How to combine multiple PNGs into one big PNG file?

I have approx. 6000 PNG files (256*256 pixels) and want to combine them into a big PNG holding all of them programmatically.

What's the best/fastest way to do that?

(The purpose is printing on paper, so using some web-technology is not an option and having one, single picture file will eliminate many usage errors.)

I tried fahd's suggestion but I get a NullPointerException when I try to create a BufferedImage with 24576 pixels wide and 15360 pixels high. Any ideas?

Create a large image which you will write to. Work out its dimensions based on how many rows and columns you want.

    BufferedImage result = new BufferedImage(
                               width, height, //work these out
                               BufferedImage.TYPE_INT_RGB);
    Graphics g = result.getGraphics();

Now loop through your images and draw them:

    for(String image : images){
        BufferedImage bi = ImageIO.read(new File(image));
        g.drawImage(bi, x, y, null);
        x += 256;
        if(x > result.getWidth()){
            x = 0;
            y += bi.getHeight();
        }
    }

Finally write it out to file:

    ImageIO.write(result,"png",new File("result.png"));

I had some similar need some time ago (huge images -and, I my case with 16 bitdepth- to have them fully in memory was not an option). And I ended coding a PNG library to do the read/write in a sequential way. In case someone find it useful, it's here.

Updated: here's a sample code:

/**
 * Takes several tiles and join them in a single image
 * 
 * @param tiles            Filenames of PNG files to tile
 * @param dest            Destination PNG filename
 * @param nTilesX            How many tiles per row?
 */
public class SampleTileImage {

        public static void doTiling(String tiles[], String dest, int nTilesX) {
                int ntiles = tiles.length;
                int nTilesY = (ntiles + nTilesX - 1) / nTilesX; // integer ceil
                ImageInfo imi1, imi2; // 1:small tile   2:big image
                PngReader pngr = new PngReader(new File(tiles[0]));
                imi1 = pngr.imgInfo;
                PngReader[] readers = new PngReader[nTilesX];
                imi2 = new ImageInfo(imi1.cols * nTilesX, imi1.rows * nTilesY, imi1.bitDepth, imi1.alpha, imi1.greyscale,
                                imi1.indexed);
                PngWriter pngw = new PngWriter(new File(dest), imi2, true);
                // copy palette and transparency if necessary (more chunks?)
                pngw.copyChunksFrom(pngr.getChunksList(), ChunkCopyBehaviour.COPY_PALETTE
                                | ChunkCopyBehaviour.COPY_TRANSPARENCY);
                pngr.readSkippingAllRows(); // reads only metadata             
                pngr.end(); // close, we'll reopen it again soon
                ImageLineInt line2 = new ImageLineInt(imi2);
                int row2 = 0;
                for (int ty = 0; ty < nTilesY; ty++) {
                        int nTilesXcur = ty < nTilesY - 1 ? nTilesX : ntiles - (nTilesY - 1) * nTilesX;
                        Arrays.fill(line2.getScanline(), 0);
                        for (int tx = 0; tx < nTilesXcur; tx++) { // open several readers
                                readers[tx] = new PngReader(new File(tiles[tx + ty * nTilesX]));
                                readers[tx].setChunkLoadBehaviour(ChunkLoadBehaviour.LOAD_CHUNK_NEVER);
                                if (!readers[tx].imgInfo.equals(imi1))
                                        throw new RuntimeException("different tile ? " + readers[tx].imgInfo);
                        }
                        for (int row1 = 0; row1 < imi1.rows; row1++, row2++) {
                                for (int tx = 0; tx < nTilesXcur; tx++) {
                                        ImageLineInt line1 = (ImageLineInt) readers[tx].readRow(row1); // read line
                                        System.arraycopy(line1.getScanline(), 0, line2.getScanline(), line1.getScanline().length * tx,
                                                        line1.getScanline().length);
                                }
                                pngw.writeRow(line2, row2); // write to full image
                        }
                        for (int tx = 0; tx < nTilesXcur; tx++)
                                readers[tx].end(); // close readers
                }
                pngw.end(); // close writer
        }

        public static void main(String[] args) {
                doTiling(new String[] { "t1.png", "t2.png", "t3.png", "t4.png", "t5.png", "t6.png" }, "tiled.png", 2);
                System.out.println("done");
        }
}