Add preserve_tone option to autocontrast

Tests/test_imageops.py

@@ -29,6 +29,7 @@ def test_sanity():
     ImageOps.autocontrast(hopper("L"), cutoff=(2, 10))
     ImageOps.autocontrast(hopper("L"), ignore=[0, 255])
     ImageOps.autocontrast(hopper("L"), mask=hopper("L"))
+    ImageOps.autocontrast(hopper("L"), preserve_tone=True)
 
     ImageOps.colorize(hopper("L"), (0, 0, 0), (255, 255, 255))
     ImageOps.colorize(hopper("L"), "black", "white")
@@ -336,7 +337,7 @@ def test_autocontrast_mask_toy_input():
         assert ImageStat.Stat(result_nomask).median == [128]
 
 
-def test_auto_contrast_mask_real_input():
+def test_autocontrast_mask_real_input():
     # Test the autocontrast with a rectangular mask
     with Image.open("Tests/images/iptc.jpg") as img:
 
@@ -362,3 +363,46 @@ def test_auto_contrast_mask_real_input():
             threshold=2,
             msg="autocontrast without mask pixel incorrect",
         )
+
+
+def test_autocontrast_preserve_gradient():
+    gradient = Image.linear_gradient("L")
+
+    # test with a grayscale gradient that extends to 0,255.
+    # Should be a noop.
+    out = ImageOps.autocontrast(gradient, cutoff=0, preserve_tone=True)
+
+    assert_image_equal(gradient, out)
+
+    # cutoff the top and bottom
+    # autocontrast should make the first and list histogram entries equal
+    # and should be 10% of the image pixels (+-, because integers)
+    out = ImageOps.autocontrast(gradient, cutoff=10, preserve_tone=True)
+    hist = out.histogram()
+    assert hist[0] == hist[-1]
+    assert hist[-1] == 256 * round(256 * 0.10)
+
+    # in rgb
+    img = gradient.convert("RGB")
+    out = ImageOps.autocontrast(img, cutoff=0, preserve_tone=True)
+    assert_image_equal(img, out)
+
+
+def test_autocontrast_preserve_onecolor():
+    def _test_one_color(color):
+        img = Image.new("RGB", (10, 10), color)
+
+        # single color images shouldn't change
+        out = ImageOps.autocontrast(img, cutoff=0, preserve_tone=True)
+        assert_image_equal(img, out)  # single color, no cutoff
+
+        # even if there is a cutoff
+        out = ImageOps.autocontrast(
+            img, cutoff=0, preserve_tone=True
+        )  # single color 10 cutoff
+        assert_image_equal(img, out)
+
+    _test_one_color((255, 255, 255))
+    _test_one_color((127, 255, 0))
+    _test_one_color((127, 127, 127))
+    _test_one_color((0, 0, 0))

src/PIL/ImageOps.py

@@ -61,7 +61,7 @@ def _lut(image, lut):
 # actions
 
 
-def autocontrast(image, cutoff=0, ignore=None, mask=None):
+def autocontrast(image, cutoff=0, ignore=None, mask=None, preserve_tone=False):
     """
     Maximize (normalize) image contrast. This function calculates a
     histogram of the input image (or mask region), removes ``cutoff`` percent of the
@@ -77,9 +77,14 @@ def autocontrast(image, cutoff=0, ignore=None, mask=None):
     :param mask: Histogram used in contrast operation is computed using pixels
                  within the mask. If no mask is given the entire image is used
                  for histogram computation.
+    :param preserve_tone: Preserve image tone in Photoshop-like style autocontrast.
     :return: An image.
     """
-    histogram = image.histogram(mask)
+    if preserve_tone:
+        histogram = image.convert("L").histogram(mask)
+    else:
+        histogram = image.histogram(mask)
+
     lut = []
     for layer in range(0, len(histogram), 256):
         h = histogram[layer : layer + 256]