From 60c6ee92e59ac3c57dfcf1fc2775e9b3bfb485ee Mon Sep 17 00:00:00 2001
From: RunasSudo <runassudo@yingtongli.me>
Date: Thu, 8 May 2025 23:15:05 +1000
Subject: [PATCH] Implement PNG compression
---
pdf_segmented/__main__.py | 4 +--
pdf_segmented/compression/__init__.py | 6 ++++
pdf_segmented/output/pdf.py | 44 +++++++++++++++++++++++++--
3 files changed, 49 insertions(+), 5 deletions(-)
diff --git a/pdf_segmented/__main__.py b/pdf_segmented/__main__.py
index f6bda5b..0efabcd 100644
--- a/pdf_segmented/__main__.py
+++ b/pdf_segmented/__main__.py
@@ -28,8 +28,8 @@ parser.add_argument('input_file')
parser.add_argument('output_file')
parser.add_argument('--input-format', choices=['xcf'])
parser.add_argument('--output-format', choices=['pdf'])
-parser.add_argument('--fg-compression', default='jbig2', choices=['jbig2'])
-parser.add_argument('--bg-compression', default='jpeg', choices=['jpeg', 'jp2'])
+parser.add_argument('--fg-compression', default='jbig2', choices=['jbig2', 'png'])
+parser.add_argument('--bg-compression', default='jpeg', choices=['jpeg', 'jp2', 'png'])
parser.add_argument('--jp2-lossless', action='store_true')
parser.add_argument('--jp2-rate', type=float)
parser.add_argument('--jpeg-quality', type=float)
diff --git a/pdf_segmented/compression/__init__.py b/pdf_segmented/compression/__init__.py
index d3028a0..d28496c 100644
--- a/pdf_segmented/compression/__init__.py
+++ b/pdf_segmented/compression/__init__.py
@@ -24,6 +24,7 @@ class CompressedLayer:
from .jbig2 import jbig2_compress_layer
from .jp2 import jp2_compress_layer
from .jpeg import jpeg_compress_layer
+from .png import png_compress_layer
from ..segmentation import SegmentedPage
from PIL import Image
@@ -73,12 +74,14 @@ def compress_page(
fg=compress_layer(
layer=segmented_page.fg,
compression=fg_compression,
+ is_foreground=True,
options=options,
tempdir=tempdir
),
bg=compress_layer(
layer=segmented_page.bg,
compression=bg_compression,
+ is_foreground=False,
options=options,
tempdir=tempdir
)
@@ -87,6 +90,7 @@ def compress_page(
def compress_layer(
layer: Image,
compression: str,
+ is_foreground: bool,
options: CompressionOptions,
tempdir: str
) -> CompressedLayer:
@@ -98,5 +102,7 @@ def compress_layer(
return jp2_compress_layer(layer=layer, jp2_lossless=options.jp2_lossless, jp2_rate=options.jp2_rate)
elif compression == 'jpeg':
return jpeg_compress_layer(layer=layer, jpeg_quality=options.jpeg_quality)
+ elif compression == 'png':
+ return png_compress_layer(layer=layer, is_foreground=is_foreground)
else:
raise NotImplementedError()
diff --git a/pdf_segmented/output/pdf.py b/pdf_segmented/output/pdf.py
index 9df79c9..f098b39 100644
--- a/pdf_segmented/output/pdf.py
+++ b/pdf_segmented/output/pdf.py
@@ -18,9 +18,10 @@ from ..compression import CompressedLayer, CompressedPage
from ..compression.jbig2 import JBIG2Layer
from ..compression.jp2 import JP2Layer
from ..compression.jpeg import JPEGLayer
+from ..compression.png import PNGLayer
from ..input import InputPages
-from pikepdf import ContentStreamInstruction, Name, Operator, Page, Pdf, Stream, unparse_content_stream
+from pikepdf import ContentStreamInstruction, Dictionary, Name, Operator, Page, Pdf, Stream, unparse_content_stream
from typing import Generator
@@ -42,8 +43,8 @@ def pdf_write_pages(
# Write each layer to the page
content_instructions = []
- pdf_write_layer(input_pages=input_pages, pdf=pdf, page=page, layer=compressed_page.bg, content_instructions=content_instructions)
- pdf_write_layer(input_pages=input_pages, pdf=pdf, page=page, layer=compressed_page.fg, content_instructions=content_instructions)
+ pdf_write_layer(input_pages=input_pages, pdf=pdf, page=page, layer=compressed_page.bg, is_foreground=False, content_instructions=content_instructions)
+ pdf_write_layer(input_pages=input_pages, pdf=pdf, page=page, layer=compressed_page.fg, is_foreground=True, content_instructions=content_instructions)
# Generate content stream
wrapped_instructions = [
@@ -63,6 +64,7 @@ def pdf_write_layer(
pdf: Pdf,
page: Page,
layer: CompressedLayer,
+ is_foreground: bool,
content_instructions,
) -> None:
@@ -101,6 +103,42 @@ def pdf_write_layer(
Filter=Name.DCTDecode,
BitsPerComponent=8
)
+ elif isinstance(layer, PNGLayer):
+ if is_foreground:
+ # See PDF 1.7 section 7.4.4.3
+ # See also the implementation in img2pdf
+ pdf_write_image(
+ input_pages=input_pages,
+ pdf=pdf,
+ page=page,
+ value=layer.get_flate_data(),
+ content_instructions=content_instructions,
+ ColorSpace=Name.DeviceGray,
+ Filter=Name.FlateDecode,
+ BitsPerComponent=1,
+ Mask=[1, 1], # Layer mask
+ DecodeParms=Dictionary(
+ Predictor=15, # PNG prediction (on encoding, PNG optimum) - this is the only allowed value in a PNG file
+ BitsPerComponent=1, # Default is 8 so must set this here
+ Columns=input_pages.width
+ )
+ )
+ else:
+ pdf_write_image(
+ input_pages=input_pages,
+ pdf=pdf,
+ page=page,
+ value=layer.get_flate_data(),
+ content_instructions=content_instructions,
+ ColorSpace=Name.DeviceRGB,
+ Filter=Name.FlateDecode,
+ BitsPerComponent=8,
+ DecodeParms=Dictionary(
+ Predictor=15,
+ Colors=3, # Default is 1 so must set this here
+ Columns=input_pages.width
+ )
+ )
else:
raise NotImplementedError()