fix contrast()

The mean pixel value should be weighted average of the histogram.

Author: EIFY

official/vision/ops/augment.py

@@ -844,6 +844,8 @@ def color(image: tf.Tensor, factor: float) -> tf.Tensor:
 
 def contrast(image: tf.Tensor, factor: float) -> tf.Tensor:
   """Equivalent of PIL Contrast."""
+  image_height = tf.shape(image)[0]
+  image_width = tf.shape(image)[1]
   degenerate = tf.image.rgb_to_grayscale(image)
   # Cast before calling tf.histogram.
   degenerate = tf.cast(degenerate, tf.int32)
@@ -852,7 +854,8 @@ def contrast(image: tf.Tensor, factor: float) -> tf.Tensor:
   # and create a constant image size of that value.  Use that as the
   # blending degenerate target of the original image.
   hist = tf.histogram_fixed_width(degenerate, [0, 255], nbins=256)
-  mean = tf.reduce_sum(tf.cast(hist, tf.float32)) / 256.0
+  mean = tf.reduce_sum(
+      tf.cast(hist, tf.float32) * tf.linspace(0., 255., 256)) / float(image_height * image_width)
   degenerate = tf.ones_like(degenerate, dtype=tf.float32) * mean
   degenerate = tf.clip_by_value(degenerate, 0.0, 255.0)
   degenerate = tf.image.grayscale_to_rgb(tf.cast(degenerate, tf.uint8))