Java BufferedImage沒有注冊透明像素？

Question

我已經使用方法ImageIO.read(File file); 讀取PNG圖像文件。 但是，當我在其上使用getRGB(int x, int y)方法提取Alpha時，無論像素是否透明，它總是返回255。 我該如何解決這種不便？

Answer 1

將打包的int顏色轉換為Color對象時，需要告訴它是否應該計算alpha值。

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

有關更多詳細信息Color(int, boolean)請參見Color(int, boolean)

Answer 2

只是想指出，使用方法getRGB（x，y）效率極低。 如果要獲取圖像的像素，可以從每個像素提取顏色，然后將像素存儲在一個int數組中。 也要感謝mota解釋為什么這效率低下， 請參閱他的文章 。 下面的例子：

/**===============================================================================================
     * 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;
    }