Fix PNG transparency: add format=auto to overlay filter for proper alpha blending

app.py

@@ -487,16 +487,16 @@ def burn_subtitles_and_watermark(video_path: str, output_path: str, subtitle_pat
 
         # Build filter complex
         if subtitle_path and watermark_path and os.path.exists(subtitle_path) and os.path.exists(watermark_path):
-            # Both subtitles and watermark: subtitles first, then overlay watermark at position 0,0 (full control in PNG)
-            filter_complex = f"[0:v]subtitles={subtitle_path}:force_style='FontName=Montserrat SemiBold'[v];[v][1:v]overlay=0:0"
+            # Both subtitles and watermark: subtitles first, then overlay watermark with alpha at position 0,0
+            filter_complex = f"[0:v]subtitles={subtitle_path}:force_style='FontName=Montserrat SemiBold'[v];[v][1:v]overlay=0:0:format=auto"
             cmd.extend(['-filter_complex', filter_complex])
         elif subtitle_path and os.path.exists(subtitle_path):
             # Only subtitles
             filter_complex = f"subtitles={subtitle_path}:force_style='FontName=Montserrat SemiBold'"
             cmd.extend(['-vf', filter_complex])
         elif watermark_path and os.path.exists(watermark_path):
-            # Only watermark: overlay at 0,0 (full control in PNG)
-            filter_complex = "[0:v][1:v]overlay=0:0"
+            # Only watermark: overlay at 0,0 with alpha transparency support
+            filter_complex = "[0:v][1:v]overlay=0:0:format=auto"
             cmd.extend(['-filter_complex', filter_complex])
 
         cmd.extend(['-c:a', 'copy', output_path])