Use mono.jb2 for tests

This also uncovered a bug in jbig2enc where it uses the wrong unit
for resolution.

src/img2pdf.py

@@ -1318,10 +1318,16 @@ def get_imgmetadata(
         INCH_PER_METER = 39.370079
         if xres == 0:
             hdpi = default_dpi
+        elif xres < 1000:
+            # If xres is very small, it's likely accidentally expressed in dpi instead
+            # of dpm. See e.g. https://github.com/agl/jbig2enc/issues/86
+            hdpi = xres
         else:
             hdpi = int(float(xres) / INCH_PER_METER)
         if yres == 0:
             vdpi = default_dpi
+        elif yres < 1000:
+            vdpi = yres
         else:
             vdpi = int(float(yres) / INCH_PER_METER)
         ndpi = (hdpi, vdpi)
@@ -1840,9 +1846,8 @@ def read_images(
             # For now we only support single-page generic coding of JBIG2, for example as generated by
             # https://github.com/agl/jbig2enc
             #
-            # In fact, you can pipe an example image like 042.bmp from https://git.ghostscript.com/?p=tests.git;a=blob_plain;f=jbig2/042.bmp;hb=HEAD
-            # directly into img2pdf:
-            #   jbig2 042.bmp | img2pdf > 042.pdf
+            # In fact, you can pipe an example image `like src/tests/input/mono.png` directly into img2pdf:
+            #   jbig2 src/tests/input/mono.png | img2pdf -o src/tests/output/mono.png.pdf
             #
             # For this we assume that the first 13 bytes are the JBIG file header describing a document with one page,
             # followed by a "page information" segment describing the dimensions of that page.
@@ -1854,11 +1859,11 @@ def read_images(
             # \_____________________/  |  \_________/ \______
             #       magic-bytes     org/unk  pages     seg-num
             #
-            # 00 30 00 01 00 00 00 13  || 00 00 06 c0 00 00 09 23
+            # 00 30 00 01 00 00 00 13  || 00 00 00 73 00 00 00 30
             # _/ |  |   | \_________/  || \_________/ \_________/
             # type refs page seg-size  ||  width-px    height-px
             #
-            # 00 00 00 00 00 00 00 00
+            # 00 00 00 48 00 00 00 48
             # \_________/ \_________/
             #     xres       yres
             # 

src/img2pdf_test.py

@@ -6861,7 +6861,7 @@ def test_layout(layout_test_cases):
 
 @pytest.fixture(
     scope="session",
-    params=[filename for filename in os.listdir(os.path.join(os.path.dirname(__file__), "tests", "input")) if not filename.endswith(".jb2.bmp")],
+    params=os.listdir(os.path.join(os.path.dirname(__file__), "tests", "input")),
 )
 def general_input(request):
     assert os.path.isfile(
@@ -6988,9 +6988,9 @@ def test_general(general_input, engine):
     assert sorted(x.Root.Pages.keys()) == ["/Count", "/Kids", "/Type"]
     assert x.Root.Pages.Type == "/Pages"
     if f.endswith(".jb2"):
-        # PIL doens't support .jb2, so we load the original .bmp, which
+        # PIL doens't support .jb2, so we load the original .png, which
         # was converted to the .jb2 using `jbig2enc`.
-        orig_img = Image.open(f.replace(".jb2", ".jb2.bmp"))
+        orig_img = Image.open(f.replace(".jb2", ".png"))
     else:
         orig_img = Image.open(f)
     for pagenum in range(len(x.Root.Pages.Kids)):