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The world’s most efficient PNG file?

02 September 2015, Mike Williams

ZIPYou probably already know that PNG is a great lossless compression format, but have you ever wondered just how efficient it can be?

Developer David Fifield decided to find out, and the results are impressive: a 420-byte bzip2 file, which decompresses to a 5.8MB PNG, and holds a 225,000 x 225,000 pixel image.

This doesn’t have much to do with real-world image compression, of course, as the image is mostly zeroes, though “with a secret message in the center”.

Still, this kind of “unusual” file can be useful for testing. Fifield’s suggestions include “uploading as your profile picture to some online service, try to crash their image processing scripts”, or setting “as your web site’s favicon; try to crash browsers that don’t check the size”.

Corel PaintShop Pro X8

Corel PaintShop Pro X8 opens even the largest images without complaint

We just wondered, what would happen if we tried to open the file? Double-clicking it in Windows resulted in a locked-up system as Explorer tied up our hard drive, and became completely unresponsive. That’s perhaps no surprise for an image which requires 141GB RAM at 24-bit, but eventually we turned off the power and started again.

GIMP was a little more polite, also making heavy use of our hard drive, but leaving just about enough resources to do other things. It might have eventually opened the file, too, but after several hours of waiting we gave up and closed the process.

Paint.NET couldn’t display the file, either, but at least it didn’t waste our time, displaying a (non-fatal) error message almost immediately.

Corel PaintShop Pro X8 was our winner, recognising the image size, politely asking if we really wanted to open something that would use this much RAM, and loading it in a few minutes. We still couldn’t see the “hidden message” – probably because the maximum 5,000% zoom factor just wasn’t high enough – but it was still a good performance.

How will your favourite image editor perform? The test image is here, and you’ll need something like 7-Zip to decompress it, but be careful – previewing or even just right-clicking the image could lock up or crash your system. Don’t download it anywhere that isn’t fully backed up.

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