Use viewbox if width and height are not present in svg. Fixes #76

lib/fastimage.rb

@@ -706,6 +706,10 @@ class FastImage
         [@width, @width / @ratio]
       elsif @height && @ratio
         [@height * @ratio, @height]
+      elsif @viewbox_width && @viewbox_height
+        [@viewbox_width, @viewbox_height]
+      else
+        nil
       end
     end
 
@@ -728,7 +732,11 @@ class FastImage
             return if @width
           elsif attr_name.join =~ /viewbox/i
             values = attr_value.split(/\s/)
-            @ratio = values[2].to_f / values[3].to_f
+            if values[2].to_f > 0 && values[3].to_f > 0 
+              @ratio = values[2].to_f / values[3].to_f
+              @viewbox_width = values[2].to_i
+              @viewbox_height = values[3].to_i
+            end
           end
         when /\w/
           attr_name << char

test/test.rb

@@ -34,7 +34,8 @@ GoodFixtures = {
   "webp_vp8.webp" => [:webp, [550, 368]],
   "test.svg" => [:svg, [200, 300]],
   "test_partial_viewport.svg" => [:svg, [860, 400]],
-  "test2.svg" => [:svg, [366, 271]]
+  "test2.svg" => [:svg, [366, 271]],
+  "test3.svg" => [:svg, [255, 48]]
 }
 
 BadFixtures = [