document experimental uri reader and add changelog entries, resolves #17

CHANGELOG.markdown

@@ -2,6 +2,8 @@
 
 ## unreleased
 
+* Improve experimental fetching of image meta from http server by reading only required amount of data when server does not support range header [@toy](https://github.com/toy)
+
 ## v3.0.2 (2022-05-19)
 
 * Fix handling empty files [#20](https://github.com/toy/image_size/issues/20) [@toy](https://github.com/toy)
@@ -15,6 +17,7 @@
 * Read only required chunks of data for files and seekable IOs [@toy](https://github.com/toy)
 * Raise `FormatError` whenever reading data returns less data than expected [#12](https://github.com/toy/image_size/issues/12) [@toy](https://github.com/toy)
 * Add `w`/`width` and `h`/`height` accessors to `Size` [@toy](https://github.com/toy)
+* Experimental efficient fetching of image meta from http server supporting range [@toy](https://github.com/toy)
 
 ## v2.1.2 (2021-08-21)
 

README.markdown

@@ -54,7 +54,7 @@ require 'image_size'
 image_size = ImageSize.new(ARGF)
 ```
 
-Works with `open-uri` if needed:
+Works with `open-uri`, see [experimental interface below](#experimental-fetch-image-meta-from-http-server):
 
 ```ruby
 require 'image_size'
@@ -90,6 +90,43 @@ File.open('spec/images/jpeg/436x429.jpeg', 'rb') do |fh|
 end
 ```
 
+### Experimental: fetch image meta from HTTP server
+
+If server recognises Range header, only needed chunks will be fetched even for TIFF images, otherwise required amount
+of data will be fetched, in most cases first few kilobytes (TIFF images is an exception).
+
+```ruby
+require 'image_size'
+require 'image_size/uri_reader'
+
+url = 'http://upload.wikimedia.org/wikipedia/commons/b/b4/Mardin_1350660_1350692_33_images.jpg'
+p ImageSize.url(url).size
+```
+
+This interface is as fast as dedicated gem fastimage for images with meta information in the header:
+
+```ruby
+irb> url = 'http://upload.wikimedia.org/wikipedia/commons/b/b4/Mardin_1350660_1350692_33_images.jpg'
+irb> puts Benchmark.measure{ p FastImage.size(url) }
+[9545, 6623]
+  0.004176   0.001974   0.006150 (  0.282889)
+irb> puts Benchmark.measure{ p ImageSize.url(url).size }
+[9545, 6623]
+  0.005604   0.001406   0.007010 (  0.238629)
+```
+
+And considerably faster for images with meta information at the end of file:
+
+```ruby
+irb> url = "https://upload.wikimedia.org/wikipedia/commons/c/c7/Curiosity%27s_Vehicle_System_Test_Bed_%28VSTB%29_Rover_%28PIA15876%29.tif"
+irb> puts Benchmark.measure{ p FastImage.size(url) }
+[7360, 4912]
+  0.331284   0.247295   0.578579 (  6.027051)
+irb> puts Benchmark.measure{ p ImageSize.url(url).size }
+[7360, 4912]
+  0.006247   0.001045   0.007292 (  0.197631)
+```
+
 ## Licence
 
 This code is free to use under the terms of the [Ruby's licence](LICENSE.txt).