Automatically find frame sizes from a png sprite sheet

Since all images are framed with the same color, you can search for the "frame border color" columns in columns and rows in the large image.

Use the number of columns and rows derived from the image size (width x height) to determine the pixel dimensions of each sub-image.

import java.awt.*;
import java.awt.image.BufferedImage;
import javax.swing.*;
import javax.swing.border.LineBorder;
import java.net.*;
import java.net.URL;
import java.util.ArrayList;
import javax.imageio.ImageIO;

class TileSetUtility {

    /** Divide the tile into tiles based on the number of cols & rows 
     * supplied.  Exclude any images that are a solid color. */
    public static ArrayList<BufferedImage> getTiles(
            BufferedImage tile, int cols, int rows) {
        int w = tile.getWidth();
        int h = tile.getHeight();
        int wT = w / cols;
        int hT = h / rows;
        if (wT * cols != w || hT * rows != h) {
            throw new IllegalArgumentException("Tile is not an even " +
                    "multiple of pixels of WxCols or HxRows!");
        }
        ArrayList<BufferedImage> tiles = new ArrayList<BufferedImage>();
        for (int x = 0; x < cols; x++) {
            for (int y = 0; y < rows; y++) {
                BufferedImage i = tile.getSubimage(x * wT, y * hT, wT, hT);
                if (!isImageSolidColor(i)) {
                    tiles.add(i);
                }
            }
        }
        return tiles;
    }

    /** Takes an image that represents tiles of a tile set, and infers the 
     * number of columns based on the assumption that the color at 0x0 in the 
     * image represents a border color or frame for the contained tiles. */
    public static int inferNumberColumns(BufferedImage img) {
        boolean[] columnClear = new boolean[img.getWidth()];
        // after this loop, we should have a series of contiguous regions
        // of 'true' in the array.
        for (int ii = 0; ii < columnClear.length; ii++) {
            columnClear[ii] = isLineEmpty(img, ii, false);
        }
        return countContiguousRegions(columnClear);
    }

    /** Takes an image that represents tiles of a tile set, and infers the 
     * number of rows based on the assumption that the color at 0x0 in the 
     * image represents a border color or frame for the contained tiles. */
    public static int inferNumberRows(BufferedImage img) {
        boolean[] columnClear = new boolean[img.getHeight()];
        // after this loop, we should have a series of contiguous regions
        // of 'true' in the array.
        for (int ii = 0; ii < columnClear.length; ii++) {
            columnClear[ii] = isLineEmpty(img, ii, true);
        }
        return countContiguousRegions(columnClear);
    }

    /** Count the number of contiguous regions of 'true' */
    public static int countContiguousRegions(boolean[] array) {
        boolean newRegion = false;
        int count = 0;
        for (boolean bool : array) {
            if (bool) {
                if (newRegion) {
                    count++;
                }
                newRegion = false;
            } else {
                newRegion = true;
            }
        }
        return count;
    }

    /** Determine if this entire column or row of image pixels is empty. */
    public static boolean isLineEmpty(
            BufferedImage img, int pos, boolean row) {

        if (!row) {
            for (int y = 0; y < img.getHeight(); y++) {
                if (img.getRGB(pos, y) != img.getRGB(0, 0)) {
                    return false;
                }
            }
        } else {
            for (int x = 0; x < img.getWidth(); x++) {
                if (img.getRGB(x, pos) != img.getRGB(0, 0)) {
                    return false;
                }
            }
        }
        return true;
    }

    /** Determine if this image is one solid color (implies redundant tile) */
    public static boolean isImageSolidColor(BufferedImage img) {
        int c = img.getRGB(0,0);
        for (int x=0; x<img.getWidth(); x++) {
            for (int y=0; y<img.getHeight(); y++) {
                if (c!=img.getRGB(x,y)) {
                    return false;
                }
            }
        }
        return true;
    }

    public static void main(String[] args) throws Exception {
        URL url = new URL("http://i.stack.imgur.com/ttXm6.png");
        final BufferedImage tileSet = ImageIO.read(url);
        Runnable r = new Runnable() {

            @Override
            public void run() {
                JPanel gui = new JPanel(new BorderLayout(5, 5));

                int cols = inferNumberColumns(tileSet);
                System.out.println("tileSet cols: " + cols);
                int rows = inferNumberRows(tileSet);
                System.out.println("tileSet rows: " + rows);

                ArrayList<BufferedImage> tiles = getTiles(tileSet, cols, rows);
                JPanel p = new JPanel(new GridLayout(0, 7, 1, 1));
                for (BufferedImage tile : tiles) {
                    JLabel l = new JLabel(new ImageIcon(tile));
                    l.setBorder(new LineBorder(Color.BLACK));
                    p.add(l);
                }

                gui.add(new JLabel(new ImageIcon(tileSet)));

                JOptionPane.showMessageDialog(null, p);
            }
        };
        // Swing GUIs should be created and updated on the EDT
        SwingUtilities.invokeLater(r);
    }
}