4chan uses PHP to generate their thumbnails (and run their site).
PHP uses their own libgd (image processing library) which also uses their own libjpeg-8d.
Before 2017, thumbnails used the same jpeg quality: 60.
Since the start of 2017 and beyond, OP thumbnails use 50 and reply thumbnails use 40 as their jpeg image quality scale.
OP Thumbnails
- Max width:
250 - Max height:
250 - JPG quality:
50
Reply Thumbnails
- Max width:
125 - Max height:
125 - JPG quality:
40
To generate checksum-compliant thumbnails you can either use PHP v5.4.15 or later — or manually build libjpeg-8d and libgd from source — or download the pre-built binaries.
When dealing with transparent images, modify the script to reflect the right type of board where necessary.
$ php tg.php
$ b3sum op-thumb-orig.jpg thumb/ops.jpg
e039f64581ee448e8230146272621b4cc9891d1901f42359b66a76c0ee17916b op-thumb-orig.jpg
e039f64581ee448e8230146272621b4cc9891d1901f42359b66a76c0ee17916b thumb/ops.jpg
$ php tg.php
$ b3sum reply-thumb-orig.jpg thumb/replys.jpg
7715bcfa13d8bf5597daf7c51caf1dd4ebececce345829bc9700f1f321b4db93 reply-thumb-orig.jpg
7715bcfa13d8bf5597daf7c51caf1dd4ebececce345829bc9700f1f321b4db93 thumb/replys.jpg
$ php tg.php
$ b3sum alpha-thumb-orig.jpg thumb/alphas.jpg
59ddd640d38aa3f9e40ff81e35755877d2fa4b2f9371ac86572452e85f204701 alpha-thumb-orig.jpg
59ddd640d38aa3f9e40ff81e35755877d2fa4b2f9371ac86572452e85f204701 thumb/alphas.jpg
$ php tg.php
$ b3sum dance-thumb-orig.jpg thumb/dances.jpg
4270149933bcb5d5cac3bb5934c4422249b38e73c3347ee5f8377d8235cb0c3f dance-thumb-orig.jpg
4270149933bcb5d5cac3bb5934c4422249b38e73c3347ee5f8377d8235cb0c3f thumb/dances.jpgOn rare occasions, animated gifs currently produce different results.
$ ./tg
$ Generate thumbnails from 4chan
./tg <input-file> <output-file> [reply?1:0] [sfw-board?1:0]
$ ./tg op.jpg op-new.jpg 0
$ b3sum op-thumb-orig.jpg op-new.jpg
e039f64581ee448e8230146272621b4cc9891d1901f42359b66a76c0ee17916b op-thumb-orig.jpg
e039f64581ee448e8230146272621b4cc9891d1901f42359b66a76c0ee17916b op-new.jpg
$ ./tg reply.jpg reply-new.jpg
$ b3sum reply-thumb-orig.jpg reply-new.jpg
7715bcfa13d8bf5597daf7c51caf1dd4ebececce345829bc9700f1f321b4db93 reply-thumb-orig.jpg
7715bcfa13d8bf5597daf7c51caf1dd4ebececce345829bc9700f1f321b4db93 reply-new.jpg
$ ./tg alpha.png alpha-new.jpg 0
$ b3sum alpha-thumb-orig.jpg alpha-new.jpg
59ddd640d38aa3f9e40ff81e35755877d2fa4b2f9371ac86572452e85f204701 alpha-thumb-orig.jpg
59ddd640d38aa3f9e40ff81e35755877d2fa4b2f9371ac86572452e85f204701 alpha-new.jpg
$ ./tg dance.gif dance-new.jpg 1 0
$ b3sum dance-thumb-orig.jpg dance-new.jpg
4270149933bcb5d5cac3bb5934c4422249b38e73c3347ee5f8377d8235cb0c3f dance-thumb-orig.jpg
ac4988a8e7b724dbe2c31c54914d25e1cab641a9d899c26900bf8d4970f791b4 dance-new.jpgImagemagick:
(Note: Installing Imagemagick also installs libjpeg and could mess with your libjpeg-8d installation. In that scenario, use the --prefix= flag when building libjpeg-8d)
identify -verbose -features 1 -moments -unique 1583219690557s.jpg > thumb.txt
(Iɴsᴛʀᴜᴄᴛɪᴏɴs ᴄᴜʀʀᴇɴᴛʟʏ ᴏɴʟʏ ғᴏʀ Lɪɴᴜx)
Sources:
libjpeg-8d[1]libgd-2.0.35or earlierlibpng-1.2.50
$ cd libjpeg-8d
$ mkdir build
$ ./configure --prefix=$(pwd)/build
$ make
$ sudo make install # Install to prefix
$ export JPEG_INCLUDE_DIR=$(pwd)/build$ cd libpng-1.2.50
$ mkdir build
$ cp scripts/makefile.linux Makefile # Then edit the Makefile and change the prefix to $(pwd)/build
$ make
$ sudo make install # Install to prefix
$ export PNG_INCLUDE_DIR=$(pwd)/build$ cd libgd-2.0.35 # Then apply the patches from the repo
$ mkdir build
$ autoreconf -fi
$ ./configure --with-jpeg=$JPEG_INCLUDE_DIR --with-png=$PNG_INCLUDE_DIR --x-includes=$PNG_INCLUDE_DIR/include --x-libraries=$PNG_INCLUDE_DIR/lib --with-xpm=no --with-x=no --with-freetype=no --with-fontconfig=no --prefix=$(pwd)/build
$ make
$ sudo make install # Install to prefix$ gcc -o tg tg.c -lgd -lpng -lz -ljpeg -lm -static
# Or if you installed the libs locally
$ gcc -o tg tg.c -I/path/to/libgd-2.0.35/build/include -L/path/to/libgd-2.0.35/build/lib -lgd -I/path/to/libjpeg-8d/build/include -L/path/to/libjpeg-8d/build/lib -ljpeg -I/path/to/libpng-1.2.50/build/include -L/path/to/libpng-1.2.50/build/build/ -lpng -lz -lm -staticI looked at the Futaba's source code which 4chan is based from + the 4chan source code leak and used the same code they used to generate thumbnails. After finding out which PHP version (5.4.15) was used, I ran php --re gd | grep GD to get the libgd version (2.0.35) and php --ri gd to get the libjpeg (8d) & libpng (1.2.50) versions.
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