Meta/jbig2_to_pdf.py: Read jbig2 dimensions from file

Since we're parsing segment headers for random-access jbig2 inputs
already, just always do that and get the image dimensions from the
PageInformation segment data. Not all that much more code, and it
makes this script much more pleasant to use.

Meta/jbig2_to_pdf.py

@@ -4,8 +4,8 @@
 Creates a PDF that embeds a jbig2 image. Useful for viewing .jbig2 files in
 PDF viewers, since all PDF viewers support .jbig2 but few image viewers do.
 
-Usage is a bit clunky (use Build/lagom/bin/file to get the dimensions):
+Usage :
-% Meta/jbig2_to_pdf.py -o foo.pdf path/to/bitmap.jbig2 399 400
+% Meta/jbig2_to_pdf.py -o foo.pdf path/to/bitmap.jbig2
 % open foo.pdf
 """
 
@@ -15,6 +15,7 @@ import struct
 import textwrap
 
 
+PageInformation = 48
 EndOfFile = 51
 
 
@@ -28,6 +29,7 @@ class SegmentHeader:
     type: int
     bytes: bytes
     data_size: int
+    data: bytes
 
 
 def read_segment_header(data, offset):
@@ -59,45 +61,60 @@ def read_segment_header(data, offset):
     segment_header_size += 4
 
     bytes = data[offset:offset + segment_header_size]
-    return SegmentHeader(segment_header_size, type, bytes, data_size)
+    return SegmentHeader(segment_header_size, type, bytes, data_size, None)
 
 
-def random_access_to_sequential(data):
+def read_segment_headers(data, is_random_access):
     offset = 0
 
     segment_headers = []
-    while True:
+    while offset < len(data):
         segment_header = read_segment_header(data, offset)
+        offset += segment_header.segment_header_size
+
+        if not is_random_access:
+            segment_header.data = data[offset:offset + segment_header.data_size]
+            offset += segment_header.data_size
+
         segment_headers.append(segment_header)
 
-        offset += segment_header.segment_header_size
         if segment_header.type == EndOfFile:
             break
 
+    if is_random_access:
+        for segment_header in segment_headers:
+            segment_header.data = data[offset:offset + segment_header.data_size]
+            offset += segment_header.data_size
+
+    return segment_headers
+
+
+def random_access_to_sequential(segment_headers):
     out_data = bytes()
     for segment_header in segment_headers:
         out_data += segment_header.bytes
-        out_data += data[offset:offset + segment_header.data_size]
+        out_data += segment_header.data
-        offset += segment_header.data_size
-
     return out_data
 
 
+def get_dimensions(segment_headers):
+    for segment_header in segment_headers:
+        if segment_header.type != PageInformation:
+            continue
+        return struct.unpack_from('>II', segment_header.data)
+    raise Exception('did not find PageInformation')
+
+
 def main():
     parser = argparse.ArgumentParser(
         epilog=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter
     )
     parser.add_argument("image", help="Input image")
-    parser.add_argument("width", type=int, help="image width")
-    parser.add_argument("height", type=int, help="image height")
     parser.add_argument("-o", "--output", help="Path to output PDF")
     args = parser.parse_args()
 
-    width, height = args.width, args.height
-
     with open(args.image, 'rb') as f:
         image_data = f.read()
-        print(f'dims {width}x{height}')
 
     # strip jbig2 header
     image_data = image_data[8:]
@@ -106,8 +123,13 @@ def main():
         image_data = image_data[4:]
     image_data = image_data[1:]
 
+    segment_headers = read_segment_headers(image_data, is_random_access)
+
+    width, height = get_dimensions(segment_headers)
+    print(f'dims {width}x{height}')
+
     if is_random_access:
-        image_data = random_access_to_sequential(image_data)
+        image_data = random_access_to_sequential(segment_headers)
 
     start = dedent(b'''\
               %PDF-1.4