/pngohsh

Tool to compute and write the PNG original hash (ohSH) chunk

Primary LanguageObjective-CThe UnlicenseUnlicense

pngohsh

Tool to compute and write the PNG original hash (ohSH) chunk.

PNG files are often ran through a crushers/optimizers such as pngcrush or optipng. The "brute force" settings of these tools can take several minutes for large PNGs. By using the ohSH chunk, we can skip unnecessary crushes.

Overview

Assume we have two files:

  1. An uncrushed PNG file, freshly exported from our graphics design program. We may have used an "Export All" command; thus, this file may be identical to our last export.

  2. A crushed PNG file on our web server.

Is it necessary to run the crusher/optimizer on File 1? Will we end up with File 2?

Methodology

  1. Compute the ohSH chunk of the new file:
pngohsh compute new.png
  1. Read the ohSH chunk of the old file:
pngohsh read old.png

  1. If equal, we are done. *tosses confetti*

  2. If not equal, run pngcrush, optipng, pngquant, etc. Store the result to new_crushed.png.

  3. Save the ohSH chunk from Step 1 to the crushed file:

pngohsh write new_crushed.png 3432ce91ecc28194f0f41ec4b696f8352d73df29
  1. Overwrite old.png with new_crushed.png:
cp new_crushed.png old.png

Specification

The ohSH chunk is build-system specific and should represent a hash of the visible image. Portability between build systems / OS versions is not a requirement.

This tool uses macOS's CoreGraphics and hashes the following components of the PNG file:

  1. Width
  2. Height
  3. Color depth
  4. bytesPerRow/CGBitmapInfo
  5. Color space
  6. Pre-multiplied image data

A SHA-256/160 hash is used (20 bytes). Combined with PNG chunk information (12 bytes), this adds 32 bytes to the resulting file.