Add frame interpolation postprocessing filter + KC hot-settings

Frame interpolation filter (frame_interpolation):
- New PostprocessingFilter with supports_idle_frames = True
- Backward-blend algorithm: blends frame N-1 → N over one capture
  interval, producing smooth output on idle ticks at ≤1 frame of lag
- Detects new vs idle frames via cheap 64-byte signature comparison
- No options; registered alongside other built-in filters

ProcessedLiveStream idle-tick support:
- Detects supports_idle_frames filters at construction (_has_idle_filters)
- target_fps returns 2× source rate when idle filters are present
- _process_loop runs at 2× rate; idle ticks copy cached source frame
  and run full filter chain, publishing result only when a filter
  returned actual interpolated output (not a pass-through)
- Pass-through idle ticks leave _latest_frame unchanged so consumers
  correctly deduplicate via object identity

KC target hot-settings:
- brightness, smoothing, interpolation_mode now read from self._settings
  each frame instead of captured as stale locals at loop startup
- Changes take effect within one frame without stop/restart

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/wled_controller/core/filters/__init__.py

@@ -18,6 +18,7 @@ import wled_controller.core.filters.pixelate  # noqa: F401
 import wled_controller.core.filters.auto_crop  # noqa: F401
 import wled_controller.core.filters.flip  # noqa: F401
 import wled_controller.core.filters.color_correction  # noqa: F401
+import wled_controller.core.filters.frame_interpolation  # noqa: F401
 
 __all__ = [
     "FilterOptionDef",

server/src/wled_controller/core/filters/base.py

@@ -41,6 +41,7 @@ class PostprocessingFilter(ABC):
 
     filter_id: str = ""
     filter_name: str = ""
+    supports_idle_frames: bool = False
 
     def __init__(self, options: Dict[str, Any]):
         """Initialize filter with validated options."""

server/src/wled_controller/core/filters/frame_interpolation.py

@@ -0,0 +1,89 @@
+"""Frame interpolation postprocessing filter."""
+
+import time
+from typing import Any, Dict, List, Optional
+
+import numpy as np
+
+from wled_controller.core.filters.base import FilterOptionDef, PostprocessingFilter
+from wled_controller.core.filters.image_pool import ImagePool
+from wled_controller.core.filters.registry import FilterRegistry
+
+
+@FilterRegistry.register
+class FrameInterpolationFilter(PostprocessingFilter):
+    """Blends consecutive frames to fill idle loop ticks with smooth output.
+
+    When ProcessedLiveStream runs at 2× source capture rate, idle ticks
+    (where no new source frame has arrived) receive an interpolated blend of
+    the two most recently captured frames instead of a duplicate.
+
+    Algorithm (backward blend):
+    - When frame B arrives at T_b: start blending A → B over the interval
+      (T_b - T_a), the duration between the two most recent source frames.
+    - At time T: alpha = clamp((T - T_b) / (T_b - T_a), 0, 1)
+    - output = (1 - alpha)*A + alpha*B
+
+    New vs idle frames are detected by comparing a cheap 64-byte signature of
+    the filter input. Since preceding filters are deterministic and the source
+    image is unchanged between idle ticks, the signature is stable.
+
+    Introduces at most one capture-interval of lag (≤33 ms at 30 fps) —
+    imperceptible for ambient lighting use cases.
+    """
+
+    filter_id = "frame_interpolation"
+    filter_name = "Frame Interpolation"
+    supports_idle_frames = True
+
+    def __init__(self, options: Dict[str, Any]):
+        super().__init__(options)
+        self._frame_a: Optional[np.ndarray] = None  # frame N-1
+        self._frame_b: Optional[np.ndarray] = None  # frame N (latest source)
+        self._time_a: float = 0.0
+        self._time_b: float = 0.0
+        self._sig_b: Optional[bytes] = None  # 64-byte signature of frame_b input
+
+    @classmethod
+    def get_options_schema(cls) -> List[FilterOptionDef]:
+        return []
+
+    def process_image(self, image: np.ndarray, image_pool: ImagePool) -> Optional[np.ndarray]:
+        """Return interpolated blend on idle ticks; update state on new source frames.
+
+        Returns:
+            None  — image passes through unchanged (no blend needed).
+            ndarray — blended output acquired from image_pool.
+        """
+        now = time.perf_counter()
+
+        # Detect new vs idle frame via cheap 64-byte signature
+        sig = bytes(image.ravel()[:64])
+        if sig != self._sig_b:
+            # New source frame — shift A ← B, B ← current
+            self._frame_a = self._frame_b
+            self._time_a = self._time_b
+            self._frame_b = image.copy()
+            self._time_b = now
+            self._sig_b = sig
+
+        # Need at least two frames and a positive interval to interpolate
+        if self._frame_a is None or self._time_b <= self._time_a:
+            return None  # pass through unchanged
+
+        interval = self._time_b - self._time_a
+        alpha = min(1.0, (now - self._time_b) / interval)
+
+        if alpha >= 1.0:
+            return None  # fully transitioned to B, no blend needed
+
+        # Blend: output = (1 - alpha)*A + alpha*B  (integer fast path)
+        alpha_i = int(alpha * 256)
+        h, w, c = image.shape
+        out = image_pool.acquire(h, w, c)
+        blended = (
+            (256 - alpha_i) * self._frame_a.astype(np.uint16)
+            + alpha_i * image.astype(np.uint16)
+        ) >> 8
+        np.copyto(out, blended, casting="unsafe")
+        return out

server/src/wled_controller/core/processing/kc_target_processor.py

@@ -258,19 +258,14 @@ class KCTargetProcessor(TargetProcessor):
 
     async def _processing_loop(self) -> None:
         """Main processing loop for key-colors extraction."""
-        settings = self._settings
+        target_fps = self._settings.fps
 
-        target_fps = settings.fps
-        smoothing = settings.smoothing
-        brightness = settings.brightness
-
-        # Select color calculation function
+        # Lookup table for interpolation mode → function (used per-frame from live settings)
         calc_fns = {
             "average": calculate_average_color,
             "median": calculate_median_color,
             "dominant": calculate_dominant_color,
         }
-        calc_fn = calc_fns.get(settings.interpolation_mode, calculate_average_color)
 
         frame_time = 1.0 / target_fps
         fps_samples: collections.deque = collections.deque(maxlen=10)
@@ -331,11 +326,16 @@ class KCTargetProcessor(TargetProcessor):
                         continue
                     prev_capture = capture
 
+                    # Read settings fresh each frame so hot updates (brightness,
+                    # smoothing, interpolation_mode) take effect without restart.
+                    s = self._settings
+                    calc_fn = calc_fns.get(s.interpolation_mode, calculate_average_color)
+
                     # CPU-bound work in thread pool
                     colors, colors_arr, frame_timing = await asyncio.to_thread(
                         _process_kc_frame,
                         capture, rect_names, rect_bounds, calc_fn,
-                        prev_colors_arr, smoothing, brightness,
+                        prev_colors_arr, s.smoothing, s.brightness,
                     )
 
                     prev_colors_arr = colors_arr

server/src/wled_controller/core/processing/live_stream.py

@@ -168,10 +168,17 @@ class ProcessedLiveStream(LiveStream):
         self._frame_lock = threading.Lock()
         self._running = False
         self._thread: Optional[threading.Thread] = None
+        # True when at least one filter requests idle-tick processing (e.g. frame interpolation).
+        # The processing loop then runs at 2× source rate and runs the full filter chain on idle
+        # ticks so those filters can produce interpolated output.
+        self._has_idle_filters: bool = any(
+            getattr(f, "supports_idle_frames", False) for f in filters
+        )
 
     @property
     def target_fps(self) -> int:
-        return self._source.target_fps
+        base = self._source.target_fps
+        return base * 2 if self._has_idle_filters else base
 
     @property
     def display_index(self) -> Optional[int]:
@@ -212,14 +219,47 @@ class ProcessedLiveStream(LiveStream):
         # processed by a consumer), so the 3rd slot is always safe to reuse.
         _ring: List[Optional[np.ndarray]] = [None, None, None]
         _ring_idx = 0
-        frame_time = 1.0 / self._source.target_fps if self._source.target_fps > 0 else 1.0
+        # Separate buffer for idle-tick source copies (not part of the ring buffer)
+        _idle_src_buf: Optional[np.ndarray] = None
+        fps = self.target_fps
+        frame_time = 1.0 / fps if fps > 0 else 1.0
 
         while self._running:
             loop_start = time.time()
 
             source_frame = self._source.get_latest_frame()
             if source_frame is None or source_frame is cached_source_frame:
-                # Sleep until next frame is expected
+                # Idle tick — run filter chain when any filter requests idle processing
+                if self._has_idle_filters and cached_source_frame is not None:
+                    src = cached_source_frame.image
+                    h, w, c = src.shape
+                    if _idle_src_buf is None or _idle_src_buf.shape != (h, w, c):
+                        _idle_src_buf = np.empty((h, w, c), dtype=np.uint8)
+                    np.copyto(_idle_src_buf, src)
+                    idle_image = _idle_src_buf
+
+                    for f in self._filters:
+                        result = f.process_image(idle_image, self._image_pool)
+                        if result is not None:
+                            if idle_image is not _idle_src_buf:
+                                self._image_pool.release(idle_image)
+                            idle_image = result
+
+                    # Only publish a new frame when the filter chain produced actual
+                    # interpolated output (idle_image advanced past the input buffer).
+                    # If every filter passed through, idle_image is still _idle_src_buf —
+                    # leave _latest_frame unchanged so consumers that rely on object
+                    # identity for deduplication correctly detect no new content.
+                    if idle_image is not _idle_src_buf:
+                        processed = ScreenCapture(
+                            image=idle_image,
+                            width=cached_source_frame.width,
+                            height=cached_source_frame.height,
+                            display_index=cached_source_frame.display_index,
+                        )
+                        with self._frame_lock:
+                            self._latest_frame = processed
+
                 elapsed = time.time() - loop_start
                 remaining = frame_time - elapsed
                 time.sleep(max(remaining, 0.001))