Java BufferedImage Not Registering Transparent Pixels?

Question

I have used the method ImageIO.read(File file); to read a PNG image file. However, when I use the getRGB(int x, int y) method on it to extract the alpha it always returns 255 whether the pixel is transparent or not. How do I remedy this inconvenience?

What method are you using to extract the alpha value from the packed `int`? — MadProgrammer, Oct 18 '13 at 01:24

score 3 · Accepted Answer · answered Oct 18 '13 at 01:37

3

When converting packed int colors to Color objects, you need to tell it if it should calculate the alpha value or not.

new Color(image.getRGB(x, y), true).getAlpha();

See Color(int, boolean) for more details

answered Oct 18 '13 at 01:37

MadProgrammer

343,457
22
230
366

Thanks loads. This helped me a bunch. I used this for 2d gaming scanning a png for boundaries to create bounded game objects. BTW, any ideas on how I can get unbanned from questions? – Dec 14 '13 at 04:04
@StoneAgeProgrammer No idea, I think you would need to contact the admins – MadProgrammer Dec 14 '13 at 04:09

score 0 · Answer 2 · edited May 23 '17 at 12:29

Just wanted to point out that using the method getRGB(x,y) is extremely inefficient. If you want to get the pixels of an image you could extract the colours from each individual pixel and then store the pixel in an int array. Credit also to mota for explaining why this is inefficient see his post . Example below:

/**===============================================================================================
     * Method that extracts pixel data from an image 
     * @return a 2d array representing the pixels of the image
    =================================================================================================*/
    public static int[][] getImageData(BufferedImage img) {
            int height = img.getHeight();
            int width = img.getWidth();
            final byte[] imgPixels = ((DataBufferByte) img.getRaster().getDataBuffer()).getData();
            final boolean is_Alpha_Present = img.getAlphaRaster() != null;
            int[][] imgArr = new int[height][width];

            if (is_Alpha_Present) {
                    final int pixelLength = 4; //number of bytes used to represent a pixel if alpha value present
                    for (int pixel = 0, row = 0, col = 0; pixel < imgPixels.length; pixel = pixel + pixelLength) {
                            int argb = 0;
                            argb += (((int) imgPixels[pixel] & 0xff) << 24); //getting the alpha for the pixel
                            argb += ((int) imgPixels[pixel + 1] & 0xff); //getting the blue colour
                            argb += (((int) imgPixels[pixel + 2] & 0xff) << 8); //getting the green colour
                            argb += (((int) imgPixels[pixel + 3] & 0xff) << 16); //getting the red colour
                            imgArr[row][col] = argb;
                            col++;
                            if (col == width) {
                                    col = 0;
                                    row++;
                            }
                    }
            }
            else {
                    final int pixelLength = 3;
                    for (int pixel = 0, row = 0, col = 0; pixel < imgPixels.length; pixel = pixel + pixelLength) {
                            int argb = 0;
                            argb += Integer.MIN_VALUE;
                            argb += ((int) imgPixels[pixel] & 0xff); //getting the blue colour
                            argb += (((int) imgPixels[pixel+1] & 0xff) << 8); //getting the green colour
                            argb += (((int) imgPixels[pixel+2] & 0xff) << 16); //getting the red colour
                            imgArr[row][col] = argb;
                            col++;
                            if (col == width) {
                                    col = 0;
                                    row++;
                            }       
                    }
            }
            return imgArr;
    }

Java BufferedImage Not Registering Transparent Pixels?

2 Answers2