Add support for JBIG2 (generic coding)

Implements the proposal detailed at
#112 (comment)

This is a limited implementation of JBIG2, which can be extended to
support multiple pages, symbol tables, and other features of the format
in the future.

To test, I included a test fixture. You can also download 042.bmp (the same
one as @josch already downloaded in #112 (comment)
from https://git.ghostscript.com/?p=tests.git;a=blob_plain;f=jbig2/042.bmp;hb=HEAD
and run the following command:

  jbig2 042.bmp | img2pdf > 042.pdf

This results in a small PDF, just as @josch originally found in the
comment mentioned above.

This is my first contribution to this repository so let me know if
something else is needed. Thanks for a great library!

README.md

@@ -33,12 +33,14 @@ input file format and image color space.
 | JPEG2000                              | any                            | direct        |
 | PNG (non-interlaced, no transparency) | any                            | direct        |
 | TIFF (CCITT Group 4)                  | monochrome                     | direct        |
+| JBIG2 (single-page generic coding)    | bi-level                       | direct        |
 | any                                   | any except CMYK and monochrome | PNG Paeth     |
 | any                                   | monochrome                     | CCITT Group 4 |
 | any                                   | CMYK                           | flate         |
 
-For JPEG, JPEG2000, non-interlaced PNG and TIFF images with CCITT Group 4
-encoded data, img2pdf directly embeds the image data into the PDF without
+For JPEG, JPEG2000, non-interlaced PNG, TIFF images with CCITT Group 4
+encoded data, and JBIG2 with single-page generic coding (e.g. using `jbig2enc`),
+img2pdf directly embeds the image data into the PDF without
 re-encoding it. It thus treats the PDF format merely as a container format for
 the image data. In these cases, img2pdf only increases the filesize by the size
 of the PDF container (typically around 500 to 700 bytes). Since data is only

src/img2pdf.py

@@ -128,7 +128,7 @@ PageOrientation = Enum("PageOrientation", "portrait landscape")
 Colorspace = Enum("Colorspace", "RGB RGBA L LA 1 CMYK CMYK;I P PA other")
 
 ImageFormat = Enum(
-    "ImageFormat", "JPEG JPEG2000 CCITTGroup4 PNG GIF TIFF MPO MIFF other"
+    "ImageFormat", "JPEG JPEG2000 CCITTGroup4 PNG GIF TIFF MPO MIFF JBIG2 other"
 )
 
 PageMode = Enum("PageMode", "none outlines thumbs")
@@ -918,6 +918,11 @@ class pdfdoc(object):
             self.output_version = "1.5"  # jpeg2000 needs pdf 1.5
         elif imgformat is ImageFormat.CCITTGroup4:
             ofilter = [PdfName.CCITTFaxDecode]
+        elif imgformat is ImageFormat.JBIG2:
+            ofilter = PdfName.JBIG2Decode
+            # JBIG2Decode requires PDF 1.4
+            if self.output_version < "1.4":
+                self.output_version = "1.4"
         else:
             ofilter = PdfName.FlateDecode
 
@@ -1308,6 +1313,19 @@ def get_imgmetadata(
         if vdpi is None:
             vdpi = default_dpi
         ndpi = (hdpi, vdpi)
+    elif imgformat == ImageFormat.JBIG2:
+        imgwidthpx, imgheightpx, xres, yres = struct.unpack('>IIII', rawdata[24:40])
+        INCH_PER_METER = 39.370079
+        if xres == 0:
+            hdpi = default_dpi
+        else:
+            hdpi = int(float(xres) / INCH_PER_METER)
+        if yres == 0:
+            vdpi = default_dpi
+        else:
+            vdpi = int(float(yres) / INCH_PER_METER)
+        ndpi = (hdpi, vdpi)
+        ics = "1"
     else:
         imgwidthpx, imgheightpx = imgdata.size
 
@@ -1334,7 +1352,7 @@ def get_imgmetadata(
 
     # GIF and PNG files with transparency are supported
     if imgformat in [ImageFormat.PNG, ImageFormat.GIF, ImageFormat.JPEG2000] and (
-        ics in ["RGBA", "LA"] or "transparency" in imgdata.info
+        ics in ["RGBA", "LA"] or (imgdata is not None and "transparency" in imgdata.info)
     ):
         # Must check the IHDR chunk for the bit depth, because PIL would lossily
         # convert 16-bit RGBA/LA images to 8-bit.
@@ -1350,7 +1368,7 @@ def get_imgmetadata(
                 raise AlphaChannelError(
                     "Refusing to work with multiple >8bit channels."
                 )
-    elif ics in ["LA", "PA", "RGBA"] or "transparency" in imgdata.info:
+    elif ics in ["LA", "PA", "RGBA"] or (imgdata is not None and "transparency" in imgdata.info):
         raise AlphaChannelError("This function must not be called on images with alpha")
 
     # Since commit 07a96209597c5e8dfe785c757d7051ce67a980fb or release 4.1.0
@@ -1455,7 +1473,7 @@ def get_imgmetadata(
         logger.debug("input colorspace = %s", color.name)
 
     iccp = None
-    if "icc_profile" in imgdata.info:
+    if imgdata is not None and "icc_profile" in imgdata.info:
         iccp = imgdata.info.get("icc_profile")
     # GIMP saves bilevel TIFF images and palette PNG images with only black and
     # white in the palette with an RGB ICC profile which is useless
@@ -1805,8 +1823,6 @@ def parse_miff(data):
                 results.extend(parse_miff(rest[lenpal + lenimgdata :]))
     return results
 # fmt: on
-
-
 def read_images(
     rawdata, colorspace, first_frame_only=False, rot=None, include_thumbnails=False
 ):
@@ -1820,7 +1836,42 @@ def read_images(
         if rawdata[:12] == b"\x00\x00\x00\x0C\x6A\x50\x20\x20\x0D\x0A\x87\x0A":
             # image is jpeg2000
             imgformat = ImageFormat.JPEG2000
-        if rawdata[:14].lower() == b"id=imagemagick":
+        elif rawdata[:8] == b"\x97\x4a\x42\x32\x0d\x0a\x1a\x0a":
+            # For now we only support single-page generic coding of JBIG2, for example as generated by
+            # https://github.com/agl/jbig2enc
+            #
+            # In fact, you can pipe an example image like 042.bmp from https://git.ghostscript.com/?p=tests.git;a=blob_plain;f=jbig2/042.bmp;hb=HEAD
+            # directly into img2pdf:
+            #   jbig2 042.bmp | img2pdf > 042.pdf
+            #
+            # For this we assume that the first 13 bytes are the JBIG file header describing a document with one page,
+            # followed by a "page information" segment describing the dimensions of that page.
+            # 
+            # The following annotated `hexdump -C 042.jb2` shows the first 40 bytes that we inspect directly.
+            # The first 24 bytes (until "||") have to match exactly, while the following 16 bytes are read by get_imgmetadata.
+            #
+            # 97 4a 42 32 0d 0a 1a 0a  01 00 00 00 01 00 00 00
+            # \_____________________/  |  \_________/ \______
+            #       magic-bytes     org/unk  pages     seg-num
+            #
+            # 00 30 00 01 00 00 00 13  || 00 00 06 c0 00 00 09 23
+            # _/ |  |   | \_________/  || \_________/ \_________/
+            # type refs page seg-size  ||  width-px    height-px
+            #
+            # 00 00 00 00 00 00 00 00
+            # \_________/ \_________/
+            #     xres       yres
+            # 
+            # For more information on the data format, see:
+            # * https://github.com/agl/jbig2enc/blob/ea05019/fcd14492.pdf
+            # For more information about the generic coding, see:
+            # * https://github.com/agl/jbig2enc/blob/ea05019/src/jbig2enc.cc#L898
+            imgformat = ImageFormat.JBIG2
+            if rawdata[:24] != b"\x97\x4a\x42\x32\x0d\x0a\x1a\x0a\x01\x00\x00\x00\x01\x00\x00\x00\x00\x30\x00\x01\x00\x00\x00\x13":
+                raise ImageOpenError(
+                    "Unsupported JBIG2 format; only single-page generic coding is supported (e.g. from `jbig2enc`)"
+                )
+        elif rawdata[:14].lower() == b"id=imagemagick":
             # image is in MIFF format
             # this is useful for 16 bit CMYK because PNG cannot do CMYK and thus
             # we need PIL but PIL cannot do 16 bit
@@ -2066,6 +2117,28 @@ def read_images(
                     )
                 ]
 
+    if imgformat == ImageFormat.JBIG2:
+        color, ndpi, imgwidthpx, imgheightpx, rotation, iccp = get_imgmetadata(
+            imgdata, imgformat, default_dpi, colorspace, rawdata, rot
+        )
+        streamdata = rawdata[13:] # Strip file header
+        return [
+            (
+                color,
+                ndpi,
+                imgformat,
+                streamdata,
+                None,
+                imgwidthpx,
+                imgheightpx,
+                [],
+                False,
+                1,
+                rotation,
+                iccp,
+            )
+        ]
+
     if imgformat == ImageFormat.MIFF:
         return parse_miff(rawdata)
 

src/img2pdf_test.py

@@ -6861,7 +6861,7 @@ def test_layout(layout_test_cases):
 
 @pytest.fixture(
     scope="session",
-    params=os.listdir(os.path.join(os.path.dirname(__file__), "tests", "input")),
+    params=[filename for filename in os.listdir(os.path.join(os.path.dirname(__file__), "tests", "input")) if not filename.endswith(".jb2.bmp")],
 )
 def general_input(request):
     assert os.path.isfile(
@@ -6987,7 +6987,12 @@ def test_general(general_input, engine):
     assert x.Root.Type == "/Catalog"
     assert sorted(x.Root.Pages.keys()) == ["/Count", "/Kids", "/Type"]
     assert x.Root.Pages.Type == "/Pages"
-    orig_img = Image.open(f)
+    if f.endswith(".jb2"):
+        # PIL doens't support .jb2, so we load the original .bmp, which
+        # was converted to the .jb2 using `jbig2enc`.
+        orig_img = Image.open(f.replace(".jb2", ".jb2.bmp"))
+    else:
+        orig_img = Image.open(f)
     for pagenum in range(len(x.Root.Pages.Kids)):
         # retrieve the original image frame that this page was
         # generated from
@@ -6995,6 +7000,8 @@ def test_general(general_input, engine):
         cur_page = x.Root.Pages.Kids[pagenum]
 
         ndpi = orig_img.info.get("dpi", (96.0, 96.0))
+        if ndpi[0] <= 0.001 or ndpi[1] <= 0.001:
+            ndpi = (96.0, 96.0)
         # In python3, the returned dpi value for some tiff images will
         # not be an integer but a float. To make the behaviour of
         # img2pdf the same between python2 and python3, we convert that
@@ -7044,6 +7051,7 @@ def test_general(general_input, engine):
             "/JPXDecode",
             "/FlateDecode",
             pikepdf.Array([pikepdf.Name.CCITTFaxDecode]),
+            "/JBIG2Decode",
         ]
 
         # test if the image has correct size
@@ -7053,6 +7061,8 @@ def test_general(general_input, engine):
         # verbatim into the PDF
         if imgprops.Filter in ["/DCTDecode", "/JPXDecode"]:
             assert cur_page.Resources.XObject.Im0.read_raw_bytes() == orig_imgdata
+        elif imgprops.Filter == "/JBIG2Decode":
+            assert cur_page.Resources.XObject.Im0.read_raw_bytes() == orig_imgdata[13:] # Strip file header
         elif imgprops.Filter == pikepdf.Array([pikepdf.Name.CCITTFaxDecode]):
             tiff_header = tiff_header_for_ccitt(
                 int(imgprops.Width), int(imgprops.Height), int(imgprops.Length), 4