java - 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?

Answer 1

将打包int颜色转换为Color对象时，您需要告诉它是否应该计算 alpha 值。

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

查看Color(int, boolean)更多详情

Answer 2

只是想指出，使用 getRGB(x,y) 方法效率极低。如果要获取图像的像素，可以从每个单独的像素中提取颜色，然后将像素存储在 int 数组中。还要感谢 mota 解释为什么这是低效的，请参阅他的帖子。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;
    }