如何使用pdfbox检查文本是否透明

如何使用pdfbox检查文本是否透明

事实证明， 透明文本 实际上根本不是透明的，而只是被图像覆盖：在201103 SA的关键吸烟统计数据2010
FINAL.pdf中
，图像“
SA — 2004的关键吸烟统计数据”已被图像覆盖。显示TC标志。

下面显示了文本剥离程序类的概念证明，该类将忽略图像覆盖的文本。

public class VisibleTextStripper extends PDFTextStripper{    public VisibleTextStripper() throws IOException    {        super();        registerOperatorProcessor("Do", new Invoke());    }    //    // Hiding operations    //    void hide(String name)    {        Matrix ctm = getGraphicsState().getCurrentTransformationMatrix();        float x = ctm.getXPosition();        float y = ctm.getYPosition();        float scaledWidth = ctm.getXScale();        float scaledHeight = ctm.getYScale();        for(List<TextPosition> characters : charactersByArticle)        { Collection<TextPosition> toRemove = new ArrayList<TextPosition>(); for (TextPosition character : characters) {     Matrix matrix = character.getTextPos();     float cx = matrix.getXPosition();     float cy = matrix.getYPosition();     float cw = character.getWidth();     float ch = character.getHeight();     if (overlaps(x, scaledWidth, cx, cw) && overlaps(y, scaledHeight, cy, cw))     {         System.out.printf("Hidden by '%s': X: %f; Y: %f; Width: %f; Height: %f; Char: '%s'n", name, cx, cy, cw, ch, character.getCharacter());         toRemove.add(character);     } } characters.removeAll(toRemove);        }    }    private boolean overlaps(float start1, float width1, float start2, float width2)    {        if (width1 < 0)        { start1 += width1; width1 = -width1;        }        if (width2 < 0)        { start2 += width2; width2 = -width2;        }        if (start1 < start2)        { return start1 + width1 >= start2;        }        else        { return start2 + width2 >= start1;        }    }    //    // operator processors    //    public static class Invoke extends OperatorProcessor    {                private static final Log LOG = LogFactory.getLog(Invoke.class);                public void process(PDFOperator operator, List<COSbase> arguments) throws IOException        { VisibleTextStripper drawer = (VisibleTextStripper)context; COSName objectName = (COSName)arguments.get( 0 ); Map<String, PDXObject> xobjects = drawer.getResources().getXObjects(); PDXObject xobject = (PDXObject)xobjects.get( objectName.getName() ); if ( xobject == null ) {     LOG.warn("Can't find the XObject for '"+objectName.getName()+"'"); } else if( xobject instanceof PDXObjectImage ) {     drawer.hide(objectName.getName()); } else if(xobject instanceof PDXObjectForm) {     PDXObjectForm form = (PDXObjectForm)xobject;     COSStream formContentstream = form.getCOSStream();     // if there is an optional form matrix, we have to map the form space to the user space     Matrix matrix = form.getMatrix();     if (matrix != null)      {         Matrix xobjectCTM = matrix.multiply( context.getGraphicsState().getCurrentTransformationMatrix());         context.getGraphicsState().setCurrentTransformationMatrix(xobjectCTM);     }     // find some optional resources, instead of using the current resources     PDResources pdResources = form.getResources();     context.processSubStream( context.getCurrentPage(), pdResources, formContentstream ); }        }    }}

它适合您的示例文档。

支票

if (overlaps(x, scaledWidth, cx, cw) && overlaps(y, scaledHeight, cy, cw))

不幸的是，假设不涉及文本和图像的旋转（所有转换都汇总）。

对于通用解决方案，您必须将此测试更改为某种检查，以检查由

Matrix ctm =getGraphicsState().getCurrentTransformationMatrix()

重叠转换的1x1正方形是否与由

Matrixmatrix = character.getTextPos()

固定宽度，高度

cw = character.getWidth()

和的字符框重叠

ch =character.getHeight()

。也许简单的重叠是不够的，您可能希望充分覆盖字符框。

此外，该测试忽略了图像遮罩，即图像的透明度。