要正确提取这些下标和上标，需要一种不同的方法来检查两个文本块是否在同一行上。以下类代表了一种这样的方法。
我更熟悉 Java/iText；因此，我首先在 Java 中实现了这种方法，然后才将其翻译为 C#/iTextSharp。
使用 Java 和 iText 的方法
我正在使用当前的开发分支 iText 5.5.8-SNAPSHOT。
一种识别线的方法
假设文本行是水平的，并且不同行上字形的边界框的垂直延伸不重叠，可以尝试使用RenderListener 来识别行，如下所示：
public class TextLineFinder implements RenderListener
{
@Override
public void beginTextBlock() { }
@Override
public void endTextBlock() { }
@Override
public void renderImage(ImageRenderInfo renderInfo) { }
/*
* @see RenderListener#renderText(TextRenderInfo)
*/
@Override
public void renderText(TextRenderInfo renderInfo)
{
LineSegment ascentLine = renderInfo.getAscentLine();
LineSegment descentLine = renderInfo.getDescentLine();
float[] yCoords = new float[]{
ascentLine.getStartPoint().get(Vector.I2),
ascentLine.getEndPoint().get(Vector.I2),
descentLine.getStartPoint().get(Vector.I2),
descentLine.getEndPoint().get(Vector.I2)
};
Arrays.sort(yCoords);
addVerticalUseSection(yCoords[0], yCoords[3]);
}
/**
* This method marks the given interval as used.
*/
void addVerticalUseSection(float from, float to)
{
if (to < from)
{
float temp = to;
to = from;
from = temp;
}
int i=0, j=0;
for (; i<verticalFlips.size(); i++)
{
float flip = verticalFlips.get(i);
if (flip < from)
continue;
for (j=i; j<verticalFlips.size(); j++)
{
flip = verticalFlips.get(j);
if (flip < to)
continue;
break;
}
break;
}
boolean fromOutsideInterval = i%2==0;
boolean toOutsideInterval = j%2==0;
while (j-- > i)
verticalFlips.remove(j);
if (toOutsideInterval)
verticalFlips.add(i, to);
if (fromOutsideInterval)
verticalFlips.add(i, from);
}
final List<Float> verticalFlips = new ArrayList<Float>();
}
(TextLineFinder.java)
此RenderListener 尝试通过将文本边界框投影到 y 轴来识别水平文本行。它假设这些投影对于来自不同行的文本不会重叠，即使在下标和上标的情况下也是如此。
这个类本质上是this answer中使用的PageVerticalAnalyzer的简化形式。
按这些行对文本块进行排序
识别出上述行后，可以调整 iText 的 LocationTextExtractionStrategy 以像这样对这些行进行排序：
public class HorizontalTextExtractionStrategy extends LocationTextExtractionStrategy
{
public class HorizontalTextChunk extends TextChunk
{
public HorizontalTextChunk(String string, Vector startLocation, Vector endLocation, float charSpaceWidth)
{
super(string, startLocation, endLocation, charSpaceWidth);
}
@Override
public int compareTo(TextChunk rhs)
{
if (rhs instanceof HorizontalTextChunk)
{
HorizontalTextChunk horRhs = (HorizontalTextChunk) rhs;
int rslt = Integer.compare(getLineNumber(), horRhs.getLineNumber());
if (rslt != 0) return rslt;
return Float.compare(getStartLocation().get(Vector.I1), rhs.getStartLocation().get(Vector.I1));
}
else
return super.compareTo(rhs);
}
@Override
public boolean sameLine(TextChunk as)
{
if (as instanceof HorizontalTextChunk)
{
HorizontalTextChunk horAs = (HorizontalTextChunk) as;
return getLineNumber() == horAs.getLineNumber();
}
else
return super.sameLine(as);
}
public int getLineNumber()
{
Vector startLocation = getStartLocation();
float y = startLocation.get(Vector.I2);
List<Float> flips = textLineFinder.verticalFlips;
if (flips == null || flips.isEmpty())
return 0;
if (y < flips.get(0))
return flips.size() / 2 + 1;
for (int i = 1; i < flips.size(); i+=2)
{
if (y < flips.get(i))
{
return (1 + flips.size() - i) / 2;
}
}
return 0;
}
}
@Override
public void renderText(TextRenderInfo renderInfo)
{
textLineFinder.renderText(renderInfo);
LineSegment segment = renderInfo.getBaseline();
if (renderInfo.getRise() != 0){ // remove the rise from the baseline - we do this because the text from a super/subscript render operations should probably be considered as part of the baseline of the text the super/sub is relative to
Matrix riseOffsetTransform = new Matrix(0, -renderInfo.getRise());
segment = segment.transformBy(riseOffsetTransform);
}
TextChunk location = new HorizontalTextChunk(renderInfo.getText(), segment.getStartPoint(), segment.getEndPoint(), renderInfo.getSingleSpaceWidth());
getLocationalResult().add(location);
}
public HorizontalTextExtractionStrategy() throws NoSuchFieldException, SecurityException
{
locationalResultField = LocationTextExtractionStrategy.class.getDeclaredField("locationalResult");
locationalResultField.setAccessible(true);
textLineFinder = new TextLineFinder();
}
@SuppressWarnings("unchecked")
List<TextChunk> getLocationalResult()
{
try
{
return (List<TextChunk>) locationalResultField.get(this);
}
catch (IllegalArgumentException | IllegalAccessException e)
{
e.printStackTrace();
throw new RuntimeException(e);
}
}
final Field locationalResultField;
final TextLineFinder textLineFinder;
}
(HorizontalTextExtractionStrategy.java)
这个TextExtractionStrategy 使用TextLineFinder 来识别水平文本行，然后使用这些信息对文本块进行排序。
注意，此代码使用反射来访问私有父类成员。并非所有环境都允许这样做。在这种情况下，只需复制LocationTextExtractionStrategy并直接插入代码即可。
提取文本
现在可以使用这种文本提取策略来提取具有内联上标和下标的文本，如下所示：
String extract(PdfReader reader, int pageNo) throws IOException, NoSuchFieldException, SecurityException
{
return PdfTextExtractor.getTextFromPage(reader, pageNo, new HorizontalTextExtractionStrategy());
}
（来自ExtractSuperAndSubInLine.java）
OP 文档第 11 页“燃烧效率”下的示例文本现在提取如下：
monoxide (CO) in flue gas in accordance with the following formula: C.E. = [CO 2/(CO + CO 2 )]
使用 C# 和 iTextSharp 的相同方法
以 Java 为中心的部分的解释、警告和示例结果仍然适用，代码如下：
我正在使用 iTextSharp 5.5.7。
一种识别线的方法
public class TextLineFinder : IRenderListener
{
public void BeginTextBlock() { }
public void EndTextBlock() { }
public void RenderImage(ImageRenderInfo renderInfo) { }
public void RenderText(TextRenderInfo renderInfo)
{
LineSegment ascentLine = renderInfo.GetAscentLine();
LineSegment descentLine = renderInfo.GetDescentLine();
float[] yCoords = new float[]{
ascentLine.GetStartPoint()[Vector.I2],
ascentLine.GetEndPoint()[Vector.I2],
descentLine.GetStartPoint()[Vector.I2],
descentLine.GetEndPoint()[Vector.I2]
};
Array.Sort(yCoords);
addVerticalUseSection(yCoords[0], yCoords[3]);
}
void addVerticalUseSection(float from, float to)
{
if (to < from)
{
float temp = to;
to = from;
from = temp;
}
int i=0, j=0;
for (; i<verticalFlips.Count; i++)
{
float flip = verticalFlips[i];
if (flip < from)
continue;
for (j=i; j<verticalFlips.Count; j++)
{
flip = verticalFlips[j];
if (flip < to)
continue;
break;
}
break;
}
bool fromOutsideInterval = i%2==0;
bool toOutsideInterval = j%2==0;
while (j-- > i)
verticalFlips.RemoveAt(j);
if (toOutsideInterval)
verticalFlips.Insert(i, to);
if (fromOutsideInterval)
verticalFlips.Insert(i, from);
}
public List<float> verticalFlips = new List<float>();
}
按这些行对文本块进行排序
public class HorizontalTextExtractionStrategy : LocationTextExtractionStrategy
{
public class HorizontalTextChunk : TextChunk
{
public HorizontalTextChunk(String stringValue, Vector startLocation, Vector endLocation, float charSpaceWidth, TextLineFinder textLineFinder)
: base(stringValue, startLocation, endLocation, charSpaceWidth)
{
this.textLineFinder = textLineFinder;
}
override public int CompareTo(TextChunk rhs)
{
if (rhs is HorizontalTextChunk)
{
HorizontalTextChunk horRhs = (HorizontalTextChunk) rhs;
int rslt = CompareInts(getLineNumber(), horRhs.getLineNumber());
if (rslt != 0) return rslt;
return CompareFloats(StartLocation[Vector.I1], rhs.StartLocation[Vector.I1]);
}
else
return base.CompareTo(rhs);
}
public override bool SameLine(TextChunk a)
{
if (a is HorizontalTextChunk)
{
HorizontalTextChunk horAs = (HorizontalTextChunk) a;
return getLineNumber() == horAs.getLineNumber();
}
else
return base.SameLine(a);
}
public int getLineNumber()
{
Vector startLocation = StartLocation;
float y = startLocation[Vector.I2];
List<float> flips = textLineFinder.verticalFlips;
if (flips == null || flips.Count == 0)
return 0;
if (y < flips[0])
return flips.Count / 2 + 1;
for (int i = 1; i < flips.Count; i+=2)
{
if (y < flips[i])
{
return (1 + flips.Count - i) / 2;
}
}
return 0;
}
private static int CompareInts(int int1, int int2){
return int1 == int2 ? 0 : int1 < int2 ? -1 : 1;
}
private static int CompareFloats(float float1, float float2)
{
return float1 == float2 ? 0 : float1 < float2 ? -1 : 1;
}
TextLineFinder textLineFinder;
}
public override void RenderText(TextRenderInfo renderInfo)
{
textLineFinder.RenderText(renderInfo);
LineSegment segment = renderInfo.GetBaseline();
if (renderInfo.GetRise() != 0){ // remove the rise from the baseline - we do this because the text from a super/subscript render operations should probably be considered as part of the baseline of the text the super/sub is relative to
Matrix riseOffsetTransform = new Matrix(0, -renderInfo.GetRise());
segment = segment.TransformBy(riseOffsetTransform);
}
TextChunk location = new HorizontalTextChunk(renderInfo.GetText(), segment.GetStartPoint(), segment.GetEndPoint(), renderInfo.GetSingleSpaceWidth(), textLineFinder);
getLocationalResult().Add(location);
}
public HorizontalTextExtractionStrategy()
{
locationalResultField = typeof(LocationTextExtractionStrategy).GetField("locationalResult", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
textLineFinder = new TextLineFinder();
}
List<TextChunk> getLocationalResult()
{
return (List<TextChunk>) locationalResultField.GetValue(this);
}
System.Reflection.FieldInfo locationalResultField;
TextLineFinder textLineFinder;
}
提取文本
string extract(PdfReader reader, int pageNo)
{
return PdfTextExtractor.GetTextFromPage(reader, pageNo, new HorizontalTextExtractionStrategy());
}
更新：LocationTextExtractionStrategy 的变化
在iText 5.5.9 - 快照提交53526E4854FCB80C86CBC2E113F7A07401DC9A67（“Refactor LocationTextExtractionStrationStrategry ...”）到1AB350Beae148Be2a4bef5e663b3d67a004ff9f8（“制作textchunklocation a比较 class ...”）LocationTextExtractionStrategy architecture已更改为允许自定义这无需反思。
不幸的是，此更改破坏了上面介绍的 HorizontalTextExtractionStrategy。对于这些提交之后的 iText 版本，可以使用以下策略：
public class HorizontalTextExtractionStrategy2 extends LocationTextExtractionStrategy
{
public static class HorizontalTextChunkLocationStrategy implements TextChunkLocationStrategy
{
public HorizontalTextChunkLocationStrategy(TextLineFinder textLineFinder)
{
this.textLineFinder = textLineFinder;
}
@Override
public TextChunkLocation createLocation(TextRenderInfo renderInfo, LineSegment baseline)
{
return new HorizontalTextChunkLocation(baseline.getStartPoint(), baseline.getEndPoint(), renderInfo.getSingleSpaceWidth());
}
final TextLineFinder textLineFinder;
public class HorizontalTextChunkLocation implements TextChunkLocation
{
/** the starting location of the chunk */
private final Vector startLocation;
/** the ending location of the chunk */
private final Vector endLocation;
/** unit vector in the orientation of the chunk */
private final Vector orientationVector;
/** the orientation as a scalar for quick sorting */
private final int orientationMagnitude;
/** perpendicular distance to the orientation unit vector (i.e. the Y position in an unrotated coordinate system)
* we round to the nearest integer to handle the fuzziness of comparing floats */
private final int distPerpendicular;
/** distance of the start of the chunk parallel to the orientation unit vector (i.e. the X position in an unrotated coordinate system) */
private final float distParallelStart;
/** distance of the end of the chunk parallel to the orientation unit vector (i.e. the X position in an unrotated coordinate system) */
private final float distParallelEnd;
/** the width of a single space character in the font of the chunk */
private final float charSpaceWidth;
public HorizontalTextChunkLocation(Vector startLocation, Vector endLocation, float charSpaceWidth)
{
this.startLocation = startLocation;
this.endLocation = endLocation;
this.charSpaceWidth = charSpaceWidth;
Vector oVector = endLocation.subtract(startLocation);
if (oVector.length() == 0)
{
oVector = new Vector(1, 0, 0);
}
orientationVector = oVector.normalize();
orientationMagnitude = (int)(Math.atan2(orientationVector.get(Vector.I2), orientationVector.get(Vector.I1))*1000);
// see http://mathworld.wolfram.com/Point-LineDistance2-Dimensional.html
// the two vectors we are crossing are in the same plane, so the result will be purely
// in the z-axis (out of plane) direction, so we just take the I3 component of the result
Vector origin = new Vector(0,0,1);
distPerpendicular = (int)(startLocation.subtract(origin)).cross(orientationVector).get(Vector.I3);
distParallelStart = orientationVector.dot(startLocation);
distParallelEnd = orientationVector.dot(endLocation);
}
public int orientationMagnitude() { return orientationMagnitude; }
public int distPerpendicular() { return distPerpendicular; }
public float distParallelStart() { return distParallelStart; }
public float distParallelEnd() { return distParallelEnd; }
public Vector getStartLocation() { return startLocation; }
public Vector getEndLocation() { return endLocation; }
public float getCharSpaceWidth() { return charSpaceWidth; }
/**
* @param as the location to compare to
* @return true is this location is on the the same line as the other
*/
public boolean sameLine(TextChunkLocation as)
{
if (as instanceof HorizontalTextChunkLocation)
{
HorizontalTextChunkLocation horAs = (HorizontalTextChunkLocation) as;
return getLineNumber() == horAs.getLineNumber();
}
else
return orientationMagnitude() == as.orientationMagnitude() && distPerpendicular() == as.distPerpendicular();
}
/**
* Computes the distance between the end of 'other' and the beginning of this chunk
* in the direction of this chunk's orientation vector. Note that it's a bad idea
* to call this for chunks that aren't on the same line and orientation, but we don't
* explicitly check for that condition for performance reasons.
* @param other
* @return the number of spaces between the end of 'other' and the beginning of this chunk
*/
public float distanceFromEndOf(TextChunkLocation other)
{
float distance = distParallelStart() - other.distParallelEnd();
return distance;
}
public boolean isAtWordBoundary(TextChunkLocation previous)
{
/**
* Here we handle a very specific case which in PDF may look like:
* -.232 Tc [( P)-226.2(r)-231.8(e)-230.8(f)-238(a)-238.9(c)-228.9(e)]TJ
* The font's charSpace width is 0.232 and it's compensated with charSpacing of 0.232.
* And a resultant TextChunk.charSpaceWidth comes to TextChunk constructor as 0.
* In this case every chunk is considered as a word boundary and space is added.
* We should consider charSpaceWidth equal (or close) to zero as a no-space.
*/
if (getCharSpaceWidth() < 0.1f)
return false;
float dist = distanceFromEndOf(previous);
return dist < -getCharSpaceWidth() || dist > getCharSpaceWidth()/2.0f;
}
public int getLineNumber()
{
Vector startLocation = getStartLocation();
float y = startLocation.get(Vector.I2);
List<Float> flips = textLineFinder.verticalFlips;
if (flips == null || flips.isEmpty())
return 0;
if (y < flips.get(0))
return flips.size() / 2 + 1;
for (int i = 1; i < flips.size(); i+=2)
{
if (y < flips.get(i))
{
return (1 + flips.size() - i) / 2;
}
}
return 0;
}
@Override
public int compareTo(TextChunkLocation rhs)
{
if (rhs instanceof HorizontalTextChunkLocation)
{
HorizontalTextChunkLocation horRhs = (HorizontalTextChunkLocation) rhs;
int rslt = Integer.compare(getLineNumber(), horRhs.getLineNumber());
if (rslt != 0) return rslt;
return Float.compare(getStartLocation().get(Vector.I1), rhs.getStartLocation().get(Vector.I1));
}
else
{
int rslt;
rslt = Integer.compare(orientationMagnitude(), rhs.orientationMagnitude());
if (rslt != 0) return rslt;
rslt = Integer.compare(distPerpendicular(), rhs.distPerpendicular());
if (rslt != 0) return rslt;
return Float.compare(distParallelStart(), rhs.distParallelStart());
}
}
}
}
@Override
public void renderText(TextRenderInfo renderInfo)
{
textLineFinder.renderText(renderInfo);
super.renderText(renderInfo);
}
public HorizontalTextExtractionStrategy2() throws NoSuchFieldException, SecurityException
{
this(new TextLineFinder());
}
public HorizontalTextExtractionStrategy2(TextLineFinder textLineFinder) throws NoSuchFieldException, SecurityException
{
super(new HorizontalTextChunkLocationStrategy(textLineFinder));
this.textLineFinder = textLineFinder;
}
final TextLineFinder textLineFinder;
}
(HorizontalTextExtractionStrategy2.java)