From ce4a020fb88ce33d24e99f0ad8afec50152d26fd Mon Sep 17 00:00:00 2001
From: aleju <kontakt@ajung.name>
Date: Fri, 6 Sep 2019 16:56:31 +0200
Subject: [PATCH] Change from fixed to random seed

---
 CHANGELOG.md     | 14 ++++++++++++--
 imgaug/random.py | 17 ++++++++---------
 2 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ee5a88114..5f9731ab5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -502,10 +502,10 @@ Changes:
 * [rarely breaking] Removed `imgaug.imgaug.CURRENT_RANDOM_STATE`.
   Use `imgaug.random.get_global_rng()` instead.
 * [rarely breaking] Removed `imgaug.imgaug.SEED_MIN_VALUE`.
-  Use  `imgaug.random.SEED_MIN_VALUE` instead or sample seeds via
+  Use `imgaug.random.SEED_MIN_VALUE` instead or sample seeds via
   `imgaug.random.generate_seeds_()`.
 * [rarely breaking] Removed `imgaug.imgaug.SEED_MAX_VALUE`.
-  Use  `imgaug.random.SEED_MAX_VALUE` instead or sample seeds via
+  Use `imgaug.random.SEED_MAX_VALUE` instead or sample seeds via
   `imgaug.random.generate_seeds_()`.
 * Optimized RNG handling throughout all augmenters to minimize the number of
   RNG copies. RNGs are now re-used as often as possible. This improves
@@ -517,6 +517,16 @@ Changes:
   change.
 * [breaking] The above listed changes will lead to different values being
   sampled for the same seeds (compared to past versions of the library).
+* [breaking] The seed for `imgaug`'s global random number generator is now
+  sampled from numpy's default random number generator. That means, that every
+  run of a program using `imgaug` will by default use a different seed and
+  hence result in different samples. Previously, a fixed seed was used,
+  resulting in the same samples for each run (unless the seed was manually
+  changed to a fixed one). It also means that seeding numpy will automatically
+  also seed imgaug (not guarantueed that this behaviour will be kept in
+  future releases). The change from fixed to random seed was done, because the
+  old (fixed) behaviour didn't match the common practice (and especially not
+  numpy's standard behaviour) and hence led to confusion. #408
 
 ## Fixes
  
diff --git a/imgaug/random.py b/imgaug/random.py
index 966ab9275..dbea98462 100644
--- a/imgaug/random.py
+++ b/imgaug/random.py
@@ -66,13 +66,6 @@
 SEED_MIN_VALUE = 0
 SEED_MAX_VALUE = 2**31-1
 
-# We pick a large random seed here, that makes collisions with seeds chosen
-# by the user unlikely to happen.
-# Note that the library might switch to random-seed instead of fixed-seed
-# in the future, so it is not guarantueed that this constant lives for very
-# long.
-SEED_GLOBAL_RNG = 148194721
-
 # TODO decrease pool_size in SeedSequence to 2 or 1?
 # TODO add 'with resetted_rng(...)'
 # TODO change random_state to rng or seed
@@ -735,8 +728,14 @@ def get_global_rng():
     """
     global GLOBAL_RNG
     if GLOBAL_RNG is None:
-        # TODO replace seed by constant
-        GLOBAL_RNG = RNG(convert_seed_to_generator(SEED_GLOBAL_RNG))
+        # This uses numpy's random state to sample a seed.
+        # Alternatively, `secrets.randbits(n_bits)` (3.6+) and
+        # `os.urandom(n_bytes)` could be used.
+        # See https://stackoverflow.com/a/27286733/3760780
+        # for an explanation how random.seed() picks a random seed value.
+        seed = generate_seed_(np.random)
+
+        GLOBAL_RNG = RNG(convert_seed_to_generator(seed))
     return GLOBAL_RNG