PNG: unnecessary padding in PLTE

[pdknsk]

Pillow incorrectly enforces a minimum of 2 palette entries.

from PIL import Image
img = Image.new(size = (1, 1), mode = 'P')
img.putpalette((1, 1, 1))
img.save('file.png')

$ pngcheck -v file.png 
...
    1 x 1 image, 1-bit palette, non-interlaced
  chunk PLTE at offset 0x00025, length 6: 2 palette entries
...

And pads the palette to bit ** 2 entries.

from PIL import Image
img = Image.new(size = (3, 3), mode = 'P')
img.putpalette((1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 3, 4, 5)) # 5
img.save('file.png')

$ pngcheck -v file.png 
...
    3 x 3 image, 4-bit palette, non-interlaced
  chunk PLTE at offset 0x00025, length 48: 16 palette entries
...

It's neither required nor recommended.

It is permissible to have fewer entries than the bit depth would allow.

[pdknsk]

diff --git a/PIL/PngImagePlugin.py b/PIL/PngImagePlugin.py
index 5e5eb14..9f883c9 100644
--- a/PIL/PngImagePlugin.py
+++ b/PIL/PngImagePlugin.py
@@ -671,7 +671,7 @@ def _save(im, fp, filename, chunk=putchunk, check=0):
         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
 
@@ -740,7 +740,7 @@ def _save(im, fp, filename, chunk=putchunk, check=0):
                 chunk(fp, cid, data)
 
     if im.mode == "P":
-        palette_byte_number = (2 ** bits) * 3
+        palette_byte_number = min(2 ** bits, colors) * 3
         palette_bytes = im.im.getpalette("RGB")[:palette_byte_number]
         while len(palette_bytes) < palette_byte_number:
             palette_bytes += b'\0'
@@ -752,7 +752,7 @@ def _save(im, fp, filename, chunk=putchunk, check=0):
     if transparency or transparency == 0:
         if im.mode == "P":
             # limit to actual palette size
-            alpha_bytes = 2**bits
+            alpha_bytes = min(2**bits, colors)
             if isinstance(transparency, bytes):
                 chunk(fp, b"tRNS", transparency[:alpha_bytes])
             else:
@@ -773,7 +773,7 @@ def _save(im, fp, filename, chunk=putchunk, check=0):
     else:
         if im.mode == "P" and im.im.getpalettemode() == "RGBA":
             alpha = im.im.getpalette("RGBA", "A")
-            alpha_bytes = 2**bits
+            alpha_bytes = min(2**bits, colors)
             chunk(fp, b"tRNS", alpha[:alpha_bytes])
 
     dpi = im.encoderinfo.get("dpi")

Works, but it's not tested extensively.

img = Image.frombytes('P', (1, 1), '\x00')
img.putpalette('\xFF\x00\x00') # R
img.save('file.png')
# chunk PLTE at offset 0x00025, length 3: 1 palette entry

img = Image.frombytes('P', (3, 3), '\x00' * 3 + '\x01' * 3 + '\x02' * 3)
img.putpalette('\xFF\x00\x00' '\x00\xFF\x00' '\x00\x00\xFF') # RGB
img.save('file.png')
# chunk PLTE at offset 0x00025, length 9: 3 palette entries

img.save('file.png', transparency = 0)
# chunk PLTE at offset 0x00025, length 9: 3 palette entries
# chunk tRNS at offset 0x0003a, length 1: 1 transparency entry

img.save('file.png', transparency = '\x00\x80\xFF')
# chunk PLTE at offset 0x00025, length 9: 3 palette entries
# chunk tRNS at offset 0x0003a, length 3: 3 transparency entries

[radarhere]

Just a note - the minimum of 2 was added in 6457eed

[radarhere]

I've created #5330 to resolve this.