0Automatically generate and serve avif images with wordpress

I have spent quite a lot of time optim­ising the per­form­ance of this site, and as part of ongo­ing work I mon­it­or new tech­no­lo­gies that could help improve the speed for vis­it­ors. I’ve been track­ing the adop­tion of next-gen­er­a­tion image formats for a while and with sup­port by web browsers now fairly wide­spread it was time to fig­ure out how to make use of these new formats in wordpress.

There are mul­tiple steps required which I have broken down into head­ings below

Enable support in nginx

The first step is to enable your web serv­er (in my case nginx) to recog­nise the mime types of the new formats. To do this you need to edit mime.types which is likely to be found at /etc/nginx/mime.types. I added the fol­low­ing section

Automatically serve files where they are available

The next step is to tell nginx to serve files auto­mat­ic­ally whenev­er they exist (and to fall back to older formats where a new format file does­n’t exist)
To do this first edit the main nginx con­fig file (usu­ally /etc/nginx/nginx.conf) and add the fol­low­ing sec­tion inside the http{} sec­tion of the config

Note that I am only try­ing to serve jxl (JPEG-XL) or avif files, you could add more options (in order of pref­er­ence!) if you wish.
Next, you need to add the fol­low­ing under the serv­er{} sec­tion of the nginx con­fig (which may be in the main con­fig file or may be in a sep­ar­ate con­fig file depend­ing how you’ve set up your nginx con­fig structure)

Now nginx will look for image.jpg.jxl and then image.jpg.avif and then image.jpg.webp and finally image.jpg when it is asked for image.jpg by any browsers that sup­port the new­er formats. Next we need to enable them in wordpress

enable next-gen formats in wordpress

Add the fol­low­ing code to your functions.php (ideally do this in a child theme so that when you update your theme your changes don’t get over-written)

Note that I also enabled sup­port for SVG images at the same time

Now you can actu­ally upload next gen format images and use them dir­ectly in word­press if you want to, but I don’t recom­mend this as many older browsers don’t sup­port them yet — and we’ve already set up nginx to serve them intel­li­gently so we should make use of that. What we want to do is to auto­mat­ic­ally gen­er­ate the new formats when we upload images (there are plu­gins that do this for webp already, but noth­ing that does it for jxl or avif yet).

Install libheif

This step does require access to the com­mand line of your web host, which is straight-for­ward if you run a VPS, but may not be so simple if you’re on shared host­ing in which case you might need to ask your host for support
Run the fol­low­ing bash com­mands (these are selec­ted for Cen­tos 8, oth­er dis­tri­bu­tions may be a little different)

Add custom function to wordpress functions.php to compress all uploaded images (and their thumbnails) to avif format

As with the earli­er functions.php step I recom­mend adding this in a child theme

At present that is it — all images you upload will be con­ver­ted to avif cop­ies (with the ori­gin­als retained). You can regen­er­ate all of your images to cre­ate the avif files by using a plu­gin. Note — I haven’t yet set up JPEG-XL com­pres­sion as sup­port for it isn’t avail­able in main­stream browsers yet (although it is in prerelease browsers so it is com­ing very soon).

Whilst I’m cov­er­ing next gen formats and optim­isa­tion though I have one final tip — To com­press SVG images with gzip (or even bet­ter zop­fli and brotli). To do that requires anoth­er cus­tom function…

Bonus: SVG compression

Note you will need gzip_static and brotli_static enabled in your nginx config.

Self-compile for a newer version

I found that on Cen­tos the latest ver­sion of heif-enc is 1.7 which has quite a few bugs when cre­at­ing avifs. So I opted to com­pile my own new­er (1.12) build and use that instead. Doing so was a little com­plic­ated as it required the aom codec as a shared lib­rary. To do so run the fol­low­ing bash com­mands. Make sure to run them as a nor­mal user, not as the root user. Also note some of these com­mands may not be strictly needed, it took me a while to get it work­ing and I just made a note of what worked — I am by no means a linux expert!

Once you’ve done that, make sure it works by run­ning php -a and run­ning the fol­low­ing command

You should get a full out­put, not just a 1 line error. Assum­ing that works ok you can modi­fy your functions.php so that each of the shell-exec ref­er­ences points to /us­r/­loc­al/bin/heif-enc instead of just heif-enc

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