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Module 2: Improving mobile site speed

2.3.3 Image optimization

Images often account for most of the downloaded bytes on a web page and also often occupy a significant amount of visual space. As a result, optimizing images can often yield some of the largest byte savings and performance improvements for your website: the fewer bytes the browser has to download, the less competition there is for the client's bandwidth and the faster the browser can download and render useful content on the screen.

Image optimization is both an art and science: an art because there is no one definitive answer for how best to compress an individual image, and a science because there are many well developed techniques and algorithms that can significantly reduce the size of an image. Finding the optimal settings for your image requires careful analysis along many dimensions: format capabilities, content of encoded data, quality, pixel dimensions, and more.

Some tips and techniques to keep in mind as you work on optimizing your images:

  • Prefer vector formats: vector images are resolution and scale independent, which makes them a perfect fit for the multi-device and high-resolution world.
  • Minify and compress SVG assets: XML markup produced by most drawing applications often contains unnecessary metadata which can be removed; ensure that your servers are configured to apply GZIP compression for SVG assets.
  • Pick best raster image format: determine your functional requirements and select the one that suits each particular asset.
  • Experiment with optimal quality settings for raster formats: don't be afraid to dial down the "quality" settings, the results are often very good and byte savings are significant.
  • Remove unnecessary image metadata: many raster images contain unnecessary metadata about the asset: geo information, camera information, and so on. Use appropriate tools to strip this data.
  • Serve scaled images: resize images on the server and ensure that the "display" size is as close as possible to the "natural" size of the image. Pay close to attention to large images in particular, as they account for largest overhead when resized!
  • Automate, automate, automate: invest into automated tools and infrastructure that will ensure that all of your image assets are always optimized.

Learn more here.

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