DM: document -r

site/dev/contrib/testing.md

 Correctness Testing
 ===================
 
 Skia correctness testing is primarily served by a tool named DM.
 This is a quickstart to building and running DM.
 
 ~~~
 $ ./gyp_skia
 $ ninja -C out/Debug dm
 $ out/Debug/dm -v -w dm_output
@@ skipping 117 matching lines @@
 dm.json is used by our automated testing system, so you can ignore it if you
 like.  It contains a listing of each test run and a checksum of the image
 generated for that run.  (Boring technical detail: it is not a checksum of the
 .png file, but rather a checksum of the raw pixels used to create that .png.)
 
 Unit tests don't generally output anything but a status update when they pass.
 If a test fails, DM will print out its assertion failures, both at the time
 they happen and then again all together after everything is done running.
 These failures are also included in the dm.json file.
 
-DM does not currently have a built-in image comparison facility, though we
-might add one.  For now, you can use the skdiff tool to compare corresponding
-images from two runs of DM.
+DM has a simple facility to compare against the results of a previous run:
 ~~~
 $ ./gyp_skia
 $ ninja -C out/Debug dm
+$ out/Debug/dm -w good
+
+   (do some work)
+
+$ ./gyp_skia
+$ ninja -C out/Debug dm
+$ out/Debug/dm -r good -w bad
+~~~
+When using `-r`, DM will display a failure for any test that didn't produce the
+same image as the `good` run.
+
+For anything fancier, I suggest using skdiff:
+~~~
+$ ./gyp_skia
+$ ninja -C out/Debug dm
 $ out/Debug/dm -w good
 
    (do some work)
 
 $ ./gyp_skia
 $ ninja -C out/Debug dm
 $ out/Debug/dm -w bad
 
 $ ninja -C out/Debug skdiff
 $ mkdir diff
 $ out/Debug/skdiff good bad diff
 
   (open diff/index.html in your web browser)
 ~~~
 
 That's the basics of DM.  DM supports many other modes and flags.  Here are a
 few examples you might find handy.
 ~~~
 $ out/Debug/dm --help        # Print all flags, their defaults, and a brief explanation of each.
 $ out/Debug/dm --src tests   # Run only unit tests.
 $ out/Debug/dm --nocpu       # Test only GPU-backed work.
 $ out/Debug/dm --nogpu       # Test only CPU-backed work.
 $ out/Debug/dm --match blur  # Run only work with "blur" in its name.
 $ out/Debug/dm --dryRun      # Don't really do anything, just print out what we'd do.
 ~~~