如何使用pdfbox检查文本是否透明

本文介绍了如何使用pdfbox检查文本是否透明的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我已经 PDFStreamEngine 并且重载 processTextPosition ，我现在能够重建像 PDFTextStripper 但我不想处理透明文本，这通常是垃圾。

I subclassed PDFStreamEngine and overloaded processTextPosition, I am now able to reconstruct the text like PDFTextStripper but I don't want to process transparent text, which is often garbage.

我怎么知道某些文字是否透明？

How can I know if some text is transparent ?

推荐答案

事实证明，透明文本实际上根本不是透明的，而只是由一个image：在 201103主要吸烟统计数据中对于SA 2010 FINAL.pdf ，文字SA的2004年吸烟统计数据已经被显示TC徽标的图像所覆盖。

As turned out the transparent text actually was not transparent at all but instead merely covered by an image: In 201103 Key Smoking Statistic for SA 2010 FINAL.pdf the text "Key Smoking Statistics for SA --- 2004" has been covered by an image showing a TC logo.

下面显示了文本剥离器类的概念证明，忽略了图像覆盖的文本。

The following shows a proof of concept of a text stripper class ignoring text covered by images.

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 { /** * Log instance. */ private static final Log LOG = LogFactory.getLog(Invoke.class); /** * process : Do : Paint the specified XObject (section 4.7). * @param operator The operator that is being executed. * @param arguments List * @throws IOException If there is an error invoking the sub object. */ 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 ); } } } }

它适用于您的示例文档。

It works well with your sample document.

支票

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

遗憾地假设没有涉及旋转（所有转换聚合），包括文本和图像。

unfortunately assumes that there are no rotations (all transformations aggregated) involved, neither of the text nor of the image.

对于通用解决方案，您必须更改此测试用于检查由 Matrix ctm = getGraphicsState（）。getCurrentTransformationMatrix（）转换的1x1平方是否重叠由矩阵转换的字符框matrix = character.getTextPos（）固定宽度和高度 cw = character.getWidth（）和 ch = character。的getHeight（）。也许简单的重叠是不够的，您可能希望充分覆盖字符框。

For a generic solution you have to change this test to something that checks whether the 1x1 square transformed by the Matrix ctm = getGraphicsState().getCurrentTransformationMatrix() overlaps the character box transformed by the Matrix matrix = character.getTextPos() with fixed width and height cw = character.getWidth() and ch = character.getHeight(). And maybe simple overlapping does not suffice, you might want the character box to be covered sufficiently.

此外，此测试忽略图像蒙版，即图像的透明度。

Furthermore this test ignores image masks, i.e. transparency of the image.