Added support for black detection in video thumbnail generation

Added 3 new conf options:
* thumb_video_black_retries = 0
* thumb_video_black_retry_offset = 1
* thumb_video_black_max_colors = 4

They control how many retries a thumbnail generation should run before
failure. Each retry seeks the video another N seconds determined by
retry_offset in addition to the original thumb_video_delay setting.

A 'black' thumbnail is detected when the resulting image has max_colors
or less distinct color values in it. This is not exclusive to black, but
attempts to avoid any solid color images.

src/sigal/gallery.py

@@ -179,6 +179,9 @@ class Media:
                         s['thumb_video_delay'],
                         fit=s['thumb_fit'],
                         converter=s['video_converter'],
+                        black_retries=s['thumb_video_black_retries'],
+                        black_offset=s['thumb_video_black_retry_offset'],
+                        black_max_colors=s['thumb_video_black_max_colors']
                     )
             except Exception as e:
                 self.logger.error('Failed to generate thumbnail: %s', e)

src/sigal/settings.py

@@ -75,7 +75,10 @@ _DEFAULT_CONFIG = {
     'thumb_prefix': '',
     'thumb_size': (200, 150),
     'thumb_suffix': '',
-    'thumb_video_delay': '0',
+    'thumb_video_delay': 0,
+    'thumb_video_black_retries': 0,
+    'thumb_video_black_retry_offset': 1,
+    'thumb_video_black_max_colors': 4,
     'title': '',
     'use_orig': False,
     'user_css': None,

src/sigal/templates/sigal.conf.py

@@ -119,7 +119,13 @@ thumb_size = (280, 210)
 # thumb_fit_centering = (0.5, 0.5)
 
 # Delay in seconds to avoid black thumbnails in videos with fade-in
-# thumb_video_delay = '0'
+# thumb_video_delay = 0
+# Max retries to generate a non-black thumbnail
+# thumb_video_black_retries = 0
+# For each retry, advance another N seconds on top of original delay
+# thumb_video_black_retry_offset = 1
+# A thumbnail with more than max_colors will not be considered "all black"
+# thumb_video_black_max_colors = 4
 
 # Keep original image (default: False)
 # keep_orig = True

src/sigal/video.py

@@ -27,6 +27,8 @@ import shutil
 import subprocess
 from os.path import splitext
 
+from PIL import Image as PILImage
+
 from . import image, utils
 from .utils import is_valid_html5_video
 
@@ -172,17 +174,32 @@ def generate_video(source, outname, settings):
 
 
 def generate_thumbnail(
-    source, outname, box, delay, fit=True, options=None, converter="ffmpeg"
+    source, outname, box, delay, fit=True, options=None, converter="ffmpeg", black_retries=0, black_offset=1, black_max_colors=4
 ):
     """Create a thumbnail image for the video source, based on ffmpeg."""
     logger = logging.getLogger(__name__)
     tmpfile = outname + ".tmp.jpg"
 
-    # dump an image of the video
-    cmd = [converter, "-i", source, "-an", "-r", "1"]
-    cmd += ["-ss", str(delay), "-vframes", "1", "-y", tmpfile]
-    logger.debug("Create thumbnail for video: %s", " ".join(cmd))
-    check_subprocess(cmd, source, outname)
+    currentTry = 0
+    iDelay = int(delay)
+    while currentTry <= abs(black_retries):
+        # dump an image of the video
+        cmd = [converter, "-i", source, "-an", "-r", "1"]
+        cmd += ["-ss", str(iDelay), "-vframes", "1", "-y", tmpfile]
+        logger.debug("Create thumbnail for video: %s", " ".join(cmd))
+        check_subprocess(cmd, source, outname)
+        if os.path.isfile(tmpfile) and black_retries > 0:
+            img = PILImage.open(tmpfile)
+            colors = img.getcolors(maxcolors=black_max_colors)
+            if colors is None:
+                # There were more colors than maxcolors in the image, it looks suitable for a valid thumbnail
+                break
+            else:
+                # Only found 'maxcolors' unique colors, looks like a solid color, try again with another seek delay
+                currentTry += 1
+                iDelay += abs(black_offset)
+        else:
+            break
 
     # Sometimes ffmpeg fails with returncode zero but without producing an
     # output file Thus, we need to check if an output file was created. If
@@ -228,6 +245,9 @@ def process_video(media):
                 fit=settings["thumb_fit"],
                 options=settings["jpg_options"],
                 converter=settings["video_converter"],
+                black_retries=s['thumb_video_black_retries'],
+                black_offset=s['thumb_video_black_retry_offset'],
+                black_max_colors=s['thumb_video_black_max_colors']
             )
 
     return status.value