How do I detect a given sprite's bounds automatically using Java?

Question

Background

---

I have the following png file that contains the player sprite.

As with any png file, the non-colored pixels are transparent.

(Width: 16px; Height: 16px)

Currently, I can erect a rectangle (collision-bounds), using java.awt.Rectangle around the image using the image's width and height, as seen below.

I would like to crop the image so that the width and height are changed to the edges of the colored portion of the image, much like what is shown below.

This will allow for more accurate collision-detection between entities or objects in a game.

I want to construct a method that handles this modification dynamically, as seen below.

//Method calculates how far in from [top, bottom, left and right] 
//the [non-zero] or [transparent] pixel-data of an image are
//in order to construct a new image that starts at the edges
//of the colored portion of the image.
public BufferedImage adjustImage(BufferedImage image)
{
    BufferedImage result = null;



    return result;
}

Problem

---

However, I am uncertain as to the algorithm to use to achieve the result above.

Can anyone give a step-by-step on how to detect these bounds and how to implement such a fuction as described above?

Answer 1

You first need to scan the image for the edges. You need to loop through each pixel in the image and store the position of the pixel closest to the left, the pixel closest to the right, etc.

Pixels are stored in the

 ARGB
 8 bit alpha | 8 bit red | 8 bit green | 8 bit blue

format in integers. This means that you need to use the

int alpha = (color >> 24) & 0xFF;

method to get the alpha value of a pixel.

int minX = Integer.MAX_VALUE;
int maxX = integer.MIN_VALUE;
int minY = Integer.MAX_VALUE;
int maxY = integer.MIN_VALUE;
for (int x = 0; x < img.getWidth(); x++) {
    for (int y = 0; y < img.getHeight(); y++) {
        int color = img.getRGB(x, y);
        int alpha = (color >> 24) & 0xFF;
        if (alpha != 0) {
            // This sets the edges to the current coordinate
            // if this pixel lies outside the current boundaries
            minX = Math.min(x, minX);
            maxX = Math.max(x, maxX);
            minY = Math.min(y, minY);
            maxY = Math.max(y, maxY);
        }
    }
}

Then you need to create an image and copy the pixel data to it:

BufferedImage result = new BufferedImage(maxX - minX, maxY - minY, BufferedImage.TYPE_INT_ARGB);

for (int x = 0; x < maxX - minX; x++) {
    for (int y = 0; y < maxY - minY; y++) {
        result.setRGB(x, y, img.getRGB(x + minX, y + minY));
    }
}