Making screenshots in JOGL

OpenGL Development JOGL Development
#1

Hi. I want my engine to be capable of making screenshots. And I have experienced serious problems doing that (even HotSpot unexpected errors). I have a method that makes screenshots. Here it is:


public void makeScreenShot(File output, int width, int height) {
        ByteBuffer data = ByteBuffer.allocateDirect(width*height*3);
        gl.glReadBuffer(gl.GL_FRONT);
        gl.glReadPixels(0, 0, width, height, gl.GL_RGB, gl.GL_BYTE, data);
        BufferedImage screenshot = new BufferedImage(width, height, BufferedImage.TYPE_3BYTE_BGR);
        screenshot.setData(Raster.createRaster(new BandedSampleModel(DataBuffer.TYPE_BYTE, width, height, 3), 
        new DataBufferByte(data.array(), data.capacity()), new Point(0, 0)));
        try {
        ImageIO.write(screenshot, "png", output);
        } catch(IOException e) {
            System.out.println("Unable to write screenshot. Details: ");
            e.printStackTrace();
            System.exit(1);
        }
    }

Well, in theory, it must read image data from the front gl buffer and convert it to a ByteBuffer, than make a BufferedImage and write it to the disk.

What I get is

`
Exception in thread “AWT-EventQueue-0” java.lang.UnsupportedOperationException
at java.nio.ByteBuffer.array(Unknown Source)
at overlord.renderer.GLView.makeScreenShot(GLView.java:264)
at overlord.renderer.Demo.display(Demo.java:48)
at net.java.games.jogl.impl.GLDrawableHelper.display(GLDrawableHelper.ja
va:74)
at net.java.games.jogl.GLCanvas$DisplayAction.run(GLCanvas.java:206)
at net.java.games.jogl.impl.GLContext.invokeGL(GLContext.java:239)
at net.java.games.jogl.GLCanvas.displayImpl(GLCanvas.java:194)
at net.java.games.jogl.GLCanvas.display(GLCanvas.java:82)
at net.java.games.jogl.GLCanvas.paint(GLCanvas.java:89)
at sun.awt.RepaintArea.paintComponent(Unknown Source)
at sun.awt.RepaintArea.paint(Unknown Source)
at sun.awt.windows.WComponentPeer.handleEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source)

    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source) `

Very likely, my buffer is empty after glReadPixels. How is that possible? Please help.

#2

Hi
I use java.awt.Robot to make screenshot, like:
Robot screenRobot = new Robot();
Rectangle rectangle = new Rectangle(0,0, width, height)
BufferedImage screenshot= screenRobot.createScreenCapture(rectangle);

This will give you a bufferedImage of the parts of the screen that is in the rectangle. I dont know if it is a good way of doing it, but it works for me :slight_smile:

// Gregof

#3

Hi

This works for me:


private static void writeBufferToFile(GLDrawable drawable, File outputFile) {

      int width = drawable.getSize().width;
      int height = drawable.getSize().height;

      ByteBuffer pixelsRGB = BufferUtils.newByteBuffer(width * height * 3);

      GL gl = drawable.getGL();

      gl.glReadBuffer(GL.GL_BACK);
      gl.glPixelStorei(GL.GL_PACK_ALIGNMENT, 1);

      gl.glReadPixels(0,                    // GLint x
                  0,                    // GLint y
                  width,                     // GLsizei width
                  height,              // GLsizei height
                  GL.GL_RGB,              // GLenum format
                  GL.GL_UNSIGNED_BYTE,        // GLenum type
                  pixelsRGB);               // GLvoid *pixels

      int[] pixelInts = new int[width * height];

      // Convert RGB bytes to ARGB ints with no transparency. Flip image vertically by reading the
      // rows of pixels in the byte buffer in reverse - (0,0) is at bottom left in OpenGL.

      int p = width * height * 3; // Points to first byte (red) in each row.
      int q;                  // Index into ByteBuffer
      int i = 0;                  // Index into target int[]
      int w3 = width*3;         // Number of bytes in each row

      for (int row = 0; row < height; row++) {
            p -= w3;
            q = p;
            for (int col = 0; col < width; col++) {
                  int iR = pixelsRGB.get(q++);
                  int iG = pixelsRGB.get(q++);
                  int iB = pixelsRGB.get(q++);

                  pixelInts[i++] = 0xFF000000
                              | ((iR & 0x000000FF) << 16)
                              | ((iG & 0x000000FF) << 8)
                              | (iB & 0x000000FF);
            }

      }

      BufferedImage bufferedImage =
            new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);

      bufferedImage.setRGB(0, 0, width, height, pixelInts, 0, width);

      try {
            ImageIO.write(bufferedImage, "PNG", outputFile);
      } catch (IOException e) {
            e.printStackTrace();
      }

}

I call this at the very end of the display() method.

One thing to watch out for is:

gl.glPixelStorei(GL.GL_PACK_ALIGNMENT, 1);

Without this, and if your display width is not a multiple of 4, glReadPixels will pad each row of pixels to the next multiple of 4 bytes, with potentially disasterous results - it will probably end up trying to pack more than widthheight3 bytes into your buffer.

Also, the order of the rows of pixels in the ByteBuffer will be reversed - Row 0 in openGL is at the bottom of the image, everywhere else it’s at the top. I don’t know if BandedSampleModel etc. will be able to resolve this, so I’m just doing it manually (efficiency isn’t that important to me in this case).

Kevin

#4

Thanks for the replies and code! I’ll take time to examine it.