Add UPDATE_EXPECT envvar to update tests (#748)

1 files changed, 10 insertions, 9 deletions

diff --git a/tests/README.md b/tests/README.md
index 1da8a9c6..627c4caf 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -50,18 +50,19 @@ making changes.
 testit --pdf
 ```
 
-## Creating new tests
-To keep things small, please optimize reference images before committing them.
-When you use the approve button from the Test Helper (see the `tools` folder)
-this happens automatically if you have `oxipng` installed.
+## Update expected images
+If you created a new test or fixed a bug in an existing test, you need to update
+the reference image used for comparison. For this, you can use the
+`UPDATE_EXPECT` environment varariable or the `--update` flag:
 ```bash
-# One image
-oxipng -o max path/to/image.png
-
-# All images
-oxipng -r -o max tests/ref
+testit mytest --update
 ```
 
+If you use the VS Code test helper extension (see the `tools` folder), you can
+alternatively use the checkmark button to update the reference image. In that
+case you should also install `oxipng` on your system so that the test helper
+can optimize the reference images.
+
 ## Making an alias
 If you want to have a quicker way to run the tests, consider adding a shortcut
 to your shell profile so that you can simply write something like: