This PR creates a new class method in UrlBuilder that will generate a srcset string, which can be used in the srcset attribute on an <img> HTML element. create_srcset() takes a path and params argument similar to create_url(), and will return a String in one of two srcset formats.

Srcset Width-Pairs
The first format creates srcset width-pairs, a comma-delimited list of URLs and width descriptors corresponding to each one. This allows for responsive size switching based on the current width of the browser viewport. create_srcset() will append any params passed to the method to each URL constructed within the list. The following is an example of how to generate a width-pair srcset:

builder = imgix.UrlBuilder("demos.imgix.net", include_library_param=False)
srcset = builder.create_srcset("image.png")
print srcset

will return the following String (collapsed for brevity):

https://demos.imgix.net/image.png?w=100 100w,
https://demos.imgix.net/image.png?w=116 116w,
https://demos.imgix.net/image.png?w=134 134w,
                         ...
https://demos.imgix.net/image.png?w=7400 7400w,
https://demos.imgix.net/image.png?w=8192 8192w

Notice that a w= parameter is automatically inserted for each entry in the srcset list. This is required in order for the element to properly display the correctly-sized image corresponding to the viewport's width.

Device Pixel Ratio (DPR) Srcset
The second format this method can return is a srcset list of same-size images in varying resolutions. In this case, images are scaled using the dpr= parameter to adjust for the device pixel ratio of the browser. A DPR srcset will be automatically generated instead of a width-pair srcset if exact dimensions for the output image are specified, by providing either a w (width) or a h (height) and ar (aspect ratio) in the params argument. The following is an example of how to generate a DPR srcset:

builder = imgix.UrlBuilder("demos.imgix.net", include_library_param=False)
srcset = builder.create_srcset("image.png", {'h':800, 'ar':'3:2', 'fit':'crop'})
print srcset

which will return the following String:

https://demos.imgix.net/image.png?ar=3%3A2&fit=crop&h=800 1x,
https://demos.imgix.net/image.png?ar=3%3A2&fit=crop&h=800 2x,
https://demos.imgix.net/image.png?ar=3%3A2&fit=crop&h=800 3x,
https://demos.imgix.net/image.png?ar=3%3A2&fit=crop&h=800 4x,
https://demos.imgix.net/image.png?ar=3%3A2&fit=crop&h=800 5x

Signing URLs
create_srcset() also supports signing URLs, which can be very useful for generating server-side and then passing it to the client. This will avoid having to expose your imgix source's secure token to the client.

builder = imgix.UrlBuilder("demos.imgix.net", sign_key="my-token", include_library_param=False)
srcset = builder.create_srcset("image.png", {'w':800})
print srcset

generates a srcset with unique signatures for each URL:

https://demos.imgix.net/image.png?w=800&s=333a2140375016c2a6d2cf53e189ed90 1x,
https://demos.imgix.net/image.png?w=800&s=333a2140375016c2a6d2cf53e189ed90 2x,
https://demos.imgix.net/image.png?w=800&s=333a2140375016c2a6d2cf53e189ed90 3x,
https://demos.imgix.net/image.png?w=800&s=333a2140375016c2a6d2cf53e189ed90 4x,
https://demos.imgix.net/image.png?w=800&s=333a2140375016c2a6d2cf53e189ed90 5x

For more information on srcset, building responsive images, and resolution switching:

• Responsive images - MDN
• Srcset and Sizes - Eric Portis