add [] overload to python

index and slice image bands

ChangeLog

@@ -10,6 +10,7 @@
 - added test_foreign.py, plus more test images
 - rewritten tiff writer is about 3 - 4x faster at making pyramids
 - jpg, magick, png, tiff readers now use only 1 fd per input image
+- python: use [] to index and slice image bands
 
 6/2/15 started 7.42.3
 - bump version for back-compat ABI change

TODO

@@ -3,6 +3,14 @@
 
   python people are not going to want to use the glib arg parser, for example
 
+- need to add [] overload to Ruby bindings
+
+- overload (x, y) to be getpoint()?
+
+
+
+
+
 
 
 - support orientation tag in tiff images

cplusplus/include/vips/VImage8.h

@@ -1263,6 +1263,12 @@ public:
 		return( a * -1 );
 	}
 
+	friend VImage operator[]( VImage a, int index ) 
+		throw( VError )
+	{ 
+		return( a.extract_band( index ) ); 
+	}
+
 };
 
 VIPS_NAMESPACE_END

doc/using-python.xml

@@ -347,6 +347,26 @@ help(image.add)
 result_image = ((image * [1, 2, 3]).abs() < 128) | 4
 </programlisting>
     </para>
+
+    <para>
+      The wrapper overloads <code>[]</code> to be band indexing. You can write:
+
+<programlisting language="Python">
+result_image = image[2]
+</programlisting>
+
+      to extract the third band of the image. It implements the usual 
+      slicing and negative indexes, so you can write:
+
+<programlisting language="Python">
+result_image = image[1:]
+result_image = image[:3]
+result_image = image[-2:]
+result_image = [x.avg() for x in image]
+</programlisting>
+
+      and so on.
+    </para>
   </refsect3>
 
   <refsect3 id="python-expansions">

python/Vips.py

@@ -757,6 +757,32 @@ class Image(Vips.Image):
         else:
             return self.relational_const(other, Vips.OperationRelational.NOTEQ)
 
+    def __getitem__(self, arg):
+        if isinstance(arg, slice):
+            i = 0
+            if arg.start != None:
+                i = arg.start
+
+            n = self.bands - i
+            if arg.stop != None:
+                if arg.stop < 0:
+                    n = self.bands + arg.stop - i
+                else:
+                    n = arg.stop - i
+        elif isinstance(arg, int):
+            i = arg
+            n = 1
+        else:
+            raise TypeError
+
+        if i < 0:
+            i = self.bands + i
+
+        if i < 0 or i >= self.bands:
+            raise IndexError
+
+        return self.extract_band(i, n = n)
+
     # the cast operators int(), long() and float() must return numeric types, 
     # so we can't define them for images
 
@@ -813,7 +839,7 @@ class Image(Vips.Image):
 
     def bandsplit(self):
         """Split an n-band image into n separate images."""
-        return [self.extract_band(i) for i in range(0, self.bands)]
+        return [x for x in self]
 
     def bandjoin(self, other):
         """Join a set of images bandwise."""

python/example/try17.py

@@ -0,0 +1,18 @@
+#!/usr/bin/python3
+
+import sys
+
+import logging
+#logging.basicConfig(level = logging.DEBUG)
+
+from gi.repository import Vips
+
+#Vips.cache_set_trace(True)
+
+a = Vips.Image.new_from_file(sys.argv[1])
+
+a = a[1:]
+
+a.write_to_file(sys.argv[2])
+
+

test/test_conversion.py

@@ -80,6 +80,8 @@ rot_angle_bonds = [Vips.Angle.D0,
 # the other
 def zip_expand(x, y):
     if isinstance(x, list) and isinstance(y, list):
+        if len(x) != len(y):
+            raise Vips.Error("zip_expand list args not equal length")
         return list(zip(x, y))
     elif isinstance(x, list):
         return [[i, y] for i in x]
@@ -138,7 +140,7 @@ class TestConversion(unittest.TestCase):
     def setUp(self):
         im = Vips.Image.mask_ideal(100, 100, 0.5, reject = True, optical = True)
         self.colour = im * [1, 2, 3] + [2, 3, 4]
-        self.mono = self.colour.extract_band(1)
+        self.mono = self.colour[1]
         self.all_images = [self.mono, self.colour]
 
     def test_band_and(self):
@@ -295,6 +297,31 @@ class TestConversion(unittest.TestCase):
             pixel = sub.getpoint(30, 30)
             self.assertAlmostEqualObjects(pixel, [3, 4])
 
+    def test_slice(self):
+        test = self.colour
+        bands = [x.avg() for x in test]
+
+        x = test[0].avg()
+        self.assertEqual(x, bands[0])
+
+        x = test[-1].avg()
+        self.assertAlmostEqualObjects(x, bands[2])
+
+        x = [i.avg() for i in test[1:3]]
+        self.assertAlmostEqualObjects(x, bands[1:3])
+
+        x = [i.avg() for i in test[1:-1]]
+        self.assertAlmostEqualObjects(x, bands[1:-1])
+
+        x = [i.avg() for i in test[:2]]
+        self.assertAlmostEqualObjects(x, bands[:2])
+
+        x = [i.avg() for i in test[1:]]
+        self.assertAlmostEqualObjects(x, bands[1:])
+
+        x = [i.avg() for i in test[-1]]
+        self.assertAlmostEqualObjects(x, bands[-1])
+
     def test_crop(self):
         for fmt in all_formats:
             test = self.colour.cast(fmt)