How to combine multiple PNGs into one big PNG file?

Question

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?

Answer 1

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")); 

Answer 2

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");         } }