pillow conversion increases image size

Question

I am using the python Pillow library to do a simple image format conversion.

Here is some simple code demonstrating what I'm doing

im = Image.open("images/filename.tiff", mode="r")
im.save("images/filename.png", optimize=True)

This results in on average, a size increase of 98%. Here is the data on the input tiffs I'm trying to create:

When I explicitly convert it (by calling .convert(), the size only increases on average by 87%, but still terrible. Why is the size ballooning so much?

Here is what I have tried?

using the "quality" parameter on .save() seems to have little effect
explicitly passing the "mode" parameter to the save command has little effect
using the .convert("L") has little effect on overall size after conversion

EDIT

Here is a sample image that ran through what I described above and resulted in an 87% bigger png file, 907% bigger jpeg, and 907% bigger jpg.

https://share.getcloudapp.com/YEuR9LqP

Version of Pillow is latest: 8.2

Python version: 3.8

Running on MacOS: 10.15.7

Am I missing something obvious or this just my lack of knowledge of how the Image library works?

user_2836621 · Answer

I believe the issue is that your image is CCITT Group4 Compressed, which is particularly well suited to your image content and has no equivalent in PNG format.

exiftool image-1.tiff

Output

ExifTool Version Number         : 12.00
File Name                       : image-1.tiff
Directory                       : .
File Size                       : 95 kB
File Modification Date/Time     : 2021:05:17 11:46:57+01:00
File Access Date/Time           : 2021:05:17 11:57:15+01:00
File Inode Change Date/Time     : 2021:05:17 11:46:58+01:00
File Permissions                : rw-r--r--
File Type                       : TIFF
File Type Extension             : tif
MIME Type                       : image/tiff
Exif Byte Order                 : Little-endian (Intel, II)
Image Width                     : 2544
Image Height                    : 2230
Bits Per Sample                 : 1
Compression                     : T6/Group 4 Fax       <--- HERE IT IS
Photometric Interpretation      : WhiteIsZero
Strip Offsets                   : 180
Orientation                     : Horizontal (normal)
Rows Per Strip                  : 2230
Strip Byte Counts               : 96954
X Resolution                    : 300
Y Resolution                    : 300
Resolution Unit                 : inches
Image Size                      : 2544x2230
Megapixels                      : 5.7

On my Mac, I got an output PNG from PIL of size 138,200 bytes, even using pngcrush I could only get down to 124,810 bytes:

pngcrush -bruteforce result-opt.png crushed.png
  Recompressing IDAT chunks in result-opt.png to crushed.png
  Total length of data found in critical chunks            =    138200
  Best pngcrush method        =   4 (ws 15 fm 0 zl 9 zs 1) =    124816

pillow conversion increases image size

1 Answers