Add compositor for adding an image as a background

satpy/composites/__init__.py

@@ -1393,3 +1393,78 @@ def __call__(self, projectables, *args, **kwargs):
         rgb_img = enhance2dataset(projectables[1])
         rgb_img *= luminance
         return super(SandwichCompositor, self).__call__(rgb_img, *args, **kwargs)
+
+
+class StaticImageCompositor(GenericCompositor):
+    """A compositor that loads a static image from disk."""
+
+    def __init__(self, name, fname=None, area=None, **kwargs):
+        """Collect custom configuration values.
+
+        Args:
+            fname (str): Filename of the image to load
+            area (str): Name of area definition for the image.  Optional
+                        for images with built-in area definitions (geotiff)
+        """
+        if fname is None:
+            raise ValueError("No image configured for static image compositor")
+        self.fname = fname
+        self.area = None
+        if area is not None:
+            from satpy.resample import get_area_def
+            self.area = get_area_def(area)
+
+        super(StaticImageCompositor, self).__init__(name, **kwargs)
+
+    def __call__(self, *args, **kwargs):
+        from satpy import Scene
+        scn = Scene(reader='generic_image', filenames=[self.fname])
+        scn.load(['image'])
+        img = scn['image']
+        # Check for proper area definition.  Non-georeferenced images
+        # will raise IndexError
+        try:
+            _ = img.area.size
+        except IndexError:
+            if self.area is None:
+                raise AttributeError("Area definition needs to be configured")
+            img.attrs['area'] = self.area
+        img.attrs['sensor'] = None
+        img.attrs['mode'] = ''.join(img.bands.data)
+        img.attrs.pop('modifiers', None)
+        img.attrs.pop('calibration', None)
+        # Add start time if not present in the filename
+        if 'start_time' not in img.attrs or not img.attrs['start_time']:
+            import datetime as dt
+            img.attrs['start_time'] = dt.datetime.utcnow()
+
+        return img
+
+
+class BackgroundCompositor(GenericCompositor):
+    """A compositor that overlays one composite on top of another."""
+
+    def __call__(self, projectables, *args, **kwargs):
+        projectables = self.check_areas(projectables)
+
+        # Get enhanced images out of the composites
+        foreground = get_enhanced_image(projectables[0])
+        background = get_enhance_image(projectables[1])
+
+        # Adjust bands so that they match
+        # L/RGB -> RGB/RGB
+        # LA/RGB -> RGBA/RGBA
+        # RGB/RGBA -> RGBA/RGBA
+        foreground = add_bands(foreground, background['bands'])
+        background = add_bands(background, foreground['bands'])
+
+        # Stack the images
+        background.stack(foreground)
+
+        # Get merged metadata
+        attrs = combine_metadata
+
+        # Split to separate bands so the mode is correct
+        data = [background.sel(bands=b) for b in background['bands']]
+
+        return super(BackgroundCompositor, self).__call__(data, **kwargs)