如何使用pdfbox检查文本是否透明 [英] How to check if a text is transparent with pdfbox
问题描述
我已经 PDFStreamEngine 并且重载 processTextPosition ，我现在能够重建像
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.
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