Add Echo tunnel, a video effect that flies through Feedback's nested echoes

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/registry.cpp

@@ -16,6 +16,7 @@
 #include "effects/tiles.hpp"
 #include "effects/tileswap.hpp"
 #include "effects/tracking.hpp"
+#include "effects/tunnel.hpp"
 #include "effects/warp.hpp"
 
 namespace effector {
@@ -41,6 +42,7 @@ const std::vector<EffectPtr>& effects() {
         // Video effects, offered only in a video project.
         std::make_shared<TrackingEffect>(),
         std::make_shared<TileSwapEffect>(),
+        std::make_shared<TunnelEffect>(),
     };
     return registry;
 }

src/effects/tunnel.cpp

@@ -0,0 +1,136 @@
+#include "effects/tunnel.hpp"
+
+#include <algorithm>
+#include <cmath>
+#include <vector>
+
+#include "core/parallel.hpp"
+#include "effects/sampling.hpp"
+
+namespace effector {
+
+const std::vector<ParamSpec>& TunnelEffect::params() const {
+    static const std::vector<ParamSpec> specs = {
+        ParamSpec::makeInt("tunnel_count", "Echoes", 8, 2, 32,
+                           "How many echoes are nested inside the picture at once."),
+        ParamSpec::makeDouble("tunnel_scale", "Scale", 0.8, 0.3, 0.95,
+                              "Size of each echo against the one around it.", 0.01),
+        ParamSpec::makeDouble("tunnel_rotate", "Rotation", 6.0, -180.0, 180.0,
+                              "Turn in degrees of each echo against the one around it.", 0.5, 1),
+        ParamSpec::makeInt("tunnel_drift", "Drift", 0, 0, 4096,
+                           "How far in pixels each echo sits off the centre of the one around "
+                           "it, all in one direction of the seed's own."),
+        ParamSpec::makeDouble("tunnel_fade", "Opacity", 0.8, 0.0, 1.0,
+                              "How strongly each echo covers what lies beneath it."),
+        ParamSpec::makeDouble("tunnel_speed", "Speed", 0.5, 0.0, 10.0,
+                              "Echoes passed per second, rounded so that the clip loops without "
+                              "a jump; 0 holds still.",
+                              0.1, 1),
+        ParamSpec::makeChoice("tunnel_direction", "Direction", "in", {"in", "out"},
+                              "in flies into the tunnel, the echoes growing towards you; out "
+                              "backs away from it, the echoes shrinking into the distance."),
+    };
+    return specs;
+}
+
+namespace {
+
+struct Settings {
+    int count;
+    double scale;
+    double rotate;
+    int drift;
+    double fade;
+    double speed;
+    bool out;
+
+    explicit Settings(const ParamValues& values)
+        : count(static_cast<int>(std::clamp(paramInt(values, "tunnel_count", 8), 2L, 32L))),
+          scale(std::clamp(paramDouble(values, "tunnel_scale", 0.8), 0.3, 0.95)),
+          rotate(std::clamp(paramDouble(values, "tunnel_rotate", 6.0), -180.0, 180.0)),
+          drift(static_cast<int>(std::max(0L, paramInt(values, "tunnel_drift", 0)))),
+          fade(std::clamp(paramDouble(values, "tunnel_fade", 0.8), 0.0, 1.0)),
+          speed(std::max(0.0, paramDouble(values, "tunnel_speed", 0.5))),
+          out(paramChoice(values, "tunnel_direction", "in") == "out") {}
+};
+
+// One echo as it lies in this frame: how a pixel of the canvas finds the
+// pixel of the input it shows, and how strongly it covers what is beneath.
+struct Echo {
+    double sine;
+    double cosine;
+    double shrink;  // 1 / size
+    double dx;
+    double dy;
+    int mix;  // out of 256
+};
+
+}  // namespace
+
+Image TunnelEffect::animate(Image input, const ParamValues& values, const Moment& at) const {
+    const Settings settings(values);
+    const int width = input.width();
+    const int height = input.height();
+
+    // What stays put for the whole clip: the direction the echoes drift in.
+    Rng steady = at.steady();
+    double driftSine = 0.0;
+    double driftCosine = 1.0;
+    sinCosDegrees(unitDouble(steady.next()) * 360.0, driftSine, driftCosine);
+
+    // Where this frame is in time: how far each echo has moved on from its
+    // place at the start of the step. Echo k sits at depth k + 1 - t flying
+    // in, k + t backing out; at depth 0 it is the picture itself, so it can
+    // melt into it, and a new one fades in at the far end as the old leaves.
+    const double t = settings.speed > 0.0 ? at.cycle(settings.speed) : 0.0;
+    std::vector<Echo> echoes;
+    echoes.reserve(static_cast<std::size_t>(settings.count));
+    for (int k = 0; k < settings.count; ++k) {
+        const double depth = settings.out ? k + t : k + 1.0 - t;
+        const double weight = settings.fade * std::clamp(depth, 0.0, 1.0) *
+                              std::clamp(settings.count - depth, 0.0, 1.0);
+        const int mix = static_cast<int>(weight * 256.0 + 0.5);
+        if (mix <= 0) continue;
+        Echo echo{};
+        sinCosDegrees(settings.rotate * depth, echo.sine, echo.cosine);
+        echo.shrink = 1.0 / std::pow(settings.scale, depth);
+        echo.dx = settings.drift * depth * driftCosine;
+        echo.dy = settings.drift * depth * driftSine;
+        echo.mix = mix;
+        echoes.push_back(echo);
+    }
+    if (echoes.empty()) return input;
+
+    // The echoes are laid largest first, so the deeper ones nest inside.
+    const double cx = (width - 1) * 0.5;
+    const double cy = (height - 1) * 0.5;
+    Image frame(width, height);
+    parallelFor(static_cast<std::size_t>(height), [&](std::size_t begin, std::size_t end) {
+        for (std::size_t row = begin; row < end; ++row) {
+            const std::uint8_t* base = input.row(static_cast<int>(row));
+            std::uint8_t* out = frame.row(static_cast<int>(row));
+            for (int col = 0; col < width; ++col) {
+                const std::size_t px = static_cast<std::size_t>(col) * 3;
+                int colour[3] = {base[px], base[px + 1], base[px + 2]};
+                for (const Echo& echo : echoes) {
+                    const double rx = col - cx - echo.dx;
+                    const double ry = static_cast<double>(row) - cy - echo.dy;
+                    const double sx = cx + (rx * echo.cosine + ry * echo.sine) * echo.shrink;
+                    const double sy = cy + (-rx * echo.sine + ry * echo.cosine) * echo.shrink;
+                    if (sx < 0.0 || sy < 0.0 || sx > width - 1 || sy > height - 1) continue;
+                    std::uint8_t echoed[3];
+                    sampleBilinear(input, sx, sy, echoed);
+                    for (int c = 0; c < 3; ++c) {
+                        colour[c] += (echoed[c] - colour[c]) * echo.mix / 256;
+                    }
+                }
+                for (int c = 0; c < 3; ++c) {
+                    out[px + static_cast<std::size_t>(c)] = static_cast<std::uint8_t>(colour[c]);
+                }
+            }
+        }
+    });
+    return frame;
+}
+
+}  // namespace effector

src/effects/tunnel.hpp

@@ -0,0 +1,30 @@
+#pragma once
+
+#include "effects/video_effect.hpp"
+
+namespace effector {
+
+// The feedback effect in motion: the picture nested inside itself, each copy
+// smaller, turned and drifted further than the one around it, and the whole
+// stack flying inwards (or outwards) without end.
+//
+// A copy sits at a depth, and its size, turn and drift follow from that depth
+// alone. Every frame moves each copy on by part of one step, so after a whole
+// cycle each copy lies exactly where its neighbour started: a new one fades
+// in at the far end while the nearest fades out into the picture itself, and
+// the clip loops without a jump.
+class TunnelEffect : public VideoEffect {
+public:
+    std::string name() const override { return "tunnel"; }
+    std::string title() const override { return "Echo tunnel"; }
+    std::string description() const override {
+        return "Nests the picture inside itself, as Feedback does, and flies through the echoes "
+               "without end.";
+    }
+    const std::vector<ParamSpec>& params() const override;
+
+protected:
+    Image animate(Image input, const ParamValues& values, const Moment& at) const override;
+};
+
+}  // namespace effector