Using SVG to Shrink Your PNGs

07 Sep 2014

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.

Saporro Beer

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.

Files

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

  1. 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.
  2. No IE8 support
  3. Masks don’t work in older versions of Android

Next up: Using Image Interpolation to Create Lightweight, Scalable Raster Images

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