pageinfo: learn to extract image information from Form XObjects
This commit is contained in:
James R. Barlow
+130
-37
@@ -89,13 +89,13 @@ def _is_unit_square(shorthand):
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pairwise = zip(values, UNIT_SQUARE)
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return all([isclose(a, b, rel_tol=1e-3) for a, b in pairwise])
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RasterSettings = namedtuple('RasterSettings',
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XobjectSettings = namedtuple('XobjectSettings',
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['name', 'shorthand', 'stack_depth'])
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InlineSettings = namedtuple('InlineSettings',
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['settings', 'shorthand', 'stack_depth'])
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ContentsInfo = namedtuple('ContentsInfo', ['raster_settings', 'inline_images'])
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ContentsInfo = namedtuple('ContentsInfo', ['xobject_settings', 'inline_images'])
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def _normalize_stack(operations):
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@@ -114,7 +114,7 @@ def _normalize_stack(operations):
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yield (operands, command)
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def _interpret_contents(contentstream):
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def _interpret_contents(contentstream, initial_shorthand=UNIT_SQUARE):
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"""Interpret the PDF content stream
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The stack represents the state of the PDF graphics stack. We are only
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@@ -139,8 +139,8 @@ def _interpret_contents(contentstream):
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operations = contentstream.operations
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stack = []
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ctm = _matrix_from_shorthand(UNIT_SQUARE)
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image_raster_settings = []
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ctm = _matrix_from_shorthand(initial_shorthand)
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xobject_settings = []
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inline_images = []
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for n, op in enumerate(_normalize_stack(operations)):
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@@ -161,10 +161,10 @@ def _interpret_contents(contentstream):
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_matrix_from_shorthand(operands), ctm)
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elif command == b'Do':
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image_name = operands[0]
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raster = RasterSettings(
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settings = XobjectSettings(
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name=image_name, shorthand=_shorthand_from_matrix(ctm),
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stack_depth=len(stack))
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image_raster_settings.append(raster)
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xobject_settings.append(settings)
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elif command == b'INLINE IMAGE':
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settings = operands['settings']
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inline = InlineSettings(
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@@ -173,7 +173,7 @@ def _interpret_contents(contentstream):
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inline_images.append(inline)
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return ContentsInfo(
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raster_settings=image_raster_settings,
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xobject_settings=xobject_settings,
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inline_images=inline_images)
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@@ -241,8 +241,8 @@ def _get_dpi(ctm_shorthand, image_size):
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return (dpi_w, dpi_h)
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def _find_page_inline_images(page, pageinfo, contentsinfo):
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"Find inline images on the page"
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def _find_inline_images(contentsinfo):
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"Find inline images in the contentstream"
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for n, inline in enumerate(contentsinfo.inline_images):
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image = {}
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@@ -272,20 +272,44 @@ def _find_page_inline_images(page, pageinfo, contentsinfo):
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yield image
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def _find_page_regular_images(page, pageinfo, contentsinfo):
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"Find images stored in XObject resources"
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def _image_xobjects(container):
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"""Search for all XObject-based images in the container
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try:
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page['/Resources']['/XObject']
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except KeyError:
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Usually the container is a page, but it could also be a Form XObject
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that contains images. Filter out the Form XObjects which are dealt with
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elsewhere.
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Generate a sequence of tuples (image, xobj container), where container,
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where xobj is the name of the object and image is the object itself,
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since the object does not know its own name.
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"""
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if '/Resources' not in container:
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return
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for xobj in page['/Resources']['/XObject']:
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# PyPDF2 returns the keys as an iterator
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pdfimage = page['/Resources']['/XObject'][xobj]
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if pdfimage['/Subtype'] != '/Image':
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continue
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resources = container['/Resources']
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if '/XObject' not in resources:
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return
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for xobj in resources['/XObject']:
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candidate = resources['/XObject'][xobj]
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if candidate['/Subtype'] == '/Image':
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image = candidate
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yield (image, xobj)
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def _find_regular_images(container, contentsinfo):
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"""Find images stored in the container's /Resources /XObject
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Usually the container is a page, but it could also be a Form XObject
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that contains images.
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Generates images with their DPI at time of drawing.
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"""
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for pdfimage, xobj in _image_xobjects(container):
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image = {}
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image['name'] = str(xobj)
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image['name'] = xobj
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image['width'] = pdfimage['/Width']
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image['height'] = pdfimage['/Height']
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if '/BitsPerComponent' in pdfimage:
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@@ -329,12 +353,12 @@ def _find_page_regular_images(page, pageinfo, contentsinfo):
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image['dpi_w'] = image['dpi_h'] = 0
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for raster in contentsinfo.raster_settings:
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for xobj in contentsinfo.xobject_settings:
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# Loop in case the same image is display multiple times on a page
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if raster.name != image['name']:
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if xobj.name != image['name']:
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continue
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if raster.stack_depth == 0 and _is_unit_square(raster.shorthand):
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if xobj.stack_depth == 0 and _is_unit_square(xobj.shorthand):
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# At least one PDF in the wild (and test suite) draws an image
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# when the graphics stack depth is 0, meaning that the image
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# gets drawn into a square of 1x1 PDF units (or 1/72",
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@@ -343,7 +367,7 @@ def _find_page_regular_images(page, pageinfo, contentsinfo):
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continue
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dpi_w, dpi_h = _get_dpi(
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raster.shorthand, (image['width'], image['height']))
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xobj.shorthand, (image['width'], image['height']))
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# When image is used multiple times take the highest DPI it is
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# rendered at
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@@ -358,9 +382,85 @@ def _find_page_regular_images(page, pageinfo, contentsinfo):
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yield image
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def _find_page_images(page, pageinfo, contentsinfo):
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yield from _find_page_inline_images(page, pageinfo, contentsinfo)
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yield from _find_page_regular_images(page, pageinfo, contentsinfo)
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def _find_form_xobject_images(pdf, container, contentsinfo):
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"""Find any images that are in Form XObjects in the container
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The container may be a page, or a parent Form XObject.
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"""
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if '/Resources' not in container:
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return
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resources = container['/Resources']
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if '/XObject' not in resources:
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return
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for xobj in resources['/XObject']:
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candidate = resources['/XObject'][xobj]
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if candidate['/Subtype'] != '/Form':
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continue
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form_xobject = candidate
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for settings in contentsinfo.xobject_settings:
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if settings.name != xobj:
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continue
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# Find images once for each time this Form XObject is drawn.
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# This could be optimized to cache the multiple drawing events
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# but in practice both Form XObjects and multiple drawing of the
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# same object are both very rare.
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ctm_shorthand = settings.shorthand
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yield from _find_images(pdf, form_xobject, ctm_shorthand)
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def _find_images(pdf, container, shorthand=None):
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"""Find all individual instances of images drawn in the container
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Usually the container is a page, but it may also be a Form XObject.
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On a typical page images are stored inline or as regular images
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in an XObject.
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Form XObjects may include inline images, XObject images,
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and recursively, other Form XObjects; and also vector drawing commands.
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Every instance of an image being drawn somewhere is flattened and
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treated as a unique image, since if the same image is drawn multiple times
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on one page it may be drawn at differing resolutions, and our objective
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is to find the resolution at which the page can be rastered without
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downsampling.
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"""
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if container.get('/Type') == '/Page':
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# For a /Page the content stream is attached to the page's /Contents
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page = container
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contentstream = pypdf.pdf.ContentStream(page.getContents(), pdf)
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initial_shorthand = shorthand or UNIT_SQUARE
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elif container.get('/Type') == '/XObject' and \
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container['/Subtype'] == '/Form':
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# For a Form XObject that content stream is attached to the XObject
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contentstream = pypdf.pdf.ContentStream(container, pdf)
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# Set the CTM to the state it was when the "Do" operator was
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# encountered that is drawing this instance of the Form XObject
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ctm = _matrix_from_shorthand(shorthand or UNIT_SQUARE)
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# A Form XObject may provide its own matrix to map form space into
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# user space. Get this if one exists
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form_matrix = _matrix_from_shorthand(
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container.get('/Matrix', UNIT_SQUARE))
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# Concatenate form matrix with CTM to ensure CTM is correct for
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# drawing this instance of the XObject
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ctm = matrix_mult(form_matrix, ctm)
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initial_shorthand = _shorthand_from_matrix(ctm)
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else:
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return
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contentsinfo = _interpret_contents(contentstream, initial_shorthand)
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yield from _find_inline_images(contentsinfo)
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yield from _find_regular_images(container, contentsinfo)
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yield from _find_form_xobject_images(pdf, container, contentsinfo)
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def _page_has_text(pdf, page):
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@@ -405,15 +505,8 @@ def _pdf_get_pageinfo(infile, pageno: int):
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except KeyError:
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pageinfo['rotate'] = 0
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try:
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contentstream = pypdf.pdf.ContentStream(page.getContents(), pdf)
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except AttributeError as e:
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return pageinfo
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contentsinfo = _interpret_contents(contentstream)
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pageinfo['images'] = [im for im in _find_page_images(
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page, pageinfo, contentsinfo)]
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pageinfo['images'] = [im for im in
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_find_images(pdf, page)]
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if pageinfo['images']:
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xres = max(image['dpi_w'] for image in pageinfo['images'])
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yres = max(image['dpi_h'] for image in pageinfo['images'])
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