Adjust multisource multi layer compositing.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## 1.30.5
 
+### Improvements
+
+- When using the multisource to composite multiple images with alpha channels, use nearest neighbor for upper tiles ([#1736](../../pull/1736))
+
 ### Changes
 
 - Adjust how compositing is done on styled images by adjusting the expected full alpha value ([#1735](../../pull/1735))

sources/multi/large_image_source_multi/__init__.py

@@ -1037,7 +1037,8 @@ def _mergeTiles(self, base, tile, x, y):
             base[y:y + tile.shape[0], x:x + tile.shape[1], :] = tile
         return base
 
-    def _getTransformedTile(self, ts, transform, corners, scale, frame, crop=None):
+    def _getTransformedTile(self, ts, transform, corners, scale, frame,
+                            crop=None, firstMerge=False):
         """
         Determine where the target tile's corners are located on the source.
         Fetch that so that we have at least sqrt(2) more resolution, then use
@@ -1054,6 +1055,9 @@ def _getTransformedTile(self, ts, transform, corners, scale, frame, crop=None):
         :param crop: an optional dictionary to crop the source image in full
             resolution, untransformed coordinates.  This may contain left, top,
             right, and bottom values in pixels.
+        :param firstMerge: if False and using an alpha channel, transform
+            with nearest neighbor rather than a higher order function to
+            avoid transparency effects.
         :returns: a numpy array tile or None, x, y coordinates within the
             target tile for the placement of the numpy tile array.
         """
@@ -1145,9 +1149,12 @@ def _getTransformedTile(self, ts, transform, corners, scale, frame, crop=None):
         destShape = [min(destShape[0], outh - y), min(destShape[1], outw - x)]
         if destShape[0] <= 0 or destShape[1] <= 0:
             return None, None, None
-        # Add an alpha band if needed
+        # Add an alpha band if needed.  This has to be done before the
+        # transform if it isn't the first tile, since the unused transformed
+        # areas need to have a zero alpha value
         if srcImage.shape[2] in {1, 3}:
             _, srcImage = _makeSameChannelDepth(np.zeros((1, 1, srcImage.shape[2] + 1)), srcImage)
+        useNearest = srcImage.shape[2] in {2, 4} and not firstMerge
         # skimage.transform.warp is faster and has less artifacts than
         # scipy.ndimage.affine_transform.  It is faster than using cupy's
         # version of scipy's affine_transform when the source and destination
@@ -1157,7 +1164,7 @@ def _getTransformedTile(self, ts, transform, corners, scale, frame, crop=None):
             # provide any speed improvement
             srcImage.astype(float),
             skimage.transform.AffineTransform(np.linalg.inv(transform)),
-            order=3,
+            order=0 if useNearest else 3,
             output_shape=(destShape[0], destShape[1], srcImage.shape[2]),
         ).astype(srcImage.dtype)
         return destImage, x, y
@@ -1234,7 +1241,8 @@ def _addSourceToTile(self, tile, sourceEntry, corners, scale):
         else:
             sourceTile, x, y = self._getTransformedTile(
                 ts, transform, corners, scale, sourceEntry.get('frame', 0),
-                source.get('position', {}).get('crop'))
+                source.get('position', {}).get('crop'),
+                firstMerge=tile is None)
         if sourceTile is not None and all(dim > 0 for dim in sourceTile.shape):
             targetDtype = np.dtype(self._info.get('dtype', ts.dtype))
             changeDtype = sourceTile.dtype != targetDtype