PUG

• pug, png with a twist: lossy image format.
• pug, PUG = color.JPG + alpha.PNG + footer
• pug file format, tools and related source code is all public domain.

Features

• PUG images are smaller than PNG, and perceptually similar to JPG.
• PUG manipulation does not require additional image libraries.
• Regular image viewers should display PUG files out of the box.

Cons

• PUG is a lossy file format. Reconstructed images from PUG files will not match original images.
• Regular image viewers will ignore PUG alpha channel until fully supported.

PUG file format specification

• RGBA32 input image is split into RGB24.jpg and A8.png images, then glued together with a footer.

RGBA32.pug = [RGB24.jpg] + [optional padding] + [A8.png] + [optional padding] + [footer]

- [?? bytes]  lossy color plane (JPG stream)
- [?? bytes]  lossy color padding (optional)
- [?? bytes]  lossless alpha plane (PNG stream)
- [?? bytes]  lossless alpha padding (optional)
- [ 4 bytes]  footer chunk - sizeof( color_plane + color_padding ) (little endian)
- [ 4 bytes]  footer chunk - sizeof( alpha_plane + alpha_padding ) (little endian)
- [ 4 bytes]  footer chunk - header plus version: literal "pug1"

Pug = lossy color + lossless alpha layers

Original image	Lossy color	Lossless alpha	Pug image
PNG Image, 56 KiB	JPG plane, Q=75%	PNG plane, 8-bit	PUG Image, 13 KiB
PNG Image, 213 KiB	JPG plane, Q=75%	PNG plane, 8-bit	PUG Image, 73 KiB
PNG Image, 40 KiB	JPG plane, Q=75%	PNG plane, 8-bit	PUG Image, 20 KiB

Required pixel algebra

Assuming pixel algebra is present and pixel is a vec4f, required pixel operations are:

// pug encoding: split planes: rgba into rgb+a
   rgb.at(w,h) = rgba.at(w,h) * pixel(255,255,255,0);
     a.at(w,h) = rgba.at(w,h) * pixel(0,0,0,255);

// pug decoding: join planes: rgb+a into rgba
   rgba.at(w,h) = rgb.at(w,h) * pixel(255,255,255,0) + a.at(w,h) * pixel(0,0,0,255);

Appendix: provided tools

• pugify.exe, which does PNG <=> PUG conversions (source code)

Building:

cl pugify.cc  -I deps deps\spot\spot*.c* /Ox /Oy /MT /DNDEBUG

pugify.exe panda.png panda.pug 75
pugify.exe panda.pug panda.rebuilt.png
start panda.rebuilt.png

Appendix: notes

• Pugify uses tiny image encoders that may lead to sub-optimal JPG/PNG file sizes. Therefore, Pugify may generate sub-optimal (but fully compliant) PUG files. If you plan to use Pugify in production scenarios consider integrating better image encoders before merging RGB24 and A8 planes both together.
• panda.png taken from http://tinypng.com website.
• dices.png taken from POV-Ray source code (CCASA30U license), source
• babytux.png by Fizyplankton (public domain), source
• Uncredited pug image logo (source? help!)