Fix PNG alpha: use scale2ref filter (from pycaps) for proper transparency handling

app.py

@@ -488,19 +488,23 @@ 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 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"
+            # Both subtitles and watermark: subtitles first, then overlay with scale2ref for proper alpha
+            filter_complex = (
+                f"[0:v]subtitles={subtitle_path}:force_style='FontName=Montserrat SemiBold'[v];"
+                f"[1:v][v]scale2ref[ovr][base];"
+                f"[base][ovr]overlay=0:0"
+            )
             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 with alpha transparency support
-            filter_complex = "[0:v][1:v]overlay=0:0:format=auto"
+            # Only watermark: use scale2ref to properly handle PNG alpha channel
+            filter_complex = "[1:v][0:v]scale2ref[ovr][base];[base][ovr]overlay=0:0"
             cmd.extend(['-filter_complex', filter_complex])
 
-        cmd.extend(['-c:a', 'copy', output_path])
+        cmd.extend(['-c:a', 'copy', '-pix_fmt', 'yuv420p', output_path])
 
         print(f"[burn] Burning subtitles/watermark...")
         result = subprocess.run(cmd, capture_output=True, text=True)