Adds interpolation gradient testing

.github/workflows/main.yml

@@ -52,7 +52,7 @@ jobs:
             galsim \
             ngmix
 
-          python -m pip install tensorflow==2.5.0 tensorflow_addons==0.13.0 tensorflow_probability==0.13.0
+          python -m pip install tensorflow==2.5.0 tensorflow_addons==0.13.0 tensorflow_probability==0.13.0 numdifftools
           python -m pip install git+https://github.com/DifferentiableUniverseInitiative/GalFlow.git
           python -m pip install -e .
       - name: test

tests/test_interpolation_gradients.py

@@ -0,0 +1,61 @@
+# This module tests tfa gradients in respect to interpolation methods.
+import numpy as np
+from tensorflow_addons.image import resampler
+from scipy.misc import face
+import numdifftools
+import tensorflow as tf
+
+from numpy.testing import assert_allclose
+
+
+def facer(interpolant, warp_tf):
+  image = face(gray=True)[-512:-512+128,-512:-512+128].astype('float32')
+  image_tf = tf.convert_to_tensor(image.reshape([1,128,128, 1]))
+  #define a shift
+  shift = tf.zeros([1,2])
+
+  #calculate derivatives via tf.GradientTape
+  with tf.GradientTape() as tape:
+      tape.watch(shift)
+      ws = tf.reshape(shift,[1,1,1,2]) + warp_tf
+      o = resampler(image_tf, ws, interpolant)
+  autodiff_jacobian = tape.batch_jacobian(o, shift) 
+
+  #calculate derivatives via numdifftools
+  def fn(shift):
+      shift = tf.convert_to_tensor(shift.astype('float32'))
+      ws = tf.reshape(shift,[1,1,1,2]) + warp_tf
+      o = resampler(image_tf, ws, interpolant)
+      return o.numpy().flatten()
+
+  numdiff_jacobian = numdifftools.Jacobian(fn, order=4, step=0.04)
+  numdiff_jacobian = numdiff_jacobian(np.zeros([2])).reshape([128,128,2])
+
+  return autodiff_jacobian[0,...,0,:], numdiff_jacobian
+
+import pytest
+
+xfail = pytest.mark.xfail
+@xfail(reason="Fails because it needs the modified tensorflow_addons to work")
+def test_interpolation_gradients():
+  atol = 0.003 #taken from the bilinear case with half step warp.
+
+  interpolant = "bilinear"
+  #on pixel interpolation
+  int_warp = np.stack(np.meshgrid(np.arange(128), np.arange(128)), axis=-1).astype('float32')
+  int_warp_tf = tf.convert_to_tensor(int_warp.reshape([1,128,128,2]))
+
+  #half step interpolation
+  half_warp = np.stack(np.meshgrid(np.arange(128), np.arange(128)), axis=-1).astype('float32')
+  half_warp_tf = tf.convert_to_tensor(half_warp.reshape([1,128,128,2])+.5) #add a half-step 
+
+  autodiff_jacobian_int, numdiff_jacobian_int = facer(interpolant,int_warp_tf)
+  autodiff_jacobian_half, numdiff_jacobian_half = facer(interpolant,half_warp_tf)
+
+
+  assert_allclose(autodiff_jacobian_half,numdiff_jacobian_half, rtol=0.1, atol=atol)
+  assert_allclose(autodiff_jacobian_int, numdiff_jacobian_int, rtol=0.1, atol=atol)
+
+
+if __name__=='__main__':
+    test_interpolation_gradients()