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
