HACKING: how to bisect

Closes: #181

HACKING

@@ -27,6 +27,41 @@ Making a new release
 
  - Build and upload to pypi:
 
-    $ rm dist/*
+    $ rm -rf dist/*
     $ python3 setup.py sdist
-    $ twine upload --sign dist/*
+    $ twine upload dist/*
+
+Using debbisect to find regressions
+-----------------------------------
+
+    $ debbisect --cache=./cache  --depends="git,ca-certificates,python3,
+           ghostscript,imagemagick,mupdf-tools,poppler-utils,python3-pil,
+           python3-pytest,python3-numpy,python3-scipy,python3-pikepdf" \
+      --verbose  2023-09-16 2023-10-24 \
+      'chroot "$1" sh -c "
+           git clone https://gitlab.mister-muffin.de/josch/img2pdf.git
+           && cd img2pdf
+           && pytest 'src/img2pdf_test.py::test_jpg_2000_rgba8[internal]"'
+
+Using debbisect cache
+---------------------
+
+    $ mmdebstrap --variant=apt --aptopt='Acquire::Check-Valid-Until "false"' \
+          --include=git,ca-certificates,python3,ghostscript,imagemagick \
+          --include=mupdf-tools,poppler-utils,python3-pil,python3-pytest \
+          --include=python3-numpy,python3-scipy,python3-pikepdf \
+          --hook-dir=/usr/share/mmdebstrap/hooks/file-mirror-automount \
+          --setup-hook='mkdir -p "$1/home/josch/git/devscripts/cache/pool/"' \
+          --setup-hook='mount -o ro,bind /home/josch/git/devscripts/cache/pool/ "$1/home/josch/git/devscripts/cache/pool/"' \
+          --chrooted-customize-hook=bash
+          unstable /dev/null
+          file:///home/josch/git/devscripts/cache/archive/debian/20231022T090139Z/
+
+Bisecting imagemagick
+---------------------
+
+    $ git clean -fdx && git reset --hard
+    $ ./configure --prefix=$(pwd)/prefix
+    $ make -j$(nproc)
+    $ make install
+    $ LD_LIBRARY_PATH=$(pwd)/prefix/lib prefix/bin/compare ...

src/img2pdf.py

@@ -1311,7 +1311,19 @@ def get_imgmetadata(
     else:
         imgwidthpx, imgheightpx = imgdata.size
 
-        ndpi = imgdata.info.get("dpi", (default_dpi, default_dpi))
+        ndpi = imgdata.info.get("dpi")
+        if ndpi is None:
+            # the PNG plugin of PIL adds the undocumented "aspect" field instead of
+            # the "dpi" field if the PNG pHYs chunk unit is not set to meters
+            if imgformat == ImageFormat.PNG and imgdata.info.get("aspect") is not None:
+                aspect = imgdata.info["aspect"]
+                # make sure not to go below the default dpi
+                if aspect[0] > aspect[1]:
+                    ndpi = (default_dpi * aspect[0] / aspect[1], default_dpi)
+                else:
+                    ndpi = (default_dpi, default_dpi * aspect[1] / aspect[0])
+            else:
+                ndpi = (default_dpi, default_dpi)
         # In python3, the returned dpi value for some tiff images will
         # not be an integer but a float. To make the behaviour of
         # img2pdf the same between python2 and python3, we convert that