Fix integer stretching

trollimage/tests/test_image.py

@@ -1631,16 +1631,17 @@ def test_linear_stretch_does_not_affect_alpha(self, dtype):
 
         np.testing.assert_allclose(img.data.values, res, atol=1.e-6)
 
-    def test_linear_stretch_uint8(self):
-        """Test linear stretch with uint8 data."""
-        arr = np.arange(75, dtype=np.uint8).reshape(5, 5, 3)
+    @pytest.mark.parametrize("dtype", (np.uint8, np.uint16, int))
+    def test_linear_stretch_integer(self, dtype):
+        """Test linear stretch with integer data."""
+        arr = np.arange(75, dtype=dtype).reshape(5, 5, 3)
         arr[4, 4, :] = 255
         data = xr.DataArray(arr.copy(), dims=['y', 'x', 'bands'],
                             coords={'bands': ['R', 'G', 'B']})
         img = xrimage.XRImage(data)
         img.stretch_linear()
 
-        assert img.data.values.min() == pytest.approx(0.0)
+        assert img.data.values.min() == pytest.approx(-0.0015614156835530892)
         assert img.data.values.max() == pytest.approx(1.0960743801652901)
 
     @pytest.mark.parametrize("dtype", (np.float32, np.float64, float))

trollimage/xrimage.py

@@ -1082,6 +1082,7 @@ def crude_stretch(self, min_stretch=None, max_stretch=None):
 
         attrs = self.data.attrs
         offset = -min_stretch * scale_factor
+
         self.data = np.multiply(self.data, scale_factor, dtype=scale_factor.dtype) + offset
         self.data.attrs = attrs
         self.data.attrs.setdefault('enhancement_history', []).append({'scale': scale_factor,
@@ -1095,8 +1096,11 @@ def _check_stretch_value(self, val, kind='min'):
         if isinstance(val, (list, tuple)):
             val = self.xrify_tuples(val)
 
+        dtype = self.data.dtype
+        if np.issubdtype(dtype, np.integer):
+            dtype = np.dtype(np.float32)
         try:
-            val = val.astype(self.data.dtype)
+            val = val.astype(dtype)
         except AttributeError:
             val = self.data.dtype.type(val)