Run rendering tests with the rest of the tests

test/README.md

@@ -32,3 +32,26 @@ any time one of the source or spec files changes.
 Tip for TDD'ers: to make PhantomJS run the test suite continuously each time
 a spec file is changed you can use nosier (http://pypi.python.org/pypi/nosier)
 and do `nosier -p test -p src "make test"`.
+
+# Rendering tests
+
+The `test/rendering` directory contains rendering tests which compare a rendered map with a
+reference image using [resemble.js](http://huddle.github.io/Resemble.js/).
+
+To run the tests in the browser, make sure the development server is running
+(`make serve`) and open the URL
+[http://localhost:3000/test_rendering/index.html](http://localhost:3000/test_rendering/index.html).
+
+From the command-line the tests can be run with the build target `make test-rendering`.
+
+## Adding new tests
+When creating a new test case, a reference image has to be created. By appending `?generate`
+to the URL, a canvas with the rendered map will be shown on the page when calling
+`expectResemble`. Then the reference image can simply be created with a right-click
+and "Save image as".
+
+It is recommended to only run a single test case when generating the reference image.
+
+## Image difference
+When a test fails, an image showing the difference between the reference image and the
+rendered map can be displayed by appending `?showdiff` to the URL.