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

image/heic                                       heic;
image/heif                                       heif;
image/heic-sequence                              heics;
image/heif-sequence                              heifs;
image/avif                                       avif;
image/avif-sequence                              avis;
image/jxl                                        jxl;

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

map $http_accept $img_ext
{
   ~image/jxl   '.jxl';
   ~image/avif  '.avif';
   ~image/webp  '.webp';
   default      '';
}

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)

location ~* ^.+\.(png|jpg|jpeg)$
{
   add_header Vary Accept;
   try_files $uri$img_ext $uri =404;
}

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)

/***************************************************\
* Enable SVG support and other modern image formats *
\***************************************************/
function cc_mime_types( $mimes )
{
   $mimes['svg'] = 'image/svg+xml';
   $mimes['webp'] = 'image/webp';
   $mimes['heic'] = 'image/heic';
   $mimes['heif'] = 'image/heif';
   $mimes['heics'] = 'image/heic-sequence';
   $mimes['heifs'] = 'image/heif-sequence';
   $mimes['avif'] = 'image/avif';
   $mimes['avis'] = 'image/avif-sequence';
   $mimes['jxl'] = 'image/jxl';
   return $mimes;
}
add_filter( 'upload_mimes', 'cc_mime_types' );

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)

dnf -y install --nogpgcheck https://download1.rpmfusion.org/free/el/rpmfusion-free-release-8.noarch.rpm https://download1.rpmfusion.org/nonfree/el/rpmfusion-nonfree-release-8.noarch.rpm
dnf -y install libheif

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

/*****************************\
* Convert png and jpg to avif *
\*****************************/
add_filter( 'wp_generate_attachment_metadata', 'jps_compress_img', 10, 2 );
function jps_compress_img( $metadata, $attachment_id )
{
   // get filepath from id
   $filepath= get_attached_file($attachment_id);

   // is the file a png or jpeg
   if(pathinfo($filepath, PATHINFO_EXTENSION)=="jpg" || pathinfo($filepath, PATHINFO_EXTENSION)=="jpeg" || pathinfo($filepath, PATHINFO_EXTENSION)=="png")
   {
      // If so, run compression of the main file to avif
      shell_exec("heif-enc $filepath -o $filepath.avif -q 50 -A");
      $parts = preg_split('~/(?=[^/]*$)~', $filepath);

      // Then run compression of the thumbnails to avif
      $thumbpaths = $metadata['sizes'];
      foreach ($thumbpaths as $key => $thumb)
      {
          $thumbpath= $thumb['file'];
          $thumbfullpath= $parts[0] . "/" . $thumbpath;
          shell_exec("heif-enc $thumbfullpath -o $thumbfullpath.avif -q 50 -A");
      }
   }
   // give back what we got
   return $metadata;
}

//dont forget to delete the avifs if the attachments are deleted
function jps_delete_avif($attachment_id)
{
   $all_images= get_intermediate_image_sizes($attachment_id);
   foreach($all_images as $each_img)
   {
      $each_img_det= wp_get_attachment_image_src($attach_id,$each_img);
      $each_img_path= ABSPATH.'wp-content'.substr($each_img_det[0],strpos($each_img_det[0],"/uploads/")).'.avif';
      shell_exec("rm -f $each_img_path");
   }
}
add_action( 'delete_attachment', 'jps_delete_avif' );

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

/*************************************************\
* Compress SVG images more with brotli and zopfli *
\*************************************************/
add_filter( 'wp_generate_attachment_metadata', 'jps_compress_vectors', 10, 2 );
function jps_compress_vectors( $metadata, $attachment_id )
{
    // get filepath from id
    $filepath= get_attached_file($attachment_id);

    // is the file an svg
    if(pathinfo($filepath, PATHINFO_EXTENSION)=="svg")
    {
       // If so, run compression of it using zopfli and brotli
       shell_exec("zopfli --gzip $filepath");
       shell_exec("brotli --best --output=$filepath.br $filepath");
    }

    // give back what we got
    return $metadata;
}

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!

dnf install x265 x265-devel svt-av1
export CXXFLAGS="$CXXFLAGS -fPIC"
cd ~

git clone https://aomedia.googlesource.com/aom
mkdir -p aom_build
cd aom_build
cmake ~/aom -DBUILD_SHARED_LIBS=true
make
sudo make install
cp ./libaom.so.3 /usr/bin/local/libaom.so.3

cd ~
export PKG_CONFIG_PATH=~/aom_build/
LD_LIBRARY_PATH=$LD_LIBRARY_PATH:~/aom_build/
export LD_LIBRARY_PATH

git clone --recurse-submodules --recursive https://github.com/strukturag/libheif.git
cd libheif
./autogen.sh
./configure
make
sudo make install

LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/bin/
export LD_LIBRARY_PATH

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

shell_exec("/usr/local/bin/heif-enc -A");

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

/*****************************\
* Convert png and jpg to avif *
\*****************************/
add_filter( 'wp_generate_attachment_metadata', 'jps_compress_img', 10, 2 );
function jps_compress_img( $metadata, $attachment_id )
{
   // get filepath from id
   $filepath= get_attached_file($attachment_id);

   // is the file a png or jpeg
   if(pathinfo($filepath, PATHINFO_EXTENSION)=="jpg" || pathinfo($filepath, PATHINFO_EXTENSION)=="jpeg" || pathinfo($filepath, PATHINFO_EXTENSION)=="png")
   {
      // If so, run compression of the main file to avif
      shell_exec("/usr/local/bin/heif-enc $filepath -o $filepath.avif -q 50 -A");
      $parts = preg_split('~/(?=[^/]*$)~', $filepath);

      // Then run compression of the thumbnails to avif
      $thumbpaths = $metadata['sizes'];
      foreach ($thumbpaths as $key => $thumb)
      {
          $thumbpath= $thumb['file'];
          $thumbfullpath= $parts[0] . "/" . $thumbpath;
          shell_exec("/usr/local/bin/heif-enc $thumbfullpath -o $thumbfullpath.avif -q 50 -A");
      }
   }
   // give back what we got
   return $metadata;
}