diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 69f42cd9..d50f075b 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -73,12 +73,13 @@ For testing of **binary image outputs** we recommend using the `mae` metric (mea
 ```xml
 <expand macro="tests/binary_image_diff" name="output" value="output.tif" ftype="tiff"/>
 ```
+The macro also ensures that the image contains two distinct label values, which are not interchangable.
 
 For testing of non-binary **label map outputs** with interchangeable labels, we recommend using the `iou` metric (one minus the *intersection over the union*). With the default value of `eps` of 0.01, this asserts that there is no labeled image region with an *intersection over the union* of less than 99%:
 ```xml
 <expand macro="tests/label_image_diff" name="output" value="output.tif" ftype="tiff"/>
 ```
-At the moment it is not possible to *pin* specific labels, for example to verify that the background is assigned the correct label, but this will hopefully be added in the future.
+Label 0 is commonly connotated as the image background, and is not interchangable by default. Use `pin_labels=""` to make it interchangable.
 
 For testing of **intensity image outputs** we recommend the `rms` metric (root mean square), because it is very sensitive to large pixel value differences, but tolerates smaller differences:
 ```xml
diff --git a/macros/tests.xml b/macros/tests.xml
index e0853472..a09c7d4e 100644
--- a/macros/tests.xml
+++ b/macros/tests.xml
@@ -8,7 +8,7 @@
         token_metric="mae"
         token_eps="0.01">
 
-        <output name="@NAME@" value="@VALUE@" ftype="@FTYPE@" compare="image_diff" metric="@METRIC@" eps="@EPS@">
+        <output name="@NAME@" value="@VALUE@" ftype="@FTYPE@" compare="image_diff" metric="@METRIC@" eps="@EPS@" pin_labels="0">
             <assert_contents>
                 <has_image_n_labels n="2"/>
                 <yield/>
@@ -19,11 +19,12 @@
 
     <xml
         name="tests/label_image_diff"
-        tokens="name,value,ftype,metric,eps"
+        tokens="name,value,ftype,metric,eps,pin_labels"
         token_metric="iou"
-        token_eps="0.01">
+        token_eps="0.01"
+        token_pin_labels="0">
 
-        <output name="@NAME@" value="@VALUE@" ftype="@FTYPE@" compare="image_diff" metric="@METRIC@" eps="@EPS@">
+        <output name="@NAME@" value="@VALUE@" ftype="@FTYPE@" compare="image_diff" metric="@METRIC@" eps="@EPS@" pin_labels="@PIN_LABELS@">
             <assert_contents>
                 <yield/>
             </assert_contents>
@@ -53,7 +54,7 @@
         token_metric="mae"
         token_eps="0.01">
 
-        <element name="@NAME@" value="@VALUE@" ftype="@FTYPE@" compare="image_diff" metric="@METRIC@" eps="@EPS@">
+        <element name="@NAME@" value="@VALUE@" ftype="@FTYPE@" compare="image_diff" metric="@METRIC@" eps="@EPS@" pin_labels="0">
             <assert_contents>
                 <has_image_n_labels n="2"/>
                 <yield/>
@@ -66,9 +67,10 @@
         name="tests/label_image_diff/element"
         tokens="name,value,ftype,metric,eps"
         token_metric="iou"
-        token_eps="0.01">
+        token_eps="0.01"
+        token_pin_labels="0">
 
-        <element name="@NAME@" value="@VALUE@" ftype="@FTYPE@" compare="image_diff" metric="@METRIC@" eps="@EPS@">
+        <element name="@NAME@" value="@VALUE@" ftype="@FTYPE@" compare="image_diff" metric="@METRIC@" eps="@EPS@" pin_labels="@PIN_LABELS@">
             <assert_contents>
                 <yield/>
             </assert_contents>