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_posts/2016-02-23-the-complete-guide-to-optimising-web-images.markdown
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@ -9,19 +9,19 @@ Optimising images for your website can be a bit of a black art, but it's well wo
#### Why?
These days many of us have access to very fast broadband (we have a 100Mb/s connection here at thoughtbot London) but not everybody has access to this sort of connection. Many people are still stuck on relatively slow ADSL packages that run at no more than 2Mb/s or so.
These days many of us have access to very fast broadband (we typically have 100Mb/s connections here at thoughtbot) but not everybody has access to this sort of connection. Most people worldwide are still stuck on relatively slow ADSL packages that run at no more than 2Mb/s or so.
Mobile connections have come a long way in recent years, but the variability of speed is even worse than wired connections. 4G can be as fast as good broadband, but in areas with high contention or in areas where there is no 3G/4G it can be as slow as mud.
Mobile connections have come a long way in recent years, but the variability of speed is even worse than wired connections. 4G can be as fast as good broadband, but in areas with high contention or in areas where there is no 3G or 4G mobile connections can be as slow as mud.
Whatever the speed of the connection it's worth optimising your images as [studies have shown](https://www.nngroup.com/articles/website-response-times/) that visitors are far more likely to abandon your site if it takes more than a few seconds to load. It's also worth remembering that even fast broadband connections often have data caps.
Whatever the speed of the connection it's worth optimising your images as [visitors are far more likely to abandon your site if it takes more than a few seconds to load](https://www.nngroup.com/articles/website-response-times/). It's also worth remembering that even fast broadband connections often have data caps.
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#### Size
The first thing to do is make sure the pixel dimensions of the image are appropriate. Browsers are much better than they used to be at resizing images as they are rendered, but it is a waste of bandwidth to be sending an image that is bigger than the size it ends up at.
The first thing to do is make sure the pixel dimensions of the image are appropriate. Browsers are much better than they used to be at resizing images as they are rendered, but it is a waste of bandwidth to be sending an image that is bigger than the size that is specified by the markup or CSS.
The context in which the image is displayed should inform the size as well. For example, a full width background image without much detail may work just as well if sized a bit smaller and this is an opportunity to save some bandwidth. Conversely a full size product shot should probably be sized such that it retains plenty of detail.
The context in which the image is displayed should inform the size as well. For example, a full width background image without much detail may work just as well if sized a bit smaller and this is an opportunity to save some bandwidth. Conversely a full-size product shot should probably be sized such that it retains plenty of detail.
Sometimes it is appropriate to send an image that is bigger than the size it will be rendered. For example, you might have a 2x version of a logo that is intended for HiDPI/Retina displays, and the file size may not be significantly large that it's worth rendering a 1x version as well and adding the complexity needed to deal with swapping the images.
@ -29,11 +29,11 @@ Sometimes it is appropriate to send an image that is bigger than the size it wil
So you've finished working on your image, and made sure it's the right size. What format do you choose to export it? Well, that depends on the kind of image.
There are number of choices when it comes to image formats that work on the web but the only two you really need to worry about right now are JPEG and PNG.
When it comes to web images there are a number of formats to choose from, but the only two you really need to worry about right now are JPEG and PNG.
The JPEG file image format was created by the Joint Photographic Experts Group and, as you might expect, was optimised for storing photographic images. It uses efficient lossy compression for storing photographic images, as the artefacts that result from the compression blend well into photographs with detailed non-uniform textures. These artefacts makes it a poor choice for storing graphic images and logos however. Because the compression is lossy it means that you will lose information in the image which cannot be recovered. This means that you should keep a master copy of your image and only export to JPEG when you are ready to publish it to the web.
The PNG format was designed as a replacement for the then patent encumbered GIF format. It is a lossless format, able to store images in a variety of colour depths, with either 1 bit or 8 bit alpha transparency. This makes it ideal for storing graphic images with strong lines and flat areas of colour. It is also very good at pristinely storing photographs, but because it's lossless, the resulting file sizes end up being unwieldy for web use so it's better to use JPEG.
The PNG format was designed as a replacement for the then patent-encumbered GIF format. It is a lossless format, able to store images in a variety of colour depths, with either 1 bit or 8 bit alpha transparency. This makes it ideal for storing graphic images with strong lines and flat areas of colour. It is also very good at pristinely storing photographs, but because it's lossless, the resulting file sizes end up being unwieldy for web use so it's better to use JPEG for photos.
#### Optimising JPEG
@ -61,9 +61,11 @@ Most image editing applications will only let you export a PNG file with an 8-bi
There are some useful tools to enable you to further compress PNG images:
**ImageOptim**: An OS X application that will run your images through various lossless compression tools. OptiPNG, PNGCrush, and Gifsicle are some of the tools that it will use to strip and crush your images down in size without losing quality. The savings depend on the image you are compressing but can be anything from 10% to 75% of the original size. ImageOptim will also work with JPG images but the savings are usually not that great.
**ImageOptim**: An OS X application that will run your images through various lossless compression tools. OptiPNG, PNGCrush, and Gifsicle are some of the tools that it will use to strip and crush your images down in size without losing quality. The savings depend on the image you are compressing but can be anything from 10% to 75% of the original size. ImageOptim will also work with JPEG images but the savings are usually not that great.
If you are running Windows you can try [FileOptimizer](http://nikkhokkho.sourceforge.net/static.php?page=FileOptimizer) as an alternative.
There is a [command line version of ImageOptim](https://github.com/JamieMason/ImageOptim-CLI) available which would be useful to incorporate these optimisations as part of a build process.
If you are running Windows you can try [FileOptimizer](http://nikkhokkho.sourceforge.net/static.php?page=FileOptimizer) as an alternative. On Linux you can use [Trimage](https://trimage.org). All of these GUI tools use a similar combination of command line tools to optimise images.
Happy optimising!