Java:生成具有透明度的缩略图

Question

我在生成带有 Alpha channel (透明度)的图像缩略图时遇到问题。我使用的代码是这样的:

public void saveThumbnail(File file, String imageType) {
    if (bufferedThumb == null) {
        return;
    }

    if(bufferedImage.getColorModel().hasAlpha()) {
        logger.debug("Original image has Alpha channel");
    }

    BufferedImage bi = new BufferedImage(bufferedThumb.getWidth(null), bufferedThumb.getHeight(null), BufferedImage.TYPE_INT_ARGB);
    Graphics g = bi.getGraphics();
    g.drawImage(bufferedThumb, 0, 0, null);
    try {
        ImageIO.write(bi, imageType, file);
    } catch (IOException ioe) {
        ioe.printStackTrace();
        System.out.println("Error occured saving thumbnail");
    }
}

但是，如果我提供例如具有透明背景的 GIF 图像，我总是以黑色或彩色背景结束。

编辑:

这是使用缩略图从类中调用它的方式，我上次错过了 getThuimbnail() 方法的双参数版本:

Thumbnail th = new Thumbnail(file.getPath());
th.getThumbnail(100);

添加用于获取图像的方法:

public Thumbnail(String fileName) {
    try {
        this.bufferedImage = ImageIO.read(new File(fileName));
    } catch (IOException ex) {
        logger.error("Failed to read image file: " + ex.getMessage());
    }
}

public Image getThumbnail(int size) {
    int dir = VERTICAL;
    if (bufferedImage.getHeight() < bufferedImage.getWidth()) {
        dir = HORIZONTAL;
    }
    return getThumbnail(size, dir);
}

/**
 * Creates image with specifed max sized to a specified direction.
 * Will use Image.SCALE_SMOOTH for scaling.
 * @param size Maximum size
 * @param dir Direction of maximum size - 0 = vertical, 1 = height.
 * @return Resized image.
 */
public Image getThumbnail(int size, int dir) {
    return getThumbnail(size, dir, Image.SCALE_SMOOTH);
}

/**
 * Creates image with specified size.
 * @param size Maximum size
 * @param dir Direction of maximum size - 0 = vertical, 1 = height.
 * @param scale Image.Scale to use for conversion.
 * @return Resized image.
 */
public Image getThumbnail(int size, int dir, int scale) {
    if (dir == HORIZONTAL) {
        bufferedThumb = bufferedImage.getScaledInstance(size, -1, scale);
    } else {
        bufferedThumb = bufferedImage.getScaledInstance(-1, size, scale);
    }
    return bufferedThumb;
}

谢谢!

Answer 1

缩小具有透明颜色的 GIF 图像应该可以正常工作。我同意 jarnbjo 的观点，这个问题最有可能出现在 bufferedThumb 一代。

也许以下提示会有所帮助:

1) 创建缩略图时从源复制图像类型，例如:

BufferedImage thumb = new BufferedImage(fit, fit, image.getType());

2) 使用二维方法:

Graphics2D g = thumb.createGraphics();

这里有一些简单的拇指创建示例代码(经过测试和工作；透明度 GIF，拇指保持透明):

  public static BufferedImage thumb(BufferedImage image, int fit) {

    //image = blur(image);
    BufferedImage thumb = new BufferedImage(fit, fit, image.getType());
    Graphics2D g = thumb.createGraphics();

    try {
      int width = image.getWidth();
      int height = image.getHeight();
      int sx1;
      int sy1;
      int sx2;
      int sy2;
      int tmp;

      if (height > width) {
        tmp = height - width;
        sx1 = 0;
        sy1 = tmp / 2;
        sx2 = width;
        sy2 = height - sy1;
      } else if (width > height) {
        tmp = width - height;
        sx1 = tmp / 2;
        sy1 = 0;
        sx2 = width - sx1;
        sy2 = height;
      } else {
        sx1 = 0;
        sy1 = 0;
        sx2 = width;
        sy2 = height;
      }

      g.drawImage(
          image,
          0, 0,
          fit, fit,
          sx1, sy1,
          sx2, sy2,
          null
      );

    } finally {
      g.dispose();
    }
    return thumb;
  }//thumb

注意:简单，我的意思是如果您试图在单个步骤中缩放太多(例如 2048 像素，100 像素)，它不会产生高质量的结果。您可能需要采取多步骤方法，并且您可能应该求助于带有提示的 AffineTransformOp，而不是使用图形设备。