Update app.py

app.py

@@ -413,10 +413,17 @@ def process_relight(input_fg, input_bg, prompt, image_width, image_height, num_s
             error_msg = "❌ Please upload a background image or choose a lighting direction"
             print(error_msg)
             raise gr.Error(error_msg)
+        
+        # Handle empty prompt - provide default when using background image
         if not prompt.strip():
-            error_msg = "❌ Please enter a prompt"
-            print(error_msg)
-            raise gr.Error(error_msg)
+            if bg_source == "Use Background Image" or bg_source == "Use Flipped Background Image":
+                # When using background image as light source, use a generic default prompt
+                prompt = "best quality, detailed"
+                print(f"Using default prompt for background lighting: {prompt}")
+            else:
+                error_msg = "❌ Please enter a prompt"
+                print(error_msg)
+                raise gr.Error(error_msg)
         
         print(f"Processing with device: {device}")
         print(f"Input shapes - FG: {input_fg.shape}, BG: {input_bg.shape if input_bg is not None else 'None'}")