optimize: fix recoding of PNGs

Previously we opened pngquant-compressed PNGs with transcoding
because the transcode free function in Leptonica didn't seem to
work. This mean Leptonica may have thrown away the hard of
pngquant if didn't understand the encoding.

This change resolves the issue and allows us to open PNG encoded
data and insert it into a PDF without transcoding. Should improve
encoding quality.

src/ocrmypdf/optimize.py

@@ -271,7 +271,7 @@ def convert_to_jbig2(pike, jbig2_groups, root, log, options):
     must be lossy encoding since jbig2enc does not support refinement coding.
 
     When the JBIG2 symbolic coder is not used, each JBIG2 stands on its own
-    and needs no dictionary. Currently this is must be lossless JBIG2.
+    and needs no dictionary. Currently this must be lossless JBIG2.
     """
 
     _produce_jbig2_images(jbig2_groups, root, log, options)
@@ -319,7 +319,7 @@ def transcode_jpegs(pike, jpegs, root, log, options):
         im_obj.write(compdata.read(), filter=Name.DCTDecode)
 
 
-def transcode_pngs(pike, pngs, root, log, options):
+def transcode_pngs(pike, images, image_name_fn, root, log, options):
     if options.optimize >= 2:
         png_quality = (
             max(10, options.png_quality - 10),
@@ -328,38 +328,50 @@ def transcode_pngs(pike, pngs, root, log, options):
         with concurrent.futures.ThreadPoolExecutor(
             max_workers=options.jobs
         ) as executor:
-            for xref in pngs:
+            for xref in images:
+                log.info(image_name_fn(root, xref))
                 executor.submit(
                     pngquant.quantize,
-                    png_name(root, xref),
+                    image_name_fn(root, xref),
                     png_name(root, xref),
                     png_quality[0],
                     png_quality[1],
                 )
 
-    for xref in pngs:
+    for xref in images:
         im_obj = pike.get_object(xref, 0)
-
         # Open, transcode (!), package for PDF
         try:
-            pix = leptonica.Pix.open(png_name(root, xref))
-            if pix.depth == 1:
-                pix = pix.invert()  # PDF assumes 1 is black for monochrome
-            compdata = pix.generate_pdf_ci_data(leptonica.lept.L_FLATE_ENCODE, 0)
+            compdata = leptonica.CompressedData.open(png_name(root, xref))
         except leptonica.LeptonicaError as e:
             log.error(e)
             continue
 
-        # This is what we should be doing: open the compressed data without
-        # transcoding. However this shifts each pixel row by one for some
-        # reason.
-        # compdata = leptonica.CompressedData.open(png_name(root, xref))
+        # If re-coded image is larger don't use it
         if len(compdata) > int(im_obj.stream_dict.Length):
-            continue  # If we produced a larger image, don't use
+            log.debug(
+                f"pngquant: pngquant did not improve over original image "
+                f"{len(compdata)} > {int(im_obj.stream_dict.Length)}"
+            )
+            continue
 
-        predictor = None
-        if compdata.predictor > 0:
-            predictor = Dictionary(Predictor=compdata.predictor)
+        # We have to set the PDF predictor
+        # According to Leptonica source, PDF readers don't actually need us
+        # to specify the correct predictor, they just need a value of either
+        #   1 - there is no predictor
+        #   10-14 - there is a predictor
+        # Knowing that a predictor was used is the key information. From there
+        # the PNG decoder can infer the rest from the file.
+        # In practice the predictor should be Paeth, 14, so we'll use that.
+        # See:
+        #   - PDF RM 7.4.4.4 Table 10
+        #   - https://github.com/DanBloomberg/leptonica/blob/master/src/pdfio2.c#L757
+        predictor = 14 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
@@ -384,7 +396,7 @@ def transcode_pngs(pike, pngs, root, log, options):
             elif compdata.spp == 4:
                 cs = Name.DeviceCMYK
         im_obj.ColorSpace = cs
-        im_obj.write(compdata.read(), filter=Name.FlateDecode, decode_parms=predictor)
+        im_obj.write(compdata.read(), filter=Name.FlateDecode, decode_parms=dparms)
 
 
 def optimize(input_file, output_file, log, context):
@@ -408,7 +420,7 @@ def optimize(input_file, output_file, log, context):
 
     jpegs, pngs = extract_images_generic(pike, root, log, options)
     transcode_jpegs(pike, jpegs, root, log, options)
-    transcode_pngs(pike, pngs, root, log, options)
+    transcode_pngs(pike, pngs, png_name, root, log, options)
 
     jbig2_groups = extract_images_jbig2(pike, root, log, options)
     convert_to_jbig2(pike, jbig2_groups, root, log, options)