调整BufferedImages的大小并将其存储到文件中会导致JPG图像出现黑色背景

Question

我有以下代码:

import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;


public class JavaApplication
{
    public static void main(String[] args) throws Exception
    {
        File orig = new File ("/home/xxx/Pictures/xxx.jpg");
        BufferedImage bm1 = ImageIO.read(orig);

        Image scaled = bm1.getScaledInstance(100, 200, BufferedImage.SCALE_SMOOTH);
        BufferedImage bm2 = toBufferedImage(scaled);

        File resized = new File ("/home/xxx/Pictures/resized.jpg");
        ImageIO.write(bm2, "jpg", resized);
    }

    public static BufferedImage toBufferedImage(Image img)
    {
        if (img instanceof BufferedImage)
        {
            return (BufferedImage) img;
        }

        BufferedImage bimage = new BufferedImage(img.getWidth(null),       img.getHeight(null), BufferedImage.TYPE_INT_ARGB);

        bimage.getGraphics().drawImage(img, 0, 0 , null);
        return bimage;
      }
}

如果我在.png文件上使用此代码,它可以正常工作,并按预期调整文件大小.但是在jpg文件上,它会产生黑色背景.

如果我删除getScaledInstance()代码并只是尝试bm1使用原来重写原始文件ImageIO.write(bm1, "jpg", resized),那就可以了.只有在调整大小getScaledInstance()然后尝试将结果转换Image回来时BufferedImage,才能获得完全黑色的背景文件.

关于如何解决这个问题的想法,或者我做错了什么？

Answer 1

当我运行你的代码时,我没有得到黑色背景,但图像的颜色看起来很奇怪(通道似乎搞砸了).

当我改变图像类型中toBufferedImage(..),以BufferedImage.TYPE_INT_RGB(没有阿尔法,因为JPEG不支持透明度),一切工作正常.

ImageIO编写JPEG图像时没有考虑到这一点仍然很奇怪......

顺便说一句,异步图像缩放(就像getScaledInstance(..))不是问题,我确保在继续之前完成图像大小调整,这对结果没有影响.

要完全加载图像,请使用MediaTracker:

public static void loadCompletely (Image img) {

    MediaTracker tracker = new MediaTracker(new JPanel());
    tracker.addImage(img, 0);
    try {
        tracker.waitForID(0);
    } catch (InterruptedException ex) {
        throw new RuntimeException(ex);
    }
}

编辑
这是我用来调整图像大小,保留比例的代码(不同的调整大小方法取决于你是升级还是降尺度,以及更快的区域平均替代方法):

public static BufferedImage resizeImage (BufferedImage image, int areaWidth, int areaHeight) {
    float scaleX = (float) areaWidth / image.getWidth();
    float scaleY = (float) areaHeight / image.getHeight();
    float scale = Math.min(scaleX, scaleY);
    int w = Math.round(image.getWidth() * scale);
    int h = Math.round(image.getHeight() * scale);

    int type = image.getTransparency() == Transparency.OPAQUE ? BufferedImage.TYPE_INT_RGB : BufferedImage.TYPE_INT_ARGB;

    boolean scaleDown = scale < 1;

    if (scaleDown) {
        // multi-pass bilinear div 2
        int currentW = image.getWidth();
        int currentH = image.getHeight();
        BufferedImage resized = image;
        while (currentW > w || currentH > h) {
            currentW = Math.max(w, currentW / 2);
            currentH = Math.max(h, currentH / 2);

            BufferedImage temp = new BufferedImage(currentW, currentH, type);
            Graphics2D g2 = temp.createGraphics();
            g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);
            g2.drawImage(resized, 0, 0, currentW, currentH, null);
            g2.dispose();
            resized = temp;
        }
        return resized;
    } else {
        Object hint = scale > 2 ? RenderingHints.VALUE_INTERPOLATION_BICUBIC : RenderingHints.VALUE_INTERPOLATION_BILINEAR;

        BufferedImage resized = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
        Graphics2D g2 = resized.createGraphics();
        g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION, hint);
        g2.drawImage(image, 0, 0, w, h, null);
        g2.dispose();
        return resized;
    }
}