Make Tile swap slow at a low Swap rate, on a curved slider

A tile has to come back to the cut it started with for the clip to
loop, so every tile swapped at least twice a clip, and below that
rounding left every tile still: the bottom of the slider was either
frozen or busy. Swap rate is now the average per tile: below two swaps
a clip only a share of the tiles swap (twice each), fewer the lower it
goes, and above it each tile rounds up or down by its own draw.

ParamSpec::slider() takes an optional curve: the scale runs over the
slider's travel and sliderValue/sliderPosition map it onto the number
as position^curve. Swap rate uses a curve of 3, so the low end gets
most of the slider; the command line and project file are unchanged.

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

CHANGELOG.md

@@ -4,6 +4,19 @@ Every release of Effector, newest first. The version lives in one place, the
 `project()` line of `CMakeLists.txt`, and each release is a `vX.Y.Z` tag on the
 commit that made it. The interface shows this file under Changelog in its menu.
 
+## [0.8.2] - 2026-09-30
+
+### Changed
+- The Swap rate of Tile swap is a slider marked with its range, and goes
+  down in steps of 0.01 rather than 0.1, so a clip can swap its tiles much
+  more slowly. Its top end is now 5 swaps a second (it was 30).
+- At a low Swap rate, Tile swap now moves only some of its tiles, fewer the
+  lower the rate, instead of swapping every tile at least twice a clip (or,
+  below that, none at all). Each step down the bottom of the slider now
+  looks slower than the one above it.
+- The Swap rate slider is curved: its low end takes up most of its length
+  (1 swap a second sits just past the middle), so slow rates are easy to set.
+
 ## [0.8.1] - 2026-09-26
 
 ### Added

CLAUDE.md

@@ -80,6 +80,12 @@ travel as `ParamValues` (a
 `map<string, variant<long,double,bool,string>>`), read back with
 `paramInt`/`paramDouble`/`paramBool`/`paramChoice`.
 
+An Int or Double param marked `.slider()` is shown in
+the interface as a slider with its minimum and maximum marked instead of a spin
+button; nothing else changes. `.slider(curve)` bends it: the scale runs over the
+slider's travel and `sliderValue`/`sliderPosition` map that onto the number as
+`position^curve`, so a curve above 1 gives the low end more room.
+
 A param can also be marked `.styled()`, which says a *named style* carries it
 rather than the stage it sits on: the font and the colour of a text stage, but
 never its words or its placement. The pool a style comes from is named by

CMakeLists.txt

@@ -1,5 +1,5 @@
 cmake_minimum_required(VERSION 3.20)
-project(effector VERSION 0.8.1 LANGUAGES CXX)
+project(effector VERSION 0.8.2 LANGUAGES CXX)
 
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)

packaging/arch/PKGBUILD

@@ -2,7 +2,7 @@
 # so pkgver must follow the project() line of CMakeLists.txt (the version core
 # test checks it). From this folder: makepkg -si
 pkgname=effector
-pkgver=0.8.1
+pkgver=0.8.2
 pkgrel=1
 pkgdesc='Image effects app: a GTK4 interface and a headless CLI over pluggable effects'
 arch=('x86_64' 'aarch64')

src/core/params.cpp

@@ -1,6 +1,7 @@
 #include "core/params.hpp"
 
 #include <algorithm>
+#include <cmath>
 
 namespace effector {
 
@@ -16,6 +17,27 @@ ParamSpec ParamSpec::styled() const {
     return copy;
 }
 
+ParamSpec ParamSpec::slider(double curve) const {
+    ParamSpec copy = *this;
+    copy.onSlider = true;
+    copy.curve = std::max(1e-3, curve);
+    return copy;
+}
+
+double ParamSpec::sliderValue(double position) const {
+    const double span = maximum - minimum;
+    const double raw = minimum + span * std::pow(std::clamp(position, 0.0, 1.0), curve);
+    const double unit = type == Type::Int ? std::max(1.0, std::round(step)) : step;
+    const double stepped = unit > 0.0 ? minimum + std::round((raw - minimum) / unit) * unit : raw;
+    return std::clamp(stepped, minimum, maximum);
+}
+
+double ParamSpec::sliderPosition(double value) const {
+    const double span = maximum - minimum;
+    if (span <= 0.0) return 0.0;
+    return std::pow(std::clamp((value - minimum) / span, 0.0, 1.0), 1.0 / curve);
+}
+
 ParamSpec ParamSpec::makeInt(std::string key, std::string label, long defaultValue, long minimum,
                              long maximum, std::string help, long step) {
     ParamSpec spec;

src/core/params.hpp

@@ -27,6 +27,13 @@ struct ParamSpec {
     // Whether a named style carries this param rather than the stage it sits
     // on: see core/style.hpp.
     bool inStyle = false;
+    // Whether the interface shows this number as a slider marked with its
+    // range rather than as a spin button: see slider().
+    bool onSlider = false;
+    // How a slider's travel is spread over its range: the value is
+    // minimum + (maximum - minimum) * position^curve, so 1 is even and a
+    // larger curve gives the low end more of the slider. See slider().
+    double curve = 1.0;
 
     std::string flag() const;  // "strip_height" -> "--strip-height"
 
@@ -35,6 +42,18 @@ struct ParamSpec {
     // signatures they have.
     ParamSpec styled() const;
 
+    // A copy of this Int or Double spec shown as a slider in the interface, for
+    // a number whose place between its minimum and maximum says more than its
+    // digits do. A `curve` above 1 spreads the low end of the range over more
+    // of the slider, for a number whose small values differ the most. The
+    // command line and the project file are unchanged.
+    ParamSpec slider(double curve = 1.0) const;
+
+    // Where a slider at `position` (0 to 1) puts this number, rounded to its
+    // step and kept in range, and where on the slider `value` sits.
+    double sliderValue(double position) const;
+    double sliderPosition(double value) const;
+
     static ParamSpec makeInt(std::string key, std::string label, long defaultValue, long minimum,
                              long maximum, std::string help = {}, long step = 1);
     static ParamSpec makeDouble(std::string key, std::string label, double defaultValue,

src/effects/tileswap.cpp

@@ -25,10 +25,12 @@ const std::vector<ParamSpec>& TileSwapEffect::params() const {
                            "How many tiles are dropped on the canvas; scattered layout only."),
         ParamSpec::makeBool("tileswap_keep_order", "Cycle images in order", false,
                             "Take tiles from the images in order instead of picking randomly."),
-        ParamSpec::makeDouble("tileswap_rate", "Swap rate", 1.0, 0.0, 30.0,
-                              "Swaps per second of each tile, rounded so that the clip loops "
-                              "without a jump; 0 for none.",
-                              0.1, 1),
+        ParamSpec::makeDouble("tileswap_rate", "Swap rate", 1.0, 0.0, 5.0,
+                              "Swaps per second of each tile, on average; 0 for none. Below two "
+                              "swaps in the clip only some of the tiles swap, fewer the lower it "
+                              "goes, so that the clip still loops without a jump.",
+                              0.01, 2)
+            .slider(3.0),
     };
     return specs;
 }
@@ -56,8 +58,8 @@ struct Settings {
 
 double unit(Rng& rng) { return static_cast<double>(rng.next() >> 11) * 0x1.0p-53; }
 
-// Where a tile sits for the whole clip, when in its swaps it turns over, and
-// the seed its cuts come from.
+// Where a tile sits for the whole clip, when in its swaps it turns over, the
+// seed its cuts come from, and the draw that settles how often it swaps.
 struct Cell {
     int x;
     int y;
@@ -65,6 +67,7 @@ struct Cell {
     int h;
     double phase;
     std::uint64_t seed;
+    double share;
 };
 
 }  // namespace
@@ -80,7 +83,7 @@ Image TileSwapEffect::compose(const std::vector<Image>& sources, int width, int
     Rng steady = at.steady();
     std::vector<Cell> cells;
     auto addCell = [&](int x, int y, int w, int h) {
-        cells.push_back(Cell{x, y, w, h, 0.0, 0});
+        cells.push_back(Cell{x, y, w, h, 0.0, 0, 0.0});
     };
     if (scatter) {
         const Image& src = sources[steady.below(sources.size() - 1)];
@@ -119,27 +122,37 @@ Image TileSwapEffect::compose(const std::vector<Image>& sources, int width, int
     for (Cell& cell : cells) {
         cell.phase = unit(steady);
         cell.seed = steady.next();
+        cell.share = unit(steady);
     }
 
-    // Where this frame is in time: each tile has swapped a whole number of
-    // times, counted round the clip so that the frame after the last shows
-    // what the first does.
-    long swaps = 0;
-    double perSecond = 0.0;
-    if (settings.rate > 0.0) {
-        perSecond = at.loopRate(settings.rate);
-        swaps = std::max(1L, std::lround(perSecond * at.length()));
-    }
+    // How often each tile swaps in one pass of the clip, `each` on average. A
+    // tile has to come back to the cut it started with, so one that swaps at
+    // all swaps at least twice; below that a share of the tiles swap twice and
+    // the rest hold still, so a low rate thins out the tiles that move instead
+    // of moving every one of them. Above it, each tile rounds `each` up or
+    // down by its own draw.
+    const double length = at.length();
+    const double each = settings.rate * length;
+    auto swapsOf = [&](const Cell& cell) -> long {
+        if (each <= 0.0) return 0;
+        if (each < 2.0) return cell.share < each / 2.0 ? 2 : 0;
+        const double whole = std::floor(each);
+        return static_cast<long>(whole) + (cell.share < each - whole ? 1 : 0);
+    };
 
-    // What a tile shows depends only on the tile and its swap, so it holds
-    // still between swaps and any frame can be drawn on its own.
+    // What a tile shows depends only on the tile and how many times it has
+    // swapped, counted round the clip so that the frame after the last shows
+    // what the first does; so it holds still between swaps and any frame can
+    // be drawn on its own.
     std::vector<Tile> plan;
     plan.reserve(cells.size());
     for (std::size_t i = 0; i < cells.size(); ++i) {
         const Cell& cell = cells[i];
+        const long swaps = swapsOf(cell);
         std::uint64_t turn = 0;
         if (swaps > 0) {
-            const auto passed = static_cast<long>(std::floor(at.seconds() * perSecond + cell.phase));
+            const auto passed =
+                static_cast<long>(std::floor(at.seconds() * swaps / length + cell.phase));
             turn = static_cast<std::uint64_t>(passed % swaps);
         }
         Rng cut(cell.seed + turn * 0x9e3779b97f4a7c15ULL);

src/gui/window.cpp

@@ -5,6 +5,7 @@
 #include <cmath>
 #include <cstdint>
 #include <filesystem>
+#include <iomanip>
 #include <sstream>
 #include <utility>
 
@@ -74,14 +75,14 @@ Gtk::Label* sectionLabel(const char* text) {
     return label;
 }
 
-// A GtkSpinButton takes every wheel turn over it: it grabs the focus and
-// spins its value, so scrolling down a column of params stops dead at the
-// first spin under the pointer and quietly changes it. This hands the wheel to
+// A GtkSpinButton (or a GtkScale) takes every wheel turn over it: it grabs the
+// focus and spins its value, so scrolling down a column of params stops dead at
+// the first spin under the pointer and quietly changes it. This hands the wheel to
 // the view instead unless the spin already has the focus -- the user clicked
 // into it and means to adjust it. It runs in the capture phase, ahead of the
 // spin's own controller, and so has to scroll the enclosing ScrolledWindow
 // itself: stopping the event there stops it for the window as well.
-void scrollPastUnlessFocused(Gtk::SpinButton& spin) {
+void scrollPastUnlessFocused(Gtk::Widget& spin) {
     auto scroll = Gtk::EventControllerScroll::create();
     scroll->set_propagation_phase(Gtk::PropagationPhase::CAPTURE);
     scroll->set_flags(Gtk::EventControllerScroll::Flags::VERTICAL);
@@ -950,14 +951,42 @@ void EffectorWindow::buildParamWidgets() {
             }
             case ParamSpec::Type::Int:
             case ParamSpec::Type::Double: {
+                const double value =
+                    spec.type == ParamSpec::Type::Int
+                        ? static_cast<double>(
+                              paramInt(values, spec.key, static_cast<long>(spec.defaultValue)))
+                        : paramDouble(values, spec.key, spec.defaultValue);
+                if (spec.onSlider) {
+                    // The scale runs over the slider's travel, 0 to 1, and
+                    // the spec maps that onto the number, so a curved slider
+                    // gives its low end more room. The ends are marked with
+                    // their values, so where the number sits in its range can
+                    // be read at a glance.
+                    const int digits = spec.type == ParamSpec::Type::Int ? 0 : spec.digits;
+                    auto scale = std::make_unique<Gtk::Scale>(Gtk::Orientation::HORIZONTAL);
+                    scale->set_range(0.0, 1.0);
+                    scale->set_increments(0.01, 0.1);
+                    scale->set_draw_value(true);
+                    scale->set_value_pos(Gtk::PositionType::RIGHT);
+                    const auto mark = [digits](double at) {
+                        return Glib::ustring::format(std::fixed, std::setprecision(digits), at);
+                    };
+                    scale->set_format_value_func(
+                        [spec, mark](double position) { return mark(spec.sliderValue(position)); });
+                    scale->add_mark(0.0, Gtk::PositionType::BOTTOM, mark(spec.minimum));
+                    scale->add_mark(1.0, Gtk::PositionType::BOTTOM, mark(spec.maximum));
+                    scale->set_value(spec.sliderPosition(value));
+                    scale->signal_value_changed().connect(
+                        sigc::mem_fun(*this, &EffectorWindow::onParamChanged));
+                    scrollPastUnlessFocused(*scale);
+                    widget = std::move(scale);
+                    break;
+                }
                 auto spin = std::make_unique<Gtk::SpinButton>();
                 spin->set_range(spec.minimum, spec.maximum);
                 spin->set_increments(spec.step, spec.step * 10);
                 spin->set_digits(spec.type == ParamSpec::Type::Int ? 0 : spec.digits);
-                spin->set_value(spec.type == ParamSpec::Type::Int
-                                    ? static_cast<double>(paramInt(values, spec.key,
-                                                                  static_cast<long>(spec.defaultValue)))
-                                    : paramDouble(values, spec.key, spec.defaultValue));
+                spin->set_value(value);
                 spin->signal_value_changed().connect(
                     sigc::mem_fun(*this, &EffectorWindow::onParamChanged));
                 scrollPastUnlessFocused(*spin);
@@ -1065,12 +1094,18 @@ ParamValues EffectorWindow::paramValues() const {
                 values[entry.spec.key] = static_cast<Gtk::Switch*>(entry.widget)->get_active();
                 break;
             case ParamSpec::Type::Int:
-                values[entry.spec.key] =
-                    static_cast<long>(static_cast<Gtk::SpinButton*>(entry.widget)->get_value());
-                break;
-            case ParamSpec::Type::Double:
-                values[entry.spec.key] = static_cast<Gtk::SpinButton*>(entry.widget)->get_value();
+            case ParamSpec::Type::Double: {
+                const double value =
+                    entry.spec.onSlider
+                        ? entry.spec.sliderValue(static_cast<Gtk::Scale*>(entry.widget)->get_value())
+                        : static_cast<Gtk::SpinButton*>(entry.widget)->get_value();
+                if (entry.spec.type == ParamSpec::Type::Int) {
+                    values[entry.spec.key] = std::lround(value);
+                } else {
+                    values[entry.spec.key] = value;
+                }
                 break;
+            }
             case ParamSpec::Type::Path:
             case ParamSpec::Type::Text:
                 values[entry.spec.key] =

tests/core_tests.cpp

@@ -4,8 +4,10 @@
 #include <array>
 #include <atomic>
 #include <clocale>
+#include <cmath>
 #include <cstdio>
 #include <cstdlib>
+#include <cstring>
 #include <filesystem>
 #include <fstream>
 #include <iostream>
@@ -2079,6 +2081,37 @@ int main() {
             frozenSwap.values["tileswap_rate"] = 0.0;
             check(sameBytes(frameOf(frozenSwap, 0), frameOf(frozenSwap, 5)),
                   "at a rate of 0 it holds still");
+            // How many pixels ever show something other than frame 0.
+            auto moving = [&](const ChainStage& at) {
+                const auto first = frameOf(at, 0);
+                std::vector<bool> moved(static_cast<std::size_t>(80 * 48), false);
+                for (int index = 1; index < 8; ++index) {
+                    const auto frame = frameOf(at, index);
+                    for (int y = 0; y < 48; ++y) {
+                        for (int x = 0; x < 80; ++x) {
+                            if (std::memcmp(frame.row(y) + x * 3, first.row(y) + x * 3, 3) != 0) {
+                                moved[static_cast<std::size_t>(y * 80 + x)] = true;
+                            }
+                        }
+                    }
+                }
+                return std::count(moved.begin(), moved.end(), true);
+            };
+            ChainStage slowSwap = swapStage;
+            slowSwap.values["tileswap_rate"] = 0.3;
+            const auto slowMoving = moving(slowSwap);
+            check(slowMoving > 0 && slowMoving < moving(swapStage),
+                  "a rate below two swaps a clip moves only some of the tiles");
+
+            const auto& rate = swap->params().back();
+            check(rate.onSlider && rate.curve > 1.0, "its swap rate is a curved slider");
+            check(rate.sliderValue(0.0) == 0.0 && rate.sliderValue(1.0) == 5.0 &&
+                      rate.sliderValue(0.5) < 2.5,
+                  "which spans the whole range and gives the low end more travel");
+            check(std::abs(rate.sliderValue(rate.sliderPosition(0.37)) - 0.37) < 1e-9 &&
+                      rate.sliderValue(0.123456) * 100.0 ==
+                          std::round(rate.sliderValue(0.123456) * 100.0),
+                  "a value finds its place on the slider, and a place rounds to the step");
             ChainStage scattered = swapStage;
             scattered.values["tileswap_layout"] = std::string("scatter");
             ChainStage bricks = swapStage;

tests/gui_tests.cpp

@@ -7,6 +7,7 @@
 #include <chrono>
 #include <cstdlib>
 #include <filesystem>
+#include <iomanip>
 #include <iostream>
 #include <memory>
 #include <string>
@@ -633,6 +634,51 @@ void testFont(Gtk::Window& window, const std::filesystem::path& dir) {
           "a family chosen there is saved with the project");
 }
 
+void testSlider(Gtk::Window& window, const std::filesystem::path& dir) {
+    std::cout << "slider\n";
+    std::filesystem::create_directories(dir);
+    const auto path = dir / "swapping.json";
+    effector::Project project;
+    project.video.enabled = true;
+    effector::ChainStage stage{effector::effectByName("tileswap"), {}};
+    stage.values["tileswap_rate"] = 0.5;
+    project.stages.push_back(stage);
+    effector::saveProject(path.string(), project);
+    window.activate_action("win.open-recent",
+                           Glib::Variant<Glib::ustring>::create(Glib::filename_to_uri(path.string())));
+    pump(300);
+
+    auto* chainList = named<Gtk::ListBox>(window, "chain-list");
+    if (chainList != nullptr && chainList->get_row_at_index(0) != nullptr) {
+        chainList->select_row(*chainList->get_row_at_index(0));
+    }
+    pump(100);
+    std::vector<Gtk::Scale*> scales;
+    collect<Gtk::Scale>(window, [](Gtk::Scale& s) { return s.is_drawable(); }, scales);
+    check(scales.size() == 1, "a param marked as a slider is shown as one");
+    if (scales.size() != 1) return;
+    auto* scale = scales.front();
+    const auto& spec = effector::effectByName("tileswap")->params().back();
+    // Written the way the locale writes numbers, as the interface does.
+    const auto mark = [](double at) {
+        return std::string(Glib::ustring::format(std::fixed, std::setprecision(2), at));
+    };
+    check(showsText(*scale, mark(0.0)) && showsText(*scale, mark(5.0)),
+          "with its range marked at both ends");
+    check(spec.sliderValue(scale->get_value()) == 0.5, "and shows the value saved");
+    check(scale->get_value() > 0.5 / 5.0 + 0.3,
+          "a curved slider gives the low end of its range more of its travel");
+
+    scale->set_value(spec.sliderPosition(0.05));
+    pump(100);
+    window.activate_action("win.save-project");
+    pump(100);
+    const auto saved = effector::loadProject(path.string());
+    check(saved.stages.size() == 1 &&
+              std::abs(effector::paramDouble(saved.stages[0].values, "tileswap_rate", 0.0) - 0.05) < 1e-9,
+          "a value slid to is saved with the project");
+}
+
 bool actionEnabled(Gtk::Window& window, const std::string& name) {
     auto action = dynamic_cast<Gtk::ApplicationWindow&>(window).lookup_action(name);
     return action && action->get_enabled();
@@ -822,6 +868,7 @@ int main(int argc, char** argv) {
         testRecentProjects(*window, dataDir / "projects");
         testVideo(*window, dataDir / "video");
         testFont(*window, dataDir / "font");
+        testSlider(*window, dataDir / "slider");
         testHistory(*window, dataDir / "history");
         testMemoryGuard(*window, dataDir / "memory");
         app->remove_window(*window);