Scale maxval > 255 for P6 to 255

Tests/test_file_ppm.py

@@ -29,14 +29,34 @@ def test_arbitrary_maxval():
         px = im.load()
         assert tuple(px[x, 0] for x in range(3)) == (0, 128, 255)
 
-    # P6
+    # 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))
+        assert tuple(px[x, 0] for x in range(3)) == (
+            (0, 15, 30),
+            (120, 135, 150),
+            (225, 240, 255),
+        )
+
+    # P6 with maxval > 255
+    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():
@@ -107,18 +127,6 @@ 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")

src/PIL/PpmImagePlugin.py

@@ -16,7 +16,7 @@
 
 
 from . import Image, ImageFile
-
+from ._binary import i16be as i16
 from ._binary import o8
 
 #
@@ -115,12 +115,10 @@ def _open(self):
                     break
             elif ix == 2:  # token is maxval
                 maxval = token
-                if maxval > 255:
-                    if mode != "L":
-                        raise ValueError(f"Too many colors for band: {token}")
+                if maxval > 255 and mode == "L":
                     self.mode = "I"
                     rawmode = "I;16B" if maxval < 2**16 else "I;32B"
-                elif maxval < 255:
+                elif maxval != 255:
                     decoder_name = "ppm"
         args = (rawmode, 0, 1) if decoder_name == "raw" else (rawmode, maxval)
 
@@ -133,15 +131,19 @@ class PpmDecoder(ImageFile.PyDecoder):
 
     def decode(self, buffer):
         data = b""
-        maxval = self.args[-1]
+        maxval = min(self.args[-1], 65535)
+        in_byte_count = 1 if maxval < 256 else 2
         bands = Image.getmodebands(self.mode)
         while len(data) < self.state.xsize * self.state.ysize * bands:
-            pixels = self.fd.read(bands)
-            if len(pixels) < bands:
+            pixels = self.fd.read(in_byte_count * bands)
+            if len(pixels) < in_byte_count * bands:
                 # eof
                 break
-            for pixel in pixels:
-                value = min(255, round(pixel / maxval * 255))
+            for b in range(bands):
+                value = (
+                    pixels[b] if in_byte_count == 1 else i16(pixels, b * in_byte_count)
+                )
+                value = min(255, round(value / maxval * 255))
                 data += o8(value)
         self.set_as_raw(data, (self.mode, 0, 1))
         return -1, 0