Allow fewer palette entries than the bit depth maximum

Tests/test_file_png.py

@@ -634,6 +634,16 @@ class TestFilePng:
         with Image.open(out) as reloaded:
             assert len(reloaded.png.im_palette[1]) == 48
 
+    def test_plte_length(self, tmp_path):
+        im = Image.new("P", (1, 1))
+        im.putpalette((1, 1, 1))
+
+        out = str(tmp_path / "temp.png")
+        im.save(str(tmp_path / "temp.png"))
+
+        with Image.open(out) as reloaded:
+            assert len(reloaded.png.im_palette[1]) == 3
+
     def test_exif(self):
         # With an EXIF chunk
         with Image.open("Tests/images/exif.png") as im:

src/PIL/PngImagePlugin.py

@@ -1186,23 +1186,21 @@ def _save(im, fp, filename, chunk=putchunk, save_all=False):
         # attempt to minimize storage requirements for palette images
         if "bits" in im.encoderinfo:
             # number of bits specified by user
-            colors = 1 << im.encoderinfo["bits"]
+            colors = min(1 << im.encoderinfo["bits"], 256)
         else:
             # check palette contents
             if im.palette:
-                colors = max(min(len(im.palette.getdata()[1]) // 3, 256), 2)
+                colors = max(min(len(im.palette.getdata()[1]) // 3, 256), 1)
             else:
                 colors = 256
 
-        if colors <= 2:
-            bits = 1
-        elif colors <= 4:
-            bits = 2
-        elif colors <= 16:
-            bits = 4
-        else:
-            bits = 8
-        if bits != 8:
+        if colors <= 16:
+            if colors <= 2:
+                bits = 1
+            elif colors <= 4:
+                bits = 2
+            else:
+                bits = 4
             mode = f"{mode};{bits}"
 
     # encoder options
@@ -1270,7 +1268,7 @@ def _save(im, fp, filename, chunk=putchunk, save_all=False):
                     chunk(fp, cid, data)
 
     if im.mode == "P":
-        palette_byte_number = (2 ** bits) * 3
+        palette_byte_number = colors * 3
         palette_bytes = im.im.getpalette("RGB")[:palette_byte_number]
         while len(palette_bytes) < palette_byte_number:
             palette_bytes += b"\0"
@@ -1281,7 +1279,7 @@ def _save(im, fp, filename, chunk=putchunk, save_all=False):
     if transparency or transparency == 0:
         if im.mode == "P":
             # limit to actual palette size
-            alpha_bytes = 2 ** bits
+            alpha_bytes = colors
             if isinstance(transparency, bytes):
                 chunk(fp, b"tRNS", transparency[:alpha_bytes])
             else:
@@ -1302,7 +1300,7 @@ def _save(im, fp, filename, chunk=putchunk, save_all=False):
     else:
         if im.mode == "P" and im.im.getpalettemode() == "RGBA":
             alpha = im.im.getpalette("RGBA", "A")
-            alpha_bytes = 2 ** bits
+            alpha_bytes = colors
             chunk(fp, b"tRNS", alpha[:alpha_bytes])
 
     dpi = im.encoderinfo.get("dpi")