google
Image optimized further after running optimizer on original image
I ran image compression on original image of size 1.9 MB. The resultant image was of size 1.2 MB.
Further, I ran image compression on resultant image of size 1.2 KB and the next output image was of size 984 KB.
Seems like the complete lossless compression is not done in first run of compression on original image.
Interesting. It never occurred to me to re-run it on the same image.
Are you sure the compression is lossless? I'd be worried about gradual degradation being introduced by each subsequent pass.
I'm going to rerun guetzli on my test bank of images and see what kind of result I get. It's a large file set which was originally 849MB reduced to 586MB after one pass. I'll report back on how much I save a second time, though it may take a couple of weeks -- my VPS is running it 24/7 but isn't the most powerful.
There is generation loss in guetzli images. One should run guetzli only once. The same applies to most lossy compression.
https://en.wikipedia.org/wiki/Generation_loss
Following up on this, I ran a single test image through guetzli nine times, recompressing the output of the previous run each time. Every single run produced a smaller file:
original.jpg 2119K
first-pass.jpg 1708K
second-pass.jpg 1484K
third-pass.jpg 1309K
fourth-pass.jpg 1195K
fifth-pass.jpg 1115K
sixth-pass.jpg 1067K
seventh-pass.jpg 1041K
eight-pass.jpg 1028K
ninth-pass.jpg 1022K
The photo was a group shot taken with an iPhone SE. The compression is greatest for the first two runs, then dramatically tails off. There wasn't any noticeable quality degradation from one shot to the next that I could see.
Given that there is generation loss, I would agree with @jyrkialakuijala that it's best not to process an image more than once unless the size saving is more important than the image quality.
Given that there is generation loss, I would agree with @jyrkialakuijala that it's best not to process an image more than once unless the size saving is more important than the image quality.
Even then, don't process it multiple times: just use Guetzli at a lower quality. Using an appropriate quality and prcessing the image once gives a much better quality/size tradeoff.
just use Guetzli at a lower quality. Using an appropriate quality and prcessing the image once gives a much better quality/size tradeoff.
Good point. I haven't used that function of guetzli.
There are also lossless ways to get extra compression out of all JPEGs. Best of techniques are in ImageOptim (MacOS) & FileOptmiizer (Win).
I also found that sometimes Guetzli is unable to run on a file which is created by itself :). To be more specific: https://github.com/google/guetzli/issues/196
