Using SVG to Shrink Your PNGs

Wouldn’t it be great if you could get the compression of a JPEG with the transparency of a PNG? Well, you can, sort of. Here’s a little trick that I discovered while working on the new Sapporo Beer website.

Notice how the beer can on the Sapporo website has a transparent area around the edges (it’s hard to notice but there’s a video playing behind it). As a PNG, the beer can graphic would have been over 1.2 MB! So how did I get it down to 271KB?

First, I created two files. The first is a regular JPEG without any transparency. You can compress this one as much as you like. The second is an 8-bit PNG (alpha mask). This is just a grayscale image that represents the transparent areas of the beer can. Notice how the PNG is only 11KB; that’s because it contains so few colors and no transparency.

Next, I created a little snippet of inline SVG:

<svg viewBox= "0 0 560 1388" > <defs> <mask id= "canTopMask" > <image width= "560" height= "1388" xlink:href= "img/can-top-alpha.png" ></image> </mask> </defs> <image mask= "url(#canTopMask)" id= "canTop" width= "560" height= "1388" xlink:href= "can-top.jpg" ></image> </svg>

Here I am loading the two images inside an SVG container. The alpha channel image is included as a mask which is then applied to the JPEG to create the transparent region. The result? A transparent image with JPEG compression!

Caveats

To work in most browsers the SVG must be inline. You can’t move it into an external file and load it with an <img> tag. No IE8 support Masks don’t work in older versions of Android

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