Google Photos high quality backup -- how does Google achieve great compression and am I losing some data?

Question

I'm sure that many of us have used Google photos high quality backup which allows us to store photos online without fees.

Recently, I thought of testing just how high quality the uploads really were by uploading some pics (below 16 MP) and comparing them to the original in terms of resolution, quality (as seen by the eye) and file size.

I was surprised to note that the resolution and quality were almost the same as the original (no visual difference to the eye even zoomed in in most cases) but the file size was drastically reduced -- at least 2-3x and even around 10x in some cases. Having used RAR for various types of files in the past, I knew that it wouldn't come close and didn't bother to test it.

The question is : How does Google manage such great compression without any noticeable change in the image itself? Are they dumping location data or other data related to the image that the amateur photographer doesn't know of?

Answer 1

Well, for one thing, all photos larger than 16 megapixels are resized to 16 MP — so, for many cameras today, that's an immediate, obvious drop in resolution.

But all other photos are compressed too, using an optimizing compression algorithm. I don't think Google comes out and says exactly what they do, but it's probably their open source Guetzli algorithm. Google claims:

Guetzli-generated images are typically 20-30% smaller than images of equivalent quality generated by libjpeg.

Whatever they're doing, the results are pretty impressive — often much greater than 20-30%. (It probably follows that Google does not consider these really "equivalent quality".) You can see some side-by-side comparisons in this Phone Arena blog post.

But, to your title question: you are definitely losing some data. This algorithm isn't magic — it simply uses a perceptual model to discard visual data that conventional JPEG compression does not, resulting in higher compression rates while (hopefully) not causing any more disturbance to things that you can actually notice.

Only you can decide if you're okay with that. If you're backing up RAW originals (with adjustments also backed up) and can recreate the JPEGs at will, I wouldn't think twice about it. But if you're serious about photography and have a JPEG-centered workflow — and despite online RAW snobbery, that's fine — you probably want to at least keep the original JPEG files unaltered. Adding a terabyte of storage is only $10/month with Google. Or if you are an Amazon Prime customer ($100/year) you get actual unlimited photo storage, including of RAW files.

Answer 2

They are basically throwing processing power at the problem, using very compute intensive approaches that tries out various ways to compress the image within the JPG rules. Processing is something google likely has plenty of, especially as it also saves bandwdith.

You can do the same - the software and various services are free or cheap. Your program / camera does simply not have the processing power to come up with an "optimal" way to use JPG lossy compression.