diff --git a/tests/test_ppm.nim b/tests/test_ppm.nim
index d84ec9a..d041c12 100644
--- a/tests/test_ppm.nim
+++ b/tests/test_ppm.nim
@@ -11,3 +11,8 @@ block:
     "tests/fileformats/ppm/feep.p3.hidepth.master.ppm"
   ))
   writeFile("tests/fileformats/ppm/feep.p3.hidepth.ppm", encodePpm(image))
+
+  # produced output should be identical to P6 master
+  let p6Master = readFile("tests/fileformats/ppm/feep.p6.master.ppm")
+  for image in @["p3", "p6", "p3.hidepth"]:
+    doAssert readFile("tests/fileformats/ppm/feep." & $image & ".ppm") == p6Master