We just updated our Joomla! extension (version 0.0.2) and Magento extension (version 0.2.3) for WebP to add fix for PNG-transparency. While Google WebP offers an exciting way to decrease the required bandwidth of your site, its support for PNGs is somewhat limited: It does not support transparent PNGs, or PNGs with an alpha layer.
About Google WebP
From the Google labs, WebP is a project to add a new image-format to browsers, that replaces PNG and JPEG. Its main goal is to make your existing images smaller, and therefor decrease the bandwidth needed by your site. Because WebP takes off upto 30% of image-sizes, a site with a lot of images could surely benefit from using WebP.
There's only one downside: WebP-support is not spread widely yet. While browser-extensions exist for other browsers, the only browser that supports WebP out of the box is Google Chrome. But for those of you paying attention to browser-watches, Google Chrome is slowly beating Internet Explorer as to who is the most popular browser.
Another great point is personal use: We use Chrome as default browser, so any site we visit ourselves frequently and which has WebP installed, becomes fast. An example is our MageBridge demo-server which uses consistently less resources since we introduced WebP to its sites.
Detection of alpha-transparency
The extensions we offer for Joomla! and Magento now include a simple PHP-based check to see if a certain PNG-image is transparent or not. This is done by checking the first part of the PNG-image. If an image is a JPEG, WebP can be used. If an image is PNG but not transparent, WebP can be used. If an image is PNG but transparent, it will be skipped.
Unfortunately, we found out that WebP has - because of this - little to offer for our own Yireo site: The Yireo site uses a great-bunch of transparent-images - some of them combined in CSS Sprites, others loaded standalone - and because of this, we only gained a 3% weight loss because of WebP. But we're sure many other sites will grealy benefit from WebP.