Adjust scaling when generating lossless pyramidal files.

CHANGELOG.md

@@ -11,6 +11,7 @@
 - Make annotation schema more uniform ([#763](../../pull/763))
 - Improve TileSource class repr ([#765](../../pull/765))
 - Improve frame slider response with base quads ([#771](../../pull/771))
+- Default to nearest-neighbor scaling in lossless image conversion ([#772](../../pull/772))
 
 ## Version 1.10.0
 

large_image/tilesource/utilities.py

@@ -271,6 +271,7 @@ def _vipsParameters(forTiled=True, defaultCompression=None, **kwargs):
     :param level: compression level for zstd, 1-22 (default is 10).
     :param predictor: one of 'none', 'horizontal', or 'float' used for lzw and
         deflate.
+    :param shrinkMode: one of vips's VipsRegionShrink strings.
     :returns: a dictionary of parameters.
     """
     if not forTiled:
@@ -293,6 +294,12 @@ def _vipsParameters(forTiled=True, defaultCompression=None, **kwargs):
         'Q': 90,
         'predictor': 'horizontal',
     }
+    # For lossless modes, make sure pixel values in lower resolutions are
+    # values that exist in the upper resolutions.
+    if convertParams['compression'] in {'none', 'lzw'}:
+        convertParams['region_shrink'] = 'nearest'
+    if kwargs.get('shrinkMode') and kwargs['shrinkMode'] != 'default':
+        convertParams['region_shrink'] = kwargs['shrinkMode']
     for vkey, kwkeys in {
         'tile_width': {'tileSize'},
         'tile_height': {'tileSize'},

utilities/converter/large_image_converter/__main__.py

@@ -84,6 +84,13 @@ def get_parser():
         help='JP2K peak signal to noise ratio.  0 for lossless.')
     parser.add_argument(
         '--cr', type=int, help='JP2K compression ratio.  1 for lossless.')
+    parser.add_argument(
+        '--shrink-mode', '--shrink', '--reduce', dest='shrinkMode',
+        default=None,
+        choices=['mean', 'median', 'mode', 'max', 'min', 'nearest', 'default'],
+        help='When producing lower resolution images, use this method for '
+        'computing pixels.  This defaults to median for lossy images and '
+        'nearest for lossless images.')
     parser.add_argument(
         '--only-associated', dest='_keep_associated', action='append',
         help='Only keep associated images with the specified keys.  The value '