From 436dd6e5dd7dc8818284c8406c4eab6bdade1ee7 Mon Sep 17 00:00:00 2001
From: Jody Klymak <jklymak@gmail.com>
Date: Fri, 25 Oct 2019 12:52:59 -0700
Subject: [PATCH] FIX: fix anti-aliasing zoom bug

---
 lib/matplotlib/image.py            | 10 ++++------
 lib/matplotlib/tests/test_image.py | 18 ++++++++++++++++++
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
index 86391d4314eb..db3bf36cfee3 100644
--- a/lib/matplotlib/image.py
+++ b/lib/matplotlib/image.py
@@ -176,11 +176,10 @@ def _resample(
     if interpolation == 'antialiased':
         # don't antialias if upsampling by an integer number or
         # if zooming in more than a factor of 3
-        shape = list(data.shape)
-        if image_obj.origin == 'upper':
-            shape[0] = 0
-        dispx, dispy = transform.transform([shape[1], shape[0]])
-
+        pos = np.array([[0, 0], [data.shape[1], data.shape[0]]])
+        disp = transform.transform(pos)
+        dispx = np.abs(np.diff(disp[:, 0]))
+        dispy = np.abs(np.diff(disp[:, 1]))
         if ((dispx > 3 * data.shape[1] or
                 dispx == data.shape[1] or
                 dispx == 2 * data.shape[1]) and
@@ -190,7 +189,6 @@ def _resample(
             interpolation = 'nearest'
         else:
             interpolation = 'hanning'
-
     out = np.zeros(out_shape + data.shape[2:], data.dtype)  # 2D->2D, 3D->3D.
     if resample is None:
         resample = image_obj.get_resample()
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
index 67ac022daa1f..f227f2d3ec6b 100644
--- a/lib/matplotlib/tests/test_image.py
+++ b/lib/matplotlib/tests/test_image.py
@@ -198,6 +198,24 @@ def test_imshow_upsample3(fig_test, fig_ref):
     axs.imshow(A, interpolation='nearest')
 
 
+@check_figures_equal(extensions=['png'])
+def test_imshow_zoom(fig_test, fig_ref):
+    # should be less than 3 upsample, so should be nearest...
+    np.random.seed(19680801)
+    dpi = 100
+    A = np.random.rand(int(dpi * 3), int(dpi * 3))
+    for fig in [fig_test, fig_ref]:
+        fig.set_size_inches(2.9, 2.9)
+    axs = fig_test.subplots()
+    axs.imshow(A, interpolation='nearest')
+    axs.set_xlim([10, 20])
+    axs.set_ylim([10, 20])
+    axs = fig_ref.subplots()
+    axs.imshow(A, interpolation='antialiased')
+    axs.set_xlim([10, 20])
+    axs.set_ylim([10, 20])
+
+
 @check_figures_equal()
 def test_imshow_pil(fig_test, fig_ref):
     style.use("default")