def rewrite_png(pike: Pdf, im_obj: Object, compdata) -> None: # pragma: no cover # When a PNG is inserted into a PDF, we more or less copy the IDAT section from # the PDF and transfer the rest of the PNG headers to PDF image metadata. # One thing we have to do is tell the PDF reader whether a predictor was used # on the image before Flate encoding. (Typically one is.) # According to Leptonica source, PDF readers don't actually need us # to specify the correct predictor, they just need a value of either: # 1 - no predictor # 10-14 - there is a predictor # Leptonica's compdata->predictor only tells TRUE or FALSE # 10-14 means the actual predictor is specified in the data, so for any # number >= 10 the PDF reader will use whatever the PNG data specifies. # In practice Leptonica should use Paeth, 14, but 15 seems to be the # designated value for "optimal". So we will use 15. # See: # - PDF RM 7.4.4.4 Table 10 # - https://github.com/DanBloomberg/leptonica/blob/master/src/pdfio2.c#L757 predictor = 15 if compdata.predictor > 0 else 1 dparms = Dictionary(Predictor=predictor) if predictor > 1: dparms.BitsPerComponent = compdata.bps # Yes, this is redundant dparms.Colors = compdata.spp dparms.Columns = compdata.w im_obj.BitsPerComponent = compdata.bps im_obj.Width = compdata.w im_obj.Height = compdata.h log.debug( f"PNG {im_obj.objgen}: palette={compdata.ncolors} spp={compdata.spp} bps={compdata.bps}" ) if compdata.ncolors > 0: # .ncolors is the number of colors in the palette, not the number of # colors used in a true color image. The palette string is always # given as RGB tuples even when the image is grayscale; see # https://github.com/DanBloomberg/leptonica/blob/master/src/colormap.c#L2067 palette_pdf_string = compdata.get_palette_pdf_string() palette_data = pikepdf.Object.parse(palette_pdf_string) palette_stream = pikepdf.Stream(pike, bytes(palette_data)) palette = [ Name.Indexed, Name.DeviceRGB, compdata.ncolors - 1, palette_stream ] cs = palette else: # ncolors == 0 means we are using a colorspace without a palette if compdata.spp == 1: cs = Name.DeviceGray elif compdata.spp == 4: cs = Name.DeviceCMYK else: # spp == 3 cs = Name.DeviceRGB im_obj.ColorSpace = cs im_obj.write(compdata.read(), filter=Name.FlateDecode, decode_parms=dparms)
def rewrite_png_as_g4(pike: Pdf, im_obj: Object, compdata) -> None: # pragma: no cover im_obj.BitsPerComponent = 1 im_obj.Width = compdata.w im_obj.Height = compdata.h im_obj.write(compdata.read()) log.debug(f"PNG to G4 {im_obj.objgen}") if Name.Predictor in im_obj: del im_obj.Predictor if Name.DecodeParms in im_obj: del im_obj.DecodeParms im_obj.DecodeParms = Dictionary( K=-1, BlackIs1=bool(compdata.minisblack), Columns=compdata.w ) im_obj.Filter = Name.CCITTFaxDecode return