Png Inflate

PNG images contain deflate-compressed image data. Small changes to the raw image data can result in in disproportionately large changes in the compressed data. Especially if different editors with different compression schemes handle the data. This makes delta compression of compressed data inefficient.

This program reads a PNG image and outputs that same image, but without compression. It can read from stdin and write to stdout and thus can act as a git clean filter. If installed as such, on a git add operation, the image is piped through the program, and the resulting uncompressed image is stored in the git file store. Git does compress files places in a git file store, so a loose file in the store should be a similar size either way. However, when the images are placed in a pack file - either during a gc operation or during file transfer - the smaller delta allows git to more efficiently create a pack file.

This does not otherwise modify the image. The image's bit depth, color type, filter method and the like will not be changed, so there will still be multiple ways to represent the same image.

It may be prudent to, if using this, to also use a git smudge filter that recompresses the png image and/or add a step to a relevant build script that compresses images with a tool such as pngout or pngcrush.

How to Install a Git Filter

These instructions assume that the binary is located at /opt/png_inflate, that git's core.attributesFile config is not set, and that you don't mind smashing an existing git-filter named png_inflate (you can check for existing filters using git config -l | grep filter)

To install globally

git config --global --replace-all filter.png_inflate.clean "/opt/png_inflate --assume-filename %f"
echo "*.png filter=png_inflate" >>${XDG_CONFIG_HOME-${HOME}/.config}/git/attributes

To install for a single repository

git config --local --replace-all filter.png_inflate.clean "/opt/png_inflate --assume-filename %f"
echo "*.png filter=png_inflate" >>.git/info/attributes

Go to a git directory and run git check-attr filter -- abc.png to check that the filter is installed properly; it should say png_inflate instead of unspecified.

Similar Projects

https://github.com/hoijui/ReZipDoc does the same uncompressed repack for zip archive files