/*
num_chans must be 3 or 4. There must be w*3*h or w*4*h bytes pointed to by pImage.
The image row pitch is always w*3 or w*4 bytes.
There is no automatic determination if the image actually uses an alpha channel, so if you call it with 4 you will always get a 32bpp .PNG file.
*/

// read the Demo PNG into an Image
BufferedImage image = ImageIO.read(inputStream);

// Create the w*4*h byte array
final byte[] byteArr = new byte[image.getWidth()*4*image.getHeight()];
for (int y=0; y<image.getHeight(); y++) {
    for (int x=0; x<image.getWidth(); x++) {
        int argb = image.getRGB(x, y);

        final int alpha = 0xff & (argb >> 24);
        final int red = 0xff & (argb >> 16);
        final int green = 0xff & (argb >> 8);
        final int blue = 0xff & (argb >> 0);

        byteArr[x * 4 * y]= (byte) red;
        byteArr[x * 4 * y + 1]= (byte) green;
        byteArr[x * 4 * y + 2]= (byte) blue;
        byteArr[x * 4 * y + 3]= (byte) alpha;
    }
}

// Convert byte array to Pointer
Pointer pImage = new Memory(byteArr.length);
pImage.write(0, byteArr, 0, byteArr.length);