I’ve recently been working on a website where we are creating a tool to customize a product. We have various renders from the designers with lots of transparency and then combine these together on the frontend to produce the customized render. As a result of needing transparency we can’t use the jpeg format so we need to use PNG format, however as this is lossless it means the image sizes tend to be very big. Fortunately the WebP format can compress transparent images including the transparency layer (but this is not set by default). Running the WebP converter with light compression over our PNG assets for this projects produced a set of WebP’s which were in total only 25% of the size of the PNG assets and still a high quality. This means much faster loading for the site, especially when displaying multiple renders of the customized product and its 5-10 layers per render.

However, WebP support is only available in about 70% of the browsers today. Rather than trying to test for it on the client side, it would be great to just keep the browser-side code the same but serve different assets depending on whether the browser supports it or not.

I found a good start for apache support for transparent loading of WebPs on github, however there were a few bugs in the script. Here is the final version that I used – you need to put it under a <VirtualHost> section.

AddType image/webp .webp
<ifmodule mod_rewrite.c>
      # Does browser support WebP? 
      RewriteCond %{HTTP_ACCEPT} \bimage/webp\b

      # Capture image name
      RewriteCond %{REQUEST_URI}  (.*)(\.(jpe?g|png|gif))$

      # if you don't have all jpg/png images available
      # as webp then you want to uncomment the next line
      # so apache first checks if there is a webp file
      # otherwise leave it disabled as it removes the
      # need to query the disk
      RewriteCond %{DOCUMENT_ROOT}%1.webp -f

      # Route to WebP image 
      RewriteRule .* %1.webp [L,T=image/webp]
</ifmodule>

And here is a script to convert all png, jpg or gif files under your image directories to WebP format in such a way that they will be automatically served by the code above.

#!/bin/bash
# Convert all images to WebP
IMAGE_PATHS="assets/ imgs/"
for SRC in $(find $IMAGE_PATHS -name "*.png" -o -name "*.jpg" -o -name "*.jpeg" -o -name "*.gif"); do
    WEBP="${SRC%.*}.webp"
    if [ "$SRC" -nt "$WEBP" ]; then
        echo "Converting to $WEBP"
        convert "$SRC" -define webp:alpha-compression=1 -define webp:auto-filter=true -define webp:alpha-quality=90 -quality 95 "$WEBP"
        
    fi
done

Note the -nt comparison that only updates files if the source has changed. You could add this script to git post-checkout and post-merge hooks to automatically keep your WebP assets in sync with the images in the code (and add a .gitignore entry for *.webp – no need to keep 2 copies of each resource in the repository).

Important note: If you’re using an older version of imagemagick such as on Ubuntu 14.04 (imagemagick 6.7.7), it doesn’t pass the alpha compression arguments through correctly so if you have a lot of transparency you won’t see much in the way of compression happening. Switch the convert line to be something like the below, however you need to remove the gif support as that requires using the gif2webp command to convert:

cwebp -quiet "$SRC" -metadata none -alpha_q 80 -q 90 -o "$WEBP"

Also note that this causes some issues when you have for example a jpg and png of the same base name whose contents are different (I found a few in the old code I inherited). You can find the base name of any of these clashes clashes using the following command:

find $IMAGE_PATHS -name "*.png" -o -name "*.jpg" -o -name "*.jpeg" -o -name "*.gif" | perl -pe 's,\.[^.]+$,\n,' | sort |uniq -d