文本二进制透明度错误

Question

我遇到了以下问题，将透明的java.awt.Image嵌入到带有iText的pdf中(尝试使用2.1.7和5.5.9)。

我想在这种特殊情况下，当从ARGB图像转换为ittext图像时，ittext不正确处理二进制透明图像。

当提供的图像只包含100%和0%透明像素(即二进制透明度)，并且所有像素都是黑色的(不透明和透明的-即图像的所有像素的颜色=黑色，但alpha值为0%或100%)，透明像素的颜色值被内部检测为黑色(这在我看来是错误的)，这导致在生成的pdf中不可见的图像。

测试用例:

import java.awt.AlphaComposite;
import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import com.itextpdf.text.BadElementException;
import com.itextpdf.text.Document;
import com.itextpdf.text.DocumentException;
import com.itextpdf.text.Image;
import com.itextpdf.text.PageSize;
import com.itextpdf.text.Paragraph;
import com.itextpdf.text.pdf.PdfContentByte;
import com.itextpdf.text.pdf.PdfWriter;
public class BinaryTransparencyBug {

    private static Image bkgnd;

    public static void main(String[] args) throws Exception {
        bkgnd = Image.getInstance(new URL("http://gitlab.itextsupport.com/itext/sandbox/raw/master/resources/images/berlin2013.jpg"));
        bkgnd.scaleAbsolute(PageSize.A4);
        bkgnd.setAbsolutePosition(0, 0);

        Document document = new Document();
        File file = new File("target/binary_transparency_bug.pdf");
        FileOutputStream outputStream = new FileOutputStream(file);
        PdfWriter writer = PdfWriter.getInstance(document, outputStream);
        document.open();

        addBackground(writer);
        document.add(new Paragraph("Binary transparency bug test case"));
        document.add(new Paragraph("OK: Visible image (opaque pixels are red, non opaque pixels are black)"));
        document.add(com.itextpdf.text.Image.getInstance(createBinaryTransparentAWTImage(Color.red,false,null), null));
        document.newPage();
        addBackground(writer);
        document.add(new Paragraph("Suspected bug: invisible image (both opaque an non opaque pixels have the same color)"));
        document.add(com.itextpdf.text.Image.getInstance(createBinaryTransparentAWTImage(Color.black,false,null), null));
        document.newPage();
        addBackground(writer);
        document.add(new Paragraph("Analysis: Aliasing makes the problem disappear, because this way the image is not binary transparent any more"));
        document.add(com.itextpdf.text.Image.getInstance(createBinaryTransparentAWTImage(Color.black,true,null), null));
        document.newPage();
        addBackground(writer);
        document.add(new Paragraph("Analysis: Setting the color of the transparent pixels to anything but black makes the problem go away, too"));
        document.add(com.itextpdf.text.Image.getInstance(createBinaryTransparentAWTImage(Color.black,false,Color.red), null));
        document.close();
    }
    private static void addBackground(PdfWriter writer)
            throws BadElementException, MalformedURLException, IOException, DocumentException {
        PdfContentByte canvas = writer.getDirectContentUnder();
        canvas.saveState();
        canvas.addImage(bkgnd);
        canvas.restoreState();
    }
    // Create an ARGB AWT Image that has only 100% transparent and 0%
    // transparent pixels.
    // All 100% opaque pixels have the provided color "color"
    // All transparent pixels have the Color "backgroundColor"
    public static BufferedImage createBinaryTransparentAWTImage(Color color, boolean alias, Color backgroundColor) {
        Dimension size = new Dimension(200, 200);
        BufferedImage awtimg = new BufferedImage(size.width, size.height, BufferedImage.TYPE_INT_ARGB);
        Graphics2D g2d = awtimg.createGraphics();
        if (backgroundColor!=null)
        {
            //Usually it doen't make much sense to set the color of transparent pixels...
            //but in this case it changes the behavior of com.itextpdf.text.Image.getInstance fundamentally!
            g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC, 0f));       
            g2d.setColor(backgroundColor);
            g2d.fillRect(0, 0, size.width, size.height);
        }
        g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 1f));
        g2d.setColor(color);
        if (alias)
        {
            g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            g2d.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);
        }
        BasicStroke bs = new BasicStroke(2);
        g2d.setStroke(bs);
        for (int i = 0; i < 5; i++) {
            g2d.drawLine((size.width + 2) / 4 * i, 0, (size.width + 2) / 4 * i, size.height - 1);
            g2d.drawLine(0, (size.height + 2) / 4 * i, size.width - 1, (size.height + 2) / 4 * i);
        }
        return awtimg;
    }
}

Answer 1

下面是我解决这个问题的建议:

Image.getInstance (AWTImage、颜色、forcebw):在这种情况下forceBW=false and color=null:

for (int j = 0; j < size; j++) {
    byte alpha = smask[j] = (byte) (pixels[j] >> 24 & 0xff);
    /* bugfix by Chris Nokleberg */
    if (!shades) {
        if (alpha != 0 && alpha != -1) { 
            //as soon as there is any pixel with alpha not 0% or 100%
            //switch to smask
            shades = true;
        } else if (transparency == null) {
            //in binary transparency mode, determine the transparentPixel Color to be the 
            //value of the first Pixel we find with 100% transparency
            if (alpha == 0) {
                transparentPixel = pixels[j] & 0xffffff;
                transparency = new int[6];
                transparency[0] = transparency[1] = transparentPixel >> 16 & 0xff;
                transparency[2] = transparency[3] = transparentPixel >> 8 & 0xff;
                transparency[4] = transparency[5] = transparentPixel & 0xff;
                // vvv--- added by mkl
                // Check whether this value for transparent pixels
                // has already been used for a non-transparent one
                // before this position
                for (int jj = 0; jj < j; jj++)
                {
                    if ((pixels[jj] & 0xffffff) == transparentPixel)
                    {
                        // found a prior use of the transparentPixel color
                        // and, therefore, cannot make use of this color
                        // for transparency; we could still use an image
                        // mask but for simplicity let's use a soft mask
                        // which already is implemented here
                        shades = true;
                        break;
                    }
                }
                // ^^^--- added by mkl
            }
        } else if (((pixels[j] & 0xffffff) != transparentPixel) && (alpha==0)) {
            //TB: The above if seems incorrect to me. (EDIT: it was if ((pixels[j] & 0xffffff) != transparentPixel)
            //As soon as we find any pixel that has differnt color from  
            //transparentPixel-Color and alpha 0% or 100% 
            //switch of binary transparency mode.
            //IMHO this should only be done if alpha==0!
            //so the if clause should be 
            //((pixels[j] & 0xffffff) != transparentPixel) && (alpha==0)
            shades = true;
        }
        //TB: Proposed fix:
        else if ((pixels[j] & 0xffffff) == transparentPixel && alpha!=0) {
            //switch of binary transparency mode, if we find any pixel with the transparentPixel-Color,
            //but which is not transparent 
            shades = true;
        }                       
    }
    ...
}