OCRmyPDF/hocrTransform.py

from reportlab.pdfgen.canvas import Canvas
from reportlab.lib.units import inch
#from xml.etree.ElementTree import ElementTree
from lxml import etree as ElementTree
import Image, re, sys

class hocrTransform():
	"""
	A class for converting documents to/from the hOCR format.
	For details of the hOCR format, see:
	http://docs.google.com/View?docid=dfxcv4vc_67g844kf
	See also:
	http://code.google.com/p/hocr-tools/
	Basic usage:
	Create a PDF from an hOCR file and an image:
	hocr = hocrTransform("path/to/hOCR/file")
	hocr.to_pdf("path/to/image/file", "path/to/output/file")
	"""
	def __init__(self, hocrFileName = None, dpi=300):
		self.hocr = None
		self.dpi = dpi
		self.xmlns = ''
		self.boxPattern = re.compile('bbox((\s+\d+){4})')
		if hocrFileName is not None:
			self.parse_hocr(hocrFileName)
      		else:
			print "No hocr file provided. exiting"
			exit

	def __str__(self):
		"""
		Return the textual content of the HTML body
		"""
		if self.hocr is None:
			return ''
		body = self.hocr.find(".//%sbody"%(self.xmlns))
		if body:
			return self._get_element_text(body).encode('utf-8') # XML gives unicode
		else:
			return ''
  
	def _get_element_text(self, element):
		"""
		Return the textual content of the element and its children
		"""
		text = ''
		if element.text is not None:
			text = text + element.text
		for child in element.getchildren():
			text = text + self._get_element_text(child)
		if element.tail is not None:
			text = text + element.tail
		return text
    
	def element_coordinates(self, element):
		"""
		Returns a tuple containing the coordinates of the bounding box around
		an element
		"""
		out = (0,0,0,0)
		if 'title' in element.attrib:
			matches = self.boxPattern.search(element.attrib['title'])
			if matches:
				coords = matches.group(1).split()
				out = (int(coords[0]),int(coords[1]),int(coords[2]),int(coords[3]))
		return out
    
	def parse_hocr(self, hocrFileName):
		"""
		Reads an XML/XHTML file into an ElementTree object
		"""
		self.hocr = ElementTree.ElementTree()
		self.hocr.parse(hocrFileName)
 
		# if the hOCR file has a namespace, ElementTree requires its use to find elements
		matches = re.match('({.*})html', self.hocr.getroot().tag)
		if matches:
			self.xmlns = matches.group(1)
		else:
			self.xmlns = ''

	def pxl2pt(self, pxl):
		"""
		Returns the length in pt given length in pxl
		"""
		return float(pxl)/self.dpi*inch
			
	def to_pdf(self, imageFileName, outFileName, fontname="Courier"):
		"""
		Creates a PDF file with an image superimposed on top of the text.
		Text is positioned according to the bounding box of the lines in
		the hOCR file.
		The image need not be identical to the image used to create the hOCR file.
		It can have a lower resolution, different color mode, etc.
		"""
		im = Image.open(imageFileName)
		
		# get dimension of the OCRed image
		for div in self.hocr.findall(".//%sdiv[@class='ocr_page']"%(self.xmlns)):
			coords = self.element_coordinates(div)
			width = self.pxl2pt(coords[2]-coords[0])
			height = self.pxl2pt(coords[3]-coords[1])
			break # there shouldn't be more than one, and if there is, we don't want it

		if width is None:
			# no width and heigh definition in the ocr_image element of the hocr file
			# assuming page size is A4
			print "page width and height not available in %s. Assuming A4."%(imageFileName)
			width = 21*2.54*inch
			height = 29.6*2.54*inch

		# create the PDF file
		pdf = Canvas(outFileName, pagesize=(width, height), pageCompression=1) # page size in points (1/72 in.)

		# put the image on the page, scaled to fill the page
		#pdf.drawInlineImage(im, 0, 0, width=width, height=height)

		# check if element with class 'ocrx_word' are available
		# otherwise use 'ocr_line' as fallback
		elemclass="ocr_line"
		if self.hocr.find(".//%sspan[@class='ocrx_word']" %(self.xmlns)) is not None:
			elemclass="ocrx_word"

		# itterate all text elements
		for elem in self.hocr.findall(".//%sspan[@class='%s']" % (self.xmlns, elemclass)):

			elemtxt=self._get_element_text(elem).rstrip()
			if len(elemtxt) == 0:
				continue

			coords = self.element_coordinates(elem)
			x1=self.pxl2pt(coords[0])
			y1=self.pxl2pt(coords[1])
			x2=self.pxl2pt(coords[2])
			y2=self.pxl2pt(coords[3])
			
			# compute font size according to bbox coordinates
			fontsize=self.pxl2pt(coords[3]-coords[1])
			
			# draw the bbox border
			pdf.rect(x1, height-y2, x2-x1, y2-y1, fill=0)

			text = pdf.beginText()
			text.setFont(fontname, fontsize)
			#text.setTextRenderMode(3) # invisible

			# set cursor to bottom left corner of bbox (adjust for dpi)
			text.setTextOrigin(x1, height-y2)
       
			# scale the width of the text to fill the width of the bbox
			text.setHorizScale(100*(x2-x1)/pdf.stringWidth(elemtxt, fontname, fontsize))

			# write the text to the page
			text.textLine(elemtxt)
			pdf.drawText(text)

		# finish up the page and save it
		pdf.showPage()
		pdf.save()
  
if __name__ == "__main__":
	if len(sys.argv) < 4:
		print 'Usage: python hocrTransform.py inputHocrFile inputImageFile outputPdfFile'
		sys.exit(1)
	hocr = hocrTransform(sys.argv[1])
	hocr.to_pdf(sys.argv[2], sys.argv[3])