Java / OpenGL: Getting the image of a Canvas as a BufferedImage

Question

I've got some code that initializes OpenGL to render to a java.awt.Canvas. The problem is, I can't figure out how I can get the buffer of the canvas and turn it into a BufferedImage.

I've tried overriding getGraphics(), cloning the Raster, and replacing the CanvasPeer with a custom one.

I'm guessing OpenGL doesn't use java graphics in any way then, so how can I get OpenGL's buffer and convert it into a BufferedImage?

I am using LWJGL's code for setting parent:

Display.setParent(display_parent);
Display.create();

Answer 1

You need to copy data from OpenGL buffer. I was using this method:

FloatBuffer grabScreen(GL gl) 
{       
    int w = SCREENWITDH;
    int h = SCREENHEIGHT;
    FloatBuffer bufor = FloatBuffer.allocate(w*h*4); // 4 = rgba

    gl.glReadBuffer(GL.GL_FRONT);
    gl.glReadPixels(0, 0, w, h, GL.GL_RGBA, GL.GL_FLOAT, bufor); //Copy the image to the array imageData

    return bufor;
}

You need to use something similar according to your OpenGL wrapper. This is JOGL example.

And here for LWJGL wrapper:

private static synchronized byte[] grabScreen()
{
    int w = screenWidth;
    int h = screenHeight;
    ByteBuffer bufor = BufferUtils.createByteBuffer(w * h * 3);

    GL11.glReadPixels(0, 0, w, h, GL11.GL_RGB, GL11.GL_UNSIGNED_BYTE, bufor); //Copy the image to the array imageData

    byte[] byteimg = new byte[w * h * 3];
    bufor.get(byteimg, 0, byteimg.length);
    return byteimg;
}

EDIT

This may be useful also (it's not fully mine, should be tuned too):

BufferedImage toImage(byte[] data, int w, int h)
{
    if (data.length == 0)
        return null;

    DataBuffer buffer = new DataBufferByte(data, w * h);

    int pixelStride = 3; //assuming r, g, b, skip, r, g, b, skip...
    int scanlineStride = 3 * w; //no extra padding   
    int[] bandOffsets = { 0, 1, 2 }; //r, g, b
    WritableRaster raster = Raster.createInterleavedRaster(buffer, w, h, scanlineStride, pixelStride, bandOffsets,
            null);

    ColorSpace colorSpace = ColorSpace.getInstance(ColorSpace.CS_sRGB);
    boolean hasAlpha = false;
    boolean isAlphaPremultiplied = true;
    int transparency = Transparency.TRANSLUCENT;
    int transferType = DataBuffer.TYPE_BYTE;
    ColorModel colorModel = new ComponentColorModel(colorSpace, hasAlpha, isAlphaPremultiplied, transparency,
            transferType);

    BufferedImage image = new BufferedImage(colorModel, raster, isAlphaPremultiplied, null);

    AffineTransform flip;
    AffineTransformOp op;
    flip = AffineTransform.getScaleInstance(1, -1);
    flip.translate(0, -image.getHeight());
    op = new AffineTransformOp(flip, AffineTransformOp.TYPE_NEAREST_NEIGHBOR);
    image = op.filter(image, null);

    return image;
}

Answer 2

I don't think this is possible for your situation, and here's why:

LWJGL doesn't draw directly to the canvas (at least not in Windows). The canvas is only used to obtain a window handle to provide as the parent window to OpenGL. As such, the canvas is never directly drawn to. To capture the contents, you'll probably have to resort to a screen capture.