FEATURE: add support for responsive images in posts

When creating lightboxes we will attempt to create 1.5x and 2x thumbnails
for retina screens, this can be controlled with a new hidden site setting
called responsice_post_image_sizes, if you wish to create 3x images run

SiteSetting.responsive_post_image_sizes = "1|1.5|2|3"


The default should be good for most of the setups as it balances filesize
with quality. 3x thumbs can get big.

config/site_settings.yml

@@ -914,6 +914,9 @@ files:
     default: 'gz'
     type: list
     list_type: compact
+  responsive_post_image_sizes:
+    default: "1|1.5|2"
+    hidden: true
   crawl_images:
     default: true
   max_image_width:

lib/cooked_post_processor.rb

@@ -231,6 +231,10 @@ def get_size_from_image_sizes(src, image_sizes)
     end
   end
 
+  def add_to_size_cache(url, w, h)
+    @size_cache[url] = [w, h]
+  end
+
   def get_size(url)
     return @size_cache[url] if @size_cache.has_key?(url)
 
@@ -285,6 +289,15 @@ def convert_to_link!(img)
 
     if upload = Upload.get_from_url(src)
       upload.create_thumbnail!(width, height, crop)
+
+      each_responsive_ratio do |ratio|
+        resized_w = (width * ratio).to_i
+        resized_h = (height * ratio).to_i
+
+        if upload.width && resized_w <= upload.width
+          upload.create_thumbnail!(resized_w, resized_h, crop)
+        end
+      end
     end
 
     add_lightbox!(img, original_width, original_height, upload)
@@ -299,6 +312,15 @@ def is_a_hyperlink?(img)
     false
   end
 
+  def each_responsive_ratio
+    SiteSetting
+      .responsive_post_image_sizes
+      .split('|')
+      .map(&:to_f)
+      .sort
+      .each { |r| yield r if r > 1 }
+  end
+
   def add_lightbox!(img, original_width, original_height, upload = nil)
     # first, create a div to hold our lightbox
     lightbox = create_node("div", "lightbox-wrapper")
@@ -320,13 +342,31 @@ def add_lightbox!(img, original_width, original_height, upload = nil)
 
     if upload
       thumbnail = upload.thumbnail(w, h)
-
-      img["src"] =
-        if thumbnail && thumbnail.filesize.to_i < upload.filesize
-          upload.thumbnail(w, h).url
-        else
-          upload.url
+      if thumbnail && thumbnail.filesize.to_i < upload.filesize
+        img["src"] = upload.thumbnail(w, h).url
+
+        srcset = +""
+
+        each_responsive_ratio do |ratio|
+          resized_w = (w * ratio).to_i
+          resized_h = (h * ratio).to_i
+
+          if upload.width && resized_w > upload.width
+            cooked_url = UrlHelper.cook_url(upload.url)
+            srcset << ", #{cooked_url} #{ratio}x"
+          else
+            if t = upload.thumbnail(resized_w, resized_h)
+              cooked_url = UrlHelper.cook_url(t.url)
+              srcset << ", #{cooked_url} #{ratio}x"
+            end
+          end
         end
+
+        img["srcset"] = "#{img["src"]}#{srcset}" if srcset.length > 0
+
+      else
+        img["src"] = upload.url
+      end
     end
 
     # then, some overlay informations

spec/components/cooked_post_processor_spec.rb

@@ -53,6 +53,51 @@
 
   context ".post_process_images" do
 
+    before do
+      SiteSetting.responsive_post_image_sizes = ""
+    end
+
+    context "responsive images" do
+      it "includes responsive images on demand" do
+
+        SiteSetting.responsive_post_image_sizes = "1|1.5|3"
+
+        upload = Fabricate(:upload, width: 2000, height: 1500, filesize: 10000)
+        post = Fabricate(:post, raw: "hello <img src='#{upload.url}'>")
+
+        # fake some optimized images
+        OptimizedImage.create!(
+          url: 'http://a.b.c/666x500.jpg',
+          width: 666,
+          height: 500,
+          upload_id: upload.id,
+          sha1: SecureRandom.hex,
+          extension: '.jpg',
+          filesize: 500
+        )
+
+        # fake 3x optimized image, we lose 2 pixels here over original due to rounding on downsize
+        OptimizedImage.create!(
+          url: 'http://a.b.c/1998x1500.jpg',
+          width: 1998,
+          height: 1500,
+          upload_id: upload.id,
+          sha1: SecureRandom.hex,
+          extension: '.jpg',
+          filesize: 800
+        )
+
+        cpp = CookedPostProcessor.new(post)
+
+        cpp.add_to_size_cache(upload.url, 2000, 1500)
+        cpp.post_process_images
+
+        # 1.5x is skipped cause we have a missing thumb
+        expect(cpp.html).to include('srcset="http://a.b.c/666x500.jpg, http://a.b.c/1998x1500.jpg 3.0x"')
+
+      end
+    end
+
     shared_examples "leave dimensions alone" do
       it "doesn't use them" do
         expect(cpp.html).to match(/src="http:\/\/foo.bar\/image.png" width="" height=""/)