want, in Java, to fill only the object with black, like so:

This is a fairly trivial process in Photoshop, but this is a process I'd like to repeat frequently, ideally without making a black fill picture for each object I'd like to do it for. I've tried several edge detecting classes but found no success.

How would I accomplish this?

Additional info: this is going to be a quick-and-dirty way to create shadows. If you can think of a better way, that would solve this problem completely.

Answer 1

You could make a function that loops through all the pixels and fill them with black color.

BufferedImage image = ...
Color fillColor = new Color(0, 0, 0); // Black

for (int y = 0; y < image.getHeight(); y++) {
    for (int x = 0; x < image.getWidth(); x++) {
        int color = image.getRGB(x, y);
        int alpha = (color >> 24) & 0xff;

        if (alpha == 255) {
            image.setRGB(x, y, fillColor.getRGB());
        }
    }
}

Obviously, this will only work on fully opaque pixels. If the image on top contains some transparency, you can also change the condition to be more tolerant: if (alpha > 127). This will fill all pixels that are less than 50% transparent.