Jekyll: Get width/height of an image without using an external plugin - ruby

Question

I want to automatically add height and width attributes to all my images. It is perfectly done via this nice plugin, but I host my site on GitHub Pages where external plugins are not supported.
Question: How to prefill height/width attributes of an image without using a plugin?
Why do I need this?
My site works well even without height and width but I want to specify them because it is important from SEO point of view (you can find some details on its importance here).

Answer 1

Writing an internal filter to do this using the fastimage gem is fairly simple. You can define your filter in the _plugins directory of your project (this is outlined in the Jekyll documentation)
As a very rough example, you could do something like this:
require 'fastimage'
module ImageSizeFilter
def image_size(source, dimension = nil)
# Get the image dimensions, throw an error on failure
# NOTE: You may want to sanitize the input string provided here
# see: https://github.com/sdsykes/fastimage#security
size = FastImage.size(source, raise_on_failure: true)
# Return the requested dimension, or both dimensions if nothing was specified
return size[0] if dimension == 'w'
return size[1] if dimension == 'h'
return size unless dimension
# Fail if the requested dimension is invalid
raise 'Invalid image size dimension requested: ' + dimension
end
end
Liquid::Template.register_filter(ImageSizeFilter)
Which would be used like so:
<img src={{source}}
width="{{source | image_size: 'w'}}"
height="{{source | image_size: 'h'}}"/>
Of course this still won't work with GitHub pages directly because custom plugins are not supported. To use custom plugins in general, one solution is to build the site locally and include it in the repository.

Answer 2

This question seems to apply to content images in markdown files. These images have no width or height set by default.
The short answer
You can just set the width and height directly in HTML in your markdown, like this:
# Markdown title
paragraph
<img src="/path/to/image.jpg" width="400" height="300" />
The long answer
You cannot retreive the width and height of the image programmatically without a plugin, so when you use (pure) markdown you get an image without a width and height property. The question is WHY you wanted to add a width and a height in the first place. Setting the width and height prevents reflow, but it leaves a big gaping hole while loading. Is that truly better? It certainly does not look nice. Progressive JPG's are a very nice solution for this problem, but I do not prefer to set the width and height on them, as 'no image' looks good, and a progressive JPG also always looks good.
You say you want it for SEO reasons, but I cannot think of any.
If your website is so slow you actually want to interact with content below the image before the reflow, the logical solution is to make your website load faster.
However, if you have users with a really slow connection, you might want to manually add the image to the markdown in HTML. See the short answer for a code example.