Initial commit: video effects processing system

Add S-expression based video effects pipeline with modular effect
definitions, constructs, and recipe files.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

effects/threshold.py

@@ -0,0 +1,85 @@
+# /// script
+# requires-python = ">=3.10"
+# dependencies = ["numpy"]
+# ///
+"""
+@effect threshold
+@version 1.0.0
+@author artdag
+
+@description
+Threshold effect. Converts to high-contrast black and white.
+Creates stark, graphic look by converting grayscale to pure
+black/white based on a threshold value.
+
+@param level int
+  @range 0 255
+  @default 128
+  Threshold level. Pixels above = white, below = black.
+
+@param invert bool
+  @default false
+  Swap black and white.
+
+@param color_mode string
+  @enum bw color
+  @default bw
+  Output mode:
+  - bw: pure black and white
+  - color: keep original colors where above threshold
+
+@example
+  (effect threshold :level 100)
+
+@example
+  ;; Beat-reactive threshold
+  (effect threshold :level (bind bass :range [80 180]) :invert true)
+"""
+
+import numpy as np
+
+
+def process_frame(frame: np.ndarray, params: dict, state: dict) -> tuple:
+    """
+    Apply threshold effect to a video frame.
+
+    Args:
+        frame: Input frame as numpy array (H, W, 3) RGB uint8
+        params: Effect parameters
+            - level: threshold 0-255 (default 128)
+            - invert: swap black/white (default False)
+            - color_mode: bw or color (default bw)
+        state: Persistent state dict
+
+    Returns:
+        Tuple of (processed_frame, new_state)
+    """
+    level = int(np.clip(params.get("level", 128), 0, 255))
+    invert = params.get("invert", False)
+    color_mode = params.get("color_mode", "bw")
+
+    if state is None:
+        state = {}
+
+    # Convert to grayscale for threshold comparison
+    if len(frame.shape) == 3:
+        gray = np.mean(frame, axis=2)
+    else:
+        gray = frame
+
+    # Apply threshold
+    mask = gray > level
+
+    if invert:
+        mask = ~mask
+
+    if color_mode == "bw":
+        # Pure black and white
+        result = np.where(mask[:, :, np.newaxis], 255, 0).astype(np.uint8)
+        if len(frame.shape) == 3:
+            result = np.repeat(result, frame.shape[2], axis=2)
+    else:
+        # Keep original colors where above threshold
+        result = np.where(mask[:, :, np.newaxis], frame, 0).astype(np.uint8)
+
+    return result, state