Added support for arbitrary maxval

Tests/test_file_ppm.py

@@ -13,16 +13,64 @@
 
 def test_sanity():
     with Image.open(TEST_FILE) as im:
-        im.load()
         assert im.mode == "RGB"
         assert im.size == (128, 128)
-        assert im.format, "PPM"
+        assert im.format == "PPM"
         assert im.get_format_mimetype() == "image/x-portable-pixmap"
 
 
+def test_arbitrary_maxval():
+    # P5 L mode
+    fp = BytesIO(b"P5 3 1 4 \x00\x02\x04")
+    with Image.open(fp) as im:
+        assert im.size == (3, 1)
+        assert im.mode == "L"
+
+        px = im.load()
+        assert tuple(px[x, 0] for x in range(3)) == (0, 128, 255)
+
+    # P5 I mode
+    fp = BytesIO(b"P5 3 1 257 \x00\x00\x00\x80\x01\x01")
+    with Image.open(fp) as im:
+        assert im.size == (3, 1)
+        assert im.mode == "I"
+
+        px = im.load()
+        assert tuple(px[x, 0] for x in range(3)) == (0, 32640, 65535)
+
+    # P6 with maxval < 255
+    fp = BytesIO(b"P6 3 1 17 \x00\x01\x02\x08\x09\x0A\x0F\x10\x11")
+    with Image.open(fp) as im:
+        assert im.size == (3, 1)
+        assert im.mode == "RGB"
+
+        px = im.load()
+        assert tuple(px[x, 0] for x in range(3)) == (
+            (0, 15, 30),
+            (120, 135, 150),
+            (225, 240, 255),
+        )
+
+    # P6 with maxval > 255
+    # Scale down to 255, since there is no RGB mode with more than 8-bit
+    fp = BytesIO(
+        b"P6 3 1 257 \x00\x00\x00\x01\x00\x02"
+        b"\x00\x80\x00\x81\x00\x82\x01\x00\x01\x01\xFF\xFF"
+    )
+    with Image.open(fp) as im:
+        assert im.size == (3, 1)
+        assert im.mode == "RGB"
+
+        px = im.load()
+        assert tuple(px[x, 0] for x in range(3)) == (
+            (0, 1, 2),
+            (127, 128, 129),
+            (254, 255, 255),
+        )
+
+
 def test_16bit_pgm():
     with Image.open("Tests/images/16_bit_binary.pgm") as im:
-        im.load()
         assert im.mode == "I"
         assert im.size == (20, 100)
         assert im.get_format_mimetype() == "image/x-portable-graymap"
@@ -32,8 +80,6 @@ def test_16bit_pgm():
 
 def test_16bit_pgm_write(tmp_path):
     with Image.open("Tests/images/16_bit_binary.pgm") as im:
-        im.load()
-
         f = str(tmp_path / "temp.pgm")
         im.save(f, "PPM")
 
@@ -91,19 +137,8 @@ def test_token_too_long(tmp_path):
     assert str(e.value) == "Token too long in file header: b'01234567890'"
 
 
-def test_too_many_colors(tmp_path):
-    path = str(tmp_path / "temp.ppm")
-    with open(path, "wb") as f:
-        f.write(b"P6\n1 1\n1000\n")
-
-    with pytest.raises(ValueError) as e:
-        with Image.open(path):
-            pass
-
-    assert str(e.value) == "Too many colors for band: 1000"
-
-
 def test_truncated_file(tmp_path):
+    # Test EOF in header
     path = str(tmp_path / "temp.pgm")
     with open(path, "w") as f:
         f.write("P6")
@@ -114,6 +149,12 @@ def test_truncated_file(tmp_path):
 
     assert str(e.value) == "Reached EOF while reading header"
 
+    # Test EOF for PyDecoder
+    fp = BytesIO(b"P5 3 1 4")
+    with Image.open(fp) as im:
+        with pytest.raises(ValueError):
+            im.load()
+
 
 def test_neg_ppm():
     # Storage.c accepted negative values for xsize, ysize.  the

src/PIL/PpmImagePlugin.py

@@ -16,6 +16,9 @@
 
 
 from . import Image, ImageFile
+from ._binary import i16be as i16
+from ._binary import o8
+from ._binary import o32le as o32
 
 #
 # --------------------------------------------------------------------
@@ -102,6 +105,7 @@ def _open(self):
         else:
             self.mode = rawmode = mode
 
+        decoder_name = "raw"
         for ix in range(3):
             token = int(self._read_token())
             if ix == 0:  # token is the x size
@@ -112,18 +116,44 @@ def _open(self):
                     break
             elif ix == 2:  # token is maxval
                 maxval = token
-                if maxval > 255:
-                    if not mode == "L":
-                        raise ValueError(f"Too many colors for band: {token}")
-                    if maxval < 2**16:
-                        self.mode = "I"
-                        rawmode = "I;16B"
-                    else:
-                        self.mode = "I"
-                        rawmode = "I;32B"
+                if maxval > 255 and mode == "L":
+                    self.mode = "I"
+
+                # If maxval matches a bit depth,
+                # use the raw decoder as it is faster
+                if maxval == 65535 and mode == "L":
+                    rawmode = "I;16B"
+                elif maxval != 255:
+                    decoder_name = "ppm"
+        args = (rawmode, 0, 1) if decoder_name == "raw" else (rawmode, maxval)
 
         self._size = xsize, ysize
-        self.tile = [("raw", (0, 0, xsize, ysize), self.fp.tell(), (rawmode, 0, 1))]
+        self.tile = [(decoder_name, (0, 0, xsize, ysize), self.fp.tell(), args)]
+
+
+class PpmDecoder(ImageFile.PyDecoder):
+    _pulls_fd = True
+
+    def decode(self, buffer):
+        data = b""
+        maxval = min(self.args[-1], 65535)
+        in_byte_count = 1 if maxval < 256 else 2
+        out_byte_count = 4 if self.mode == "I" else 1
+        out_max = 65535 if self.mode == "I" else 255
+        bands = Image.getmodebands(self.mode)
+        while len(data) < self.state.xsize * self.state.ysize * bands * out_byte_count:
+            pixels = self.fd.read(in_byte_count * bands)
+            if len(pixels) < in_byte_count * bands:
+                # eof
+                break
+            for b in range(bands):
+                value = (
+                    pixels[b] if in_byte_count == 1 else i16(pixels, b * in_byte_count)
+                )
+                value = min(out_max, round(value / maxval * out_max))
+                data += o32(value) if self.mode == "I" else o8(value)
+        self.set_as_raw(data, (self.mode, 0, 1))
+        return -1, 0
 
 
 #
@@ -149,7 +179,7 @@ def _save(im, fp, filename):
     fp.write(head + ("\n%d %d\n" % im.size).encode("ascii"))
     if head == b"P6":
         fp.write(b"255\n")
-    if head == b"P5":
+    elif head == b"P5":
         if rawmode == "L":
             fp.write(b"255\n")
         elif rawmode == "I;16B":
@@ -169,6 +199,8 @@ def _save(im, fp, filename):
 Image.register_open(PpmImageFile.format, PpmImageFile, _accept)
 Image.register_save(PpmImageFile.format, _save)
 
+Image.register_decoder("ppm", PpmDecoder)
+
 Image.register_extensions(PpmImageFile.format, [".pbm", ".pgm", ".ppm", ".pnm"])
 
 Image.register_mime(PpmImageFile.format, "image/x-portable-anymap")