Added reduce operation

Tests/test_image_reduce.py

@@ -0,0 +1,222 @@
+from PIL import Image, ImageMode, ImageMath
+
+from .helper import PillowTestCase, hopper, convert_to_comparable
+
+
+class TestImageReduce(PillowTestCase):
+    # There are several internal implementations
+    remarkable_factors = [
+        1, 2, 3, 4, 5, 6,  # special implementations
+        (1, 2), (1, 3), (1, 4), (1, 7),  # 1xN implementation
+        (2, 1), (3, 1), (4, 1), (7, 1),  # Nx1 implementation
+        # general implementation with different paths
+        (4, 6), (5, 6), (4, 7), (5, 7), (19, 17),
+    ]
+
+    @classmethod
+    def setUpClass(cls):
+        cls.gradients_image = Image.open("Tests/images/radial_gradients.png")
+        cls.gradients_image.load()
+
+    def test_args_factor(self):
+        im = Image.new("L", (10, 10))
+
+        self.assertEqual((4, 4), im.reduce(3).size)
+        self.assertEqual((4, 10), im.reduce((3, 1)).size)
+        self.assertEqual((10, 4), im.reduce((1, 3)).size)
+
+        with self.assertRaises(ValueError):
+            im.reduce(0)
+        with self.assertRaises(TypeError):
+            im.reduce(2.0)
+        with self.assertRaises(ValueError):
+            im.reduce((0, 10))
+
+    def test_args_box(self):
+        im = Image.new("L", (10, 10))
+
+        self.assertEqual((5, 5), im.reduce(2, (0, 0, 10, 10)).size)
+        self.assertEqual((1, 1), im.reduce(2, (5, 5, 6, 6)).size)
+
+        with self.assertRaises(TypeError):
+            im.reduce(2, "stri")
+        with self.assertRaises(TypeError):
+            im.reduce(2, 2)
+        with self.assertRaises(ValueError):
+            im.reduce(2, (0, 0, 11, 10))
+        with self.assertRaises(ValueError):
+            im.reduce(2, (0, 0, 10, 11))
+        with self.assertRaises(ValueError):
+            im.reduce(2, (-1, 0, 10, 10))
+        with self.assertRaises(ValueError):
+            im.reduce(2, (0, -1, 10, 10))
+        with self.assertRaises(ValueError):
+            im.reduce(2, (0, 5, 10, 5))
+        with self.assertRaises(ValueError):
+            im.reduce(2, (5, 0, 5, 10))
+
+    def test_unsupported_modes(self):
+        im = Image.new("P", (10, 10))
+        with self.assertRaises(ValueError):
+            im.reduce(3)
+
+        im = Image.new("1", (10, 10))
+        with self.assertRaises(ValueError):
+            im.reduce(3)
+
+        im = Image.new("I;16", (10, 10))
+        with self.assertRaises(ValueError):
+            im.reduce(3)
+
+    def get_image(self, mode):
+        mode_info = ImageMode.getmode(mode)
+        if mode_info.basetype == 'L':
+            bands = [self.gradients_image]
+            for _ in mode_info.bands[1:]:
+                # rotate previous image
+                band = bands[-1].transpose(Image.ROTATE_90)
+                bands.append(band)
+            # Correct alpha channel to exclude completely transparent pixels.
+            # Low alpha values also emphasize error after alpha multiplication.
+            if mode.endswith('A'):
+                bands[-1] = bands[-1].point(lambda x: int(85 + x / 1.5))
+            im = Image.merge(mode, bands)
+        else:
+            assert len(mode_info.bands) == 1
+            im = self.gradients_image.convert(mode)
+        # change the height to make a not square image
+        return im.crop((0, 0, im.width, im.height - 5))
+
+    def compare_reduce_with_box(self, im, factor):
+        box = (11, 13, 146, 164)
+        reduced = im.reduce(factor, box=box)
+        reference = im.crop(box).reduce(factor)
+        self.assertEqual(reduced, reference)
+
+    def compare_reduce_with_reference(self, im, factor, average_diff=0.4, max_diff=1):
+        """Image.reduce() should look very similar to Image.resize(BOX).
+
+        A reference image is compiled from a large source area
+        and possible last column and last row.
+        +-----------+
+        |..........c|
+        |..........c|
+        |..........c|
+        |rrrrrrrrrrp|
+        +-----------+
+        """
+        reduced = im.reduce(factor)
+
+        if not isinstance(factor, (list, tuple)):
+            factor = (factor, factor)
+
+        reference = Image.new(im.mode, reduced.size)
+        area_size = (im.size[0] // factor[0], im.size[1] // factor[1])
+        area_box = (0, 0, area_size[0] * factor[0], area_size[1] * factor[1])
+        area = im.resize(area_size, Image.BOX, area_box)
+        reference.paste(area, (0, 0))
+
+        if area_size[0] < reduced.size[0]:
+            self.assertEqual(reduced.size[0] - area_size[0], 1)
+            last_column_box = (area_box[2], 0, im.size[0], area_box[3])
+            last_column = im.resize((1, area_size[1]), Image.BOX, last_column_box)
+            reference.paste(last_column, (area_size[0], 0))
+
+        if area_size[1] < reduced.size[1]:
+            self.assertEqual(reduced.size[1] - area_size[1], 1)
+            last_row_box = (0, area_box[3], area_box[2], im.size[1])
+            last_row = im.resize((area_size[0], 1), Image.BOX, last_row_box)
+            reference.paste(last_row, (0, area_size[1]))
+
+        if area_size[0] < reduced.size[0] and area_size[1] < reduced.size[1]:
+            last_pixel_box = (area_box[2], area_box[3], im.size[0], im.size[1])
+            last_pixel = im.resize((1, 1), Image.BOX, last_pixel_box)
+            reference.paste(last_pixel, area_size)
+
+        self.assert_compare_images(reduced, reference, average_diff, max_diff)
+
+    def assert_compare_images(self, a, b, max_average_diff, max_diff=255):
+        self.assertEqual(
+            a.mode, b.mode, "got mode %r, expected %r" % (a.mode, b.mode))
+        self.assertEqual(
+            a.size, b.size, "got size %r, expected %r" % (a.size, b.size))
+
+        a, b = convert_to_comparable(a, b)
+
+        bands = ImageMode.getmode(a.mode).bands
+        for band, ach, bch in zip(bands, a.split(), b.split()):
+            ch_diff = ImageMath.eval("convert(abs(a - b), 'L')", a=ach, b=bch)
+            ch_hist = ch_diff.histogram()
+
+            average_diff = (sum(i * num for i, num in enumerate(ch_hist))
+                            / float(a.size[0] * a.size[1]))
+            self.assertGreaterEqual(
+                max_average_diff, average_diff,
+                ("average pixel value difference {:.4f} > expected {:.4f} "
+                 "for '{}' band").format(average_diff, max_average_diff, band),
+            )
+
+            last_diff = [i for i, num in enumerate(ch_hist) if num > 0][-1]
+            self.assertGreaterEqual(
+                max_diff, last_diff,
+                "max pixel value difference {} > expected {} for '{}' band"
+                    .format(last_diff, max_diff, band),
+            )
+
+    def test_mode_L(self):
+        im = self.get_image("L")
+        for factor in self.remarkable_factors:
+            self.compare_reduce_with_reference(im, factor)
+            self.compare_reduce_with_box(im, factor)
+
+    def test_mode_LA(self):
+        im = self.get_image("LA")
+        for factor in self.remarkable_factors:
+            self.compare_reduce_with_reference(im, factor, 0.8, 5)
+
+        # With opaque alpha, error should be way smaller
+        im.putalpha(Image.new('L', im.size, 255))
+        for factor in self.remarkable_factors:
+            self.compare_reduce_with_reference(im, factor)
+            self.compare_reduce_with_box(im, factor)
+
+    def test_mode_La(self):
+        im = self.get_image("La")
+        for factor in self.remarkable_factors:
+            self.compare_reduce_with_reference(im, factor)
+            self.compare_reduce_with_box(im, factor)
+
+    def test_mode_RGB(self):
+        im = self.get_image("RGB")
+        for factor in self.remarkable_factors:
+            self.compare_reduce_with_reference(im, factor)
+            self.compare_reduce_with_box(im, factor)
+
+    def test_mode_RGBA(self):
+        im = self.get_image("RGBA")
+        for factor in self.remarkable_factors:
+            self.compare_reduce_with_reference(im, factor, 0.8, 5)
+
+        # With opaque alpha, error should be way smaller
+        im.putalpha(Image.new('L', im.size, 255))
+        for factor in self.remarkable_factors:
+            self.compare_reduce_with_reference(im, factor)
+            self.compare_reduce_with_box(im, factor)
+
+    def test_mode_RGBa(self):
+        im = self.get_image("RGBa")
+        for factor in self.remarkable_factors:
+            self.compare_reduce_with_reference(im, factor)
+            self.compare_reduce_with_box(im, factor)
+
+    def test_mode_I(self):
+        im = self.get_image("I")
+        for factor in self.remarkable_factors:
+            self.compare_reduce_with_reference(im, factor)
+            self.compare_reduce_with_box(im, factor)
+
+    def test_mode_F(self):
+        im = self.get_image("F")
+        for factor in self.remarkable_factors:
+            self.compare_reduce_with_reference(im, factor, 0, 0)
+            self.compare_reduce_with_box(im, factor)

setup.py

@@ -54,6 +54,7 @@ def get_version():
     "Access",
     "AlphaComposite",
     "Resample",
+    "Reduce",
     "Bands",
     "BcnDecode",
     "BitDecode",

src/PIL/Image.py

@@ -1821,6 +1821,39 @@ def resize(self, size, resample=NEAREST, box=None):
 
         return self._new(self.im.resize(size, resample, box))
 
+    def reduce(self, factor, box=None):
+        """
+        Returns reduced in `factor` times copy of the image.
+        If the size of the image is not dividable by the `factor`,
+        the resulting size will be rounded up.
+
+        :param factor: A greater than 0 integer or tuple of two integers
+            for width and height separately.
+        :param box: An optional 4-tuple of ints giving the region
+           of the source image which should be reduced.
+           The values should be within (0, 0, width, height) rectangle.
+           If omitted or None, the entire source is used.
+        """
+        if not isinstance(factor, (list, tuple)):
+            factor = (factor, factor)
+
+        if box is None:
+            box = (0, 0) + self.size
+        else:
+            box = tuple(box)
+
+        if factor == (1, 1) and box == (0, 0) + self.size:
+            return self.copy()
+
+        if self.mode in ["LA", "RGBA"]:
+            im = self.convert(self.mode[:-1] + "a")
+            im = im.reduce(factor, box)
+            return im.convert(self.mode)
+
+        self.load()
+
+        return self._new(self.im.reduce(factor, box))
+
     def rotate(
         self,
         angle,

src/_imaging.c

@@ -1787,6 +1787,51 @@ _resize(ImagingObject* self, PyObject* args)
     return PyImagingNew(imOut);
 }
 
+static PyObject*
+_reduce(ImagingObject* self, PyObject* args)
+{
+    Imaging imIn;
+    Imaging imOut;
+
+    int xscale, yscale;
+    int box[4] = {0, 0, 0, 0};
+
+    imIn = self->image;
+    box[2] = imIn->xsize;
+    box[3] = imIn->ysize;
+
+    if (!PyArg_ParseTuple(args, "(ii)|(iiii)", &xscale, &yscale,
+                          &box[0], &box[1], &box[2], &box[3]))
+        return NULL;
+
+    if (xscale < 1 || yscale < 1) {
+        return ImagingError_ValueError("scale must be > 0");
+    }
+
+    if (box[0] < 0 || box[1] < 0) {
+        return ImagingError_ValueError("box offset can't be negative");
+    }
+
+    if (box[2] > imIn->xsize || box[3] > imIn->ysize) {
+        return ImagingError_ValueError("box can't exceed original image size");
+    }
+
+    if (box[2] <= box[0] || box[3] <= box[1]) {
+        return ImagingError_ValueError("box can't be empty");
+    }
+
+    if (xscale == 1 && yscale == 1) {
+        imOut = ImagingCrop(imIn, box[0], box[1], box[2], box[3]);
+    } else {
+        // Change box format: (left, top, width, height)
+        box[2] -= box[0];
+        box[3] -= box[1];
+        imOut = ImagingReduce(imIn, xscale, yscale, box);
+    }
+
+    return PyImagingNew(imOut);
+}
+
 
 #define IS_RGB(mode)\
     (!strcmp(mode, "RGB") || !strcmp(mode, "RGBA") || !strcmp(mode, "RGBX"))
@@ -1801,7 +1846,7 @@ im_setmode(ImagingObject* self, PyObject* args)
     char* mode;
     Py_ssize_t modelen;
     if (!PyArg_ParseTuple(args, "s#:setmode", &mode, &modelen))
-    return NULL;
+        return NULL;
 
     im = self->image;
 
@@ -3235,6 +3280,7 @@ static struct PyMethodDef methods[] = {
     {"rankfilter", (PyCFunction)_rankfilter, 1},
 #endif
     {"resize", (PyCFunction)_resize, 1},
+    {"reduce", (PyCFunction)_reduce, 1},
     {"transpose", (PyCFunction)_transpose, 1},
     {"transform2", (PyCFunction)_transform2, 1},
 

src/libImaging/Imaging.h

@@ -313,6 +313,7 @@ extern Imaging ImagingRotate270(Imaging imOut, Imaging imIn);
 extern Imaging ImagingTranspose(Imaging imOut, Imaging imIn);
 extern Imaging ImagingTransverse(Imaging imOut, Imaging imIn);
 extern Imaging ImagingResample(Imaging imIn, int xsize, int ysize, int filter, float box[4]);
+extern Imaging ImagingReduce(Imaging imIn, int xscale, int yscale, int box[4]);
 extern Imaging ImagingTransform(
     Imaging imOut, Imaging imIn, int method, int x0, int y0, int x1, int y1,
     double *a, int filter, int fill);