Merge pS-fa3-waveform: gap-free per-column min/max waveform render (columnMinMax in peaks, shared across panel + editor)

src/bank_panel.cpp

@@ -13,7 +13,8 @@
 //     LICE-drawn named-banks tab-page region below (one tab per named bank, an
 //     overflow/scroll strip), and two full-height toggles that collapse the split.
 //     Each region reuses the M5 grid render loop (waveform thumbnails / empty state).
-//   * per-sample PCM read via PCM_source fed to peaks::computeEnvelope at cell width.
+//   * per-sample PCM read via PCM_source fed to peaks::computeEnvelope, one bin per
+//     drawn pixel column; drawWaveform's gap-free render comes from peaks::columnMinMax.
 //   * an in-memory thumbnail cache keyed by (sample id, draw width, bank generation).
 //   * id-keyed bank management (create / rename / delete / evacuate / activate) and
 //     sample move/copy — driven from a tab context menu and a drag — against the B1
@@ -416,7 +417,7 @@ std::vector<const Bank*> namedBanks() {
     return out;
 }
 
-// --- Thumbnail computation (unchanged from M5) --------------------------------
+// --- Thumbnail computation (M5; `width` is a BIN count since FA3 oversampling) --
 
 Envelope computeThumbnail(const std::string& absPath, int width) {
     if (width <= 0 || absPath.empty()) return {};
@@ -458,9 +459,13 @@ Envelope computeThumbnail(const std::string& absPath, int width) {
     for (std::size_t i = 0; i < sampleCount; ++i)
         pcm[i] = static_cast<float>(buf[i]);
 
+    // Clamp bins to the frame count: computeEnvelope pads binCount > frameCount with
+    // trailing empty {0,0} bins, which would render a very short sample as a comb of
+    // spikes over flat gaps.
+    const int binCount = width < got ? width : got;
     return computeEnvelope(pcm, static_cast<std::size_t>(nch),
                            static_cast<std::size_t>(got),
-                           static_cast<std::size_t>(width));
+                           static_cast<std::size_t>(binCount));
 }
 
 const Envelope& thumbnailFor(const Sample& sample, int width,
@@ -479,7 +484,7 @@ const Envelope& thumbnailFor(const Sample& sample, int width,
     return ins.first->second.envelope;
 }
 
-// --- Drawing: thumbnails (unchanged from M5) ----------------------------------
+// --- Drawing: thumbnails (via the kit's shared drawWaveform since FA3) ---------
 
 // Draws the L7 decorative metadata overlay on a card: bars.beats.subdivisions bottom-LEFT
 // (musical, from the capture-time tempo + meter stamp) and seconds.milliseconds bottom-RIGHT
@@ -529,45 +534,11 @@ void drawThumbnail(LICE_IBitmap* bmp, const CellRect& rect, const Envelope& env,
         LICE_DrawRect(bmp, rect.x + 1, rect.y + 1, rect.width - 2, rect.height - 2, ring, 1.0f, 0);
     }
 
-    // Waveform plot (peaks invariant: min<=max). The wave keeps its NORMAL accent color in
-    // every state (L7 dropped the inverted bg/base wave on the selected cell — the cell fill
-    // is no longer inverted, so no contrast swap is needed).
-    const LICE_pixel midCol = toLice(roleColor(Role::LineHairline));
-    const LICE_pixel waveCol = toLice(roleColor(Role::AccentPrimary));
-
-    if (env.empty()) {
-        const int midY = rect.y + rect.height / 2;
-        LICE_Line(bmp, rect.x + 2, midY, rect.x + rect.width - 2, midY, midCol, 1.0f, 0, false);
-        if (sample) drawCardMeta(bmp, rect, *sample);  // L7 overlay even on an empty envelope
-        return;
-    }
-
-    const int channels = static_cast<int>(env.size());
-    const int bandH = rect.height / channels;
-
-    for (int ch = 0; ch < channels; ++ch) {
-        const ChannelEnvelope& bins = env[ch];
-        const int bandTop = rect.y + ch * bandH;
-        const int midY = bandTop + bandH / 2;
-        const double halfSpan = (bandH / 2) - 2;
-
-        LICE_Line(bmp, rect.x + 2, midY, rect.x + rect.width - 2, midY, midCol, 1.0f, 0, false);
-
-        const int nbins = static_cast<int>(bins.size());
-        if (nbins <= 0) continue;
-
-        const int innerW = rect.width - 4;
-        for (int i = 0; i < nbins; ++i) {
-            const int x = rect.x + 2 + (nbins > 1 ? (i * (innerW - 1)) / (nbins - 1) : 0);
-            // min<=max always (peaks invariant). Draw a vertical line from the
-            // min sample to the max sample, clamped to the band.
-            int yMax = midY - static_cast<int>(compressAmplitudeForDisplay(bins[i].max) * halfSpan);  // max -> up
-            int yMin = midY - static_cast<int>(compressAmplitudeForDisplay(bins[i].min) * halfSpan);  // min -> down
-            if (yMax < bandTop) yMax = bandTop;
-            if (yMin > bandTop + bandH - 1) yMin = bandTop + bandH - 1;
-            LICE_Line(bmp, x, yMin, x, yMax, waveCol, 1.0f, 0, false);
-        }
-    }
+    // Waveform plot through the kit's shared primitive (FA3): the SAME per-pixel-column
+    // min/max envelope draw the VST editor hero + browser cards use — one algorithm, one
+    // look, everywhere. The oversampled env (see drawRegionGrid's binWidth) collapses per
+    // column via peaks::columnMinMax inside the kit; an empty env draws just the midline.
+    drawWaveform(bmp, cell, env);
 
     // L7 decorative metadata overlay, drawn last so it sits over the waveform.
     if (sample) drawCardMeta(bmp, rect, *sample);
@@ -1379,7 +1350,12 @@ void drawRegionGrid(LICE_IBitmap* bmp, const RECT& region, bool isBanks,
     // BankIndex insertion order. Selection/focus are keyed by the occupied-ordinal (selection
     // space); a slot maps back to its ordinal via selectionForSlot.
     const RegionDisplay disp = regionDisplay(region, isBanks, reg);
-    const int binWidth = kGrid.cellWidth - 4;
+    // FA3 gap-free: request one bin per drawn pixel column; drawWaveform's
+    // peaks::columnMinMax exact partition makes every column gap-free — overbinning
+    // produces byte-identical pixels at higher memory/CPU cost. computeThumbnail clamps
+    // the request to the frame count.
+    const int binWidth = kWaveformOversample *
+                         waveformColumnCount(KitBox{0, 0, kGrid.cellWidth, kGrid.cellHeight});
     for (const SlotCellRect& r : disp.slotRects) {
         if (r.y >= grid.bottom) continue;   // below the viewport: skip (no scroll)
         const CellRect rect{r.x, r.y, r.width, r.height};

src/component_geometry.cpp

@@ -100,4 +100,9 @@ int hitTestListRow(int px, int py, const KitBox& list, int rowHeight, int rowCou
     return row;
 }
 
+int waveformColumnCount(const KitBox& box) {
+    const int w = box.width - 4;  // fixed 2px inset each side (matches drawWaveform)
+    return w > 0 ? w : 0;
+}
+
 } // namespace reasampler

src/component_geometry.h

@@ -126,4 +126,14 @@ ListRowBox computeListRow(const KitBox& list, int index, int rowHeight);
 // a phantom row. Half-open bounds match computeListRow so the hit maps to the drawn row.
 int hitTestListRow(int px, int py, const KitBox& list, int rowHeight, int rowCount);
 
+// --- Waveform column count ---------------------------------------------------
+//
+// The number of pixel columns drawWaveform renders inside `box` (its fixed 2px side
+// insets), never negative. Callers pass this count directly as the `binCount` argument to
+// peaks::computeEnvelope — one bin per column is the correct resolution, and
+// peaks::columnMinMax's exact partition makes the render gap-free at any bins-to-pixels
+// ratio. Overbinning does NOT improve render quality (columnMinMax's frame union is
+// identical whether bins == columns or bins == k*columns) and wastes memory and CPU.
+int waveformColumnCount(const KitBox& box);
+
 } // namespace reasampler

src/draw_kit.cpp

@@ -292,6 +292,8 @@ void drawWaveform(LICE_IBitmap* bmp, const KitBox& box, const Envelope& env) {
 
     const int channels = static_cast<int>(env.size());
     const int bandH = box.height / channels;
+    const int innerW = waveformColumnCount(box);  // columns: box.x+2 .. box.x+2+innerW-1
+
     for (int ch = 0; ch < channels; ++ch) {
         const ChannelEnvelope& bins = env[static_cast<std::size_t>(ch)];
         const int bandTop = box.y + ch * bandH;
@@ -301,19 +303,22 @@ void drawWaveform(LICE_IBitmap* bmp, const KitBox& box, const Envelope& env) {
         LICE_Line(bmp, box.x + 2, midY, box.x + box.width - 2, midY,
                   midCol, 1.0f, 0, false);
 
-        const int nbins = static_cast<int>(bins.size());
-        if (nbins <= 0) continue;
+        if (bins.empty() || innerW <= 0) continue;
 
-        const int innerW = box.width - 4;
-        for (int i = 0; i < nbins; ++i) {
-            const int x = box.x + 2 +
-                          (nbins > 1 ? (i * (innerW - 1)) / (nbins - 1) : 0);
-            // Same dB display compression as the panel thumbnail (bank_grid, pure) so a
-            // waveform reads identically wherever the kit draws it.
+        // Render one filled vertical span per pixel column. peaks::columnMinMax merges
+        // all bins that project to column `col` under the exact same partition as
+        // computeEnvelope used to build the envelope, so every pixel column is covered
+        // with no gaps regardless of the bins-to-pixels ratio. With one bin per column
+        // (kWaveformOversample == 1) each span covers the true min/max of exactly the
+        // frames that fall in that column. Same dB display compression everywhere
+        // (bank_grid, pure).
+        for (int col = 0; col < innerW; ++col) {
+            const MinMax mm = columnMinMax(bins, innerW, col);
+            const int x = box.x + 2 + col;
             int yMax = midY - static_cast<int>(
-                compressAmplitudeForDisplay(bins[static_cast<std::size_t>(i)].max) * halfSpan);
+                compressAmplitudeForDisplay(mm.max) * halfSpan);
             int yMin = midY - static_cast<int>(
-                compressAmplitudeForDisplay(bins[static_cast<std::size_t>(i)].min) * halfSpan);
+                compressAmplitudeForDisplay(mm.min) * halfSpan);
             if (yMax < bandTop) yMax = bandTop;
             if (yMin > bandTop + bandH - 1) yMin = bandTop + bandH - 1;
             LICE_Line(bmp, x, yMin, x, yMax, waveCol, 1.0f, 0, false);

src/draw_kit.h

@@ -115,12 +115,28 @@ void drawSlider(LICE_IBitmap* bmp, const SliderGeometry& geom, InteractionState
 void drawListRow(LICE_IBitmap* bmp, const ListRowBox& row, const char* label,
                  int thumbWidth, InteractionState state);
 
-// A waveform envelope drawn as a min/max column plot over the bg/panel surface: a midline
-// per channel and an accent vertical line per bin (the same shape the panel thumbnail
-// draws, lifted into the kit so the panel and the L3 editor waveform share it). `box` is
-// the draw region; `env` is the per-channel min/max envelope from peaks::computeEnvelope.
-// An empty env draws just the midline. The caller fills the surface first (or passes a box
-// already filled); this draws only the wave + midline.
+// waveformColumnCount — declared in component_geometry.h (already included above). Returns
+// the drawable column count inside `box` (box.width minus the fixed 2px insets each side).
+// Callers pass this value directly as the `binCount` argument to peaks::computeEnvelope;
+// overbinning (more bins than columns) costs memory and CPU without changing a rendered
+// pixel — peaks::columnMinMax's exact partition already makes the draw gap-free.
+
+// Multiplier kept at 1 (no oversampling). kWaveformOversample is present only so existing
+// call sites `kWaveformOversample * waveformColumnCount(box)` compile unchanged; a value of
+// 1 means they request exactly one bin per column, which is correct. The gap-free render
+// comes from peaks::columnMinMax's exact partition, NOT from extra bins.
+inline constexpr int kWaveformOversample = 1;
+
+// A waveform envelope drawn as a min/max plot over the bg/panel surface: a midline per
+// channel and one accent vertical span PER PIXEL COLUMN, each column covering the true
+// extremes of every bin that projects to it (peaks::columnMinMax — gap-free at any
+// bins-to-pixels ratio because columnMinMax partitions bins exactly as computeEnvelope
+// does, so every pixel column is always covered). The ONE waveform shape in the system:
+// the dock-panel thumbnail, the browser cards, and the editor hero all render through
+// this. `box` is the draw region; `env` is the per-channel min/max envelope from
+// peaks::computeEnvelope, sized to waveformColumnCount(box) bins (clamped to frame count).
+// An empty env draws just the midline. The caller fills the surface first (or passes a
+// box already filled); this draws only the wave + midline.
 void drawWaveform(LICE_IBitmap* bmp, const KitBox& box, const Envelope& env);
 
 } // namespace reasampler

src/peaks.cpp

@@ -3,6 +3,7 @@
 #include <algorithm>
 #include <climits>
 #include <cmath>
+#include <cstdint>
 
 // peaks implementation.
 //
@@ -64,6 +65,37 @@ Envelope computeEnvelope(const std::vector<AudioSample>& interleaved,
     return envelope;
 }
 
+MinMax columnMinMax(const ChannelEnvelope& bins, int columnCount, int col) {
+    const int nbins = static_cast<int>(bins.size());
+    if (columnCount <= 0 || nbins == 0) return MinMax{};
+
+    // Clamp col to [0, columnCount-1].
+    if (col < 0) col = 0;
+    if (col >= columnCount) col = columnCount - 1;
+
+    // Half-open bin range for this column, mirroring computeEnvelope's exact partition.
+    // 64-bit products: col*nbins can exceed int range for a large oversampled envelope
+    // (same overflow discipline as computeEnvelope's frame-span arithmetic above).
+    const std::int64_t begin64 = (static_cast<std::int64_t>(col) * nbins) / columnCount;
+    const std::int64_t end64 =
+        (static_cast<std::int64_t>(col) + 1) * nbins / columnCount;
+    // col <= columnCount-1 guarantees begin64 <= (columnCount-1)*nbins/columnCount < nbins.
+    const int colBinBegin = static_cast<int>(begin64);
+
+    // When the column spans no full bin (more columns than bins), use the enclosing bin
+    // so no column is left empty.
+    const int scanEnd = (end64 > begin64) ? static_cast<int>(end64) : colBinBegin + 1;
+    const int clampedEnd = (scanEnd <= nbins) ? scanEnd : nbins;
+
+    MinMax result = bins[static_cast<std::size_t>(colBinBegin)];
+    for (int b = colBinBegin + 1; b < clampedEnd; ++b) {
+        const MinMax& mm = bins[static_cast<std::size_t>(b)];
+        if (mm.min < result.min) result.min = mm.min;
+        if (mm.max > result.max) result.max = mm.max;
+    }
+    return result;
+}
+
 std::size_t lastFrameAboveThreshold(const std::vector<AudioSample>& interleaved,
                                     std::size_t channelCount,
                                     std::size_t frameCount,

src/peaks.h

@@ -66,6 +66,21 @@ Envelope computeEnvelope(const std::vector<AudioSample>& interleaved,
                          std::size_t frameCount,
                          std::size_t binCount);
 
+// The merged min/max for display column `col` (0-based, of `columnCount` total columns)
+// of a pre-computed per-bin ChannelEnvelope: the true extremes of every bin that projects
+// to that column. This is the display-side collapse of an envelope computed at HIGHER
+// resolution than the drawn width (oversampled bins -> per-pixel-column min/max), so a
+// steep transient whose adjacent bins hold disjoint spans (e.g. {0.9,1.0} then
+// {-1.0,-0.9}) renders as one gap-free vertical span instead of two separated dots.
+//
+// Bin->column mapping mirrors computeEnvelope's half-open partition:
+//   column col owns bins [col*nbins/columnCount, (col+1)*nbins/columnCount).
+// When that range is empty (more columns than bins), the enclosing bin
+// (col*nbins/columnCount) fills the column — so no column is left empty and no bin is
+// ever dropped. columnCount <= 0 or bins.empty() returns {0, 0}; `col` is clamped to
+// [0, columnCount-1]. Pure.
+MinMax columnMinMax(const ChannelEnvelope& bins, int columnCount, int col);
+
 // Sentinel returned by lastFrameAboveThreshold when NO frame in the scanned range
 // peaks above the threshold (pure silence at that level). SIZE_MAX is unambiguous:
 // no valid frame index can equal it (a real index is < frameCount <= SIZE_MAX for

src/vst/reasampler_editor.cpp

@@ -121,10 +121,12 @@ std::string sampleLabel(const std::vector<SampleChoice>& samples, const std::str
     return "?";
 }
 
-// The bin count a card's thumbnail is computed at: the card thumbnail width, so one bin
-// per horizontal pixel.
+// The bin count a card's thumbnail is computed at: one bin per drawn pixel column — the
+// gap-free render comes from peaks::columnMinMax's exact partition, not from extra bins.
+// thumbnailFor clamps the request to the decoded frame count.
 int thumbBins(const BrowserLayout& layout) {
-    return (std::max)(1, cardThumbnailRect(layout, 0).width());
+    return (std::max)(1, kWaveformOversample *
+                             waveformColumnCount(toKitBox(cardThumbnailRect(layout, 0))));
 }
 #endif
 }  // namespace
@@ -582,8 +584,12 @@ const Envelope& ReaSamplerEditor::thumbnailFor(const std::string& sampleId, int
     const std::vector<AudioSample>& mono = monoPcmFor(sampleId);
     Envelope env;
     if (!mono.empty()) {
-        env = computeEnvelope(mono, 1, mono.size(),
-                              static_cast<std::size_t>((std::max)(1, binCount)));
+        // Clamp bins to the frame count: computeEnvelope pads binCount > frameCount with
+        // trailing empty {0,0} bins, which would render a very short sample as a comb of
+        // spikes over flat gaps.
+        const std::size_t bins =
+            (std::min)(static_cast<std::size_t>((std::max)(1, binCount)), mono.size());
+        env = computeEnvelope(mono, 1, mono.size(), bins);
     }
     auto ins = thumbCache_.emplace(key, std::move(env));
     return ins.first->second;
@@ -1017,8 +1023,15 @@ void ReaSamplerEditor::paintSample(LICE_IBitmap* bmp, int w, int h) {
     const Rect waveArea = bands.hero;
     fillSurface(bmp, toKitBox(waveArea), Role::BgBase, InteractionState::Rest);
     if (frames > 0 && waveArea.width() > 0) {
-        const int bins = (std::max)(1, waveArea.width());
-        const Envelope env = computeEnvelope(pcm, 1, pcm.size(), static_cast<std::size_t>(bins));
+        // FA3 gap-free: one bin per drawn pixel column (kWaveformOversample == 1, so this
+        // multiplies by 1). The gap-free draw comes from peaks::columnMinMax's exact
+        // partition — extra bins produce no visible change. Clamped to frame count below.
+        const std::int64_t wantBins =
+            static_cast<std::int64_t>((std::max)(1, waveformColumnCount(toKitBox(waveArea)))) *
+            kWaveformOversample;
+        const std::size_t bins =
+            static_cast<std::size_t>(wantBins < frames ? wantBins : frames);
+        const Envelope env = computeEnvelope(pcm, 1, pcm.size(), bins);
         drawEnvelope(bmp, waveArea, env);
 
         const SetupMarkers m = pickedMarkers(frames);

tests/test_component_geometry.cpp

@@ -173,6 +173,22 @@ static void testListRowLayoutHitAgreement() {
     }
 }
 
+// --- waveformColumnCount -----------------------------------------------------
+
+static void testWaveformColumnCount() {
+    // Normal box: width minus the two 2px side insets.
+    CHECK(waveformColumnCount(KitBox{0, 0, 100, 40}) == 96);
+    CHECK(waveformColumnCount(KitBox{10, 5, 50, 20}) == 46);
+    // Minimum: a 5-wide box yields exactly 1 drawable column.
+    CHECK(waveformColumnCount(KitBox{0, 0, 5, 10}) == 1);
+    // Too narrow to have any drawable columns: 4 or fewer columns returns 0.
+    CHECK(waveformColumnCount(KitBox{0, 0, 4, 10}) == 0);
+    CHECK(waveformColumnCount(KitBox{0, 0, 1, 10}) == 0);
+    // Degenerate (empty) box returns 0 — graceful suppression.
+    CHECK(waveformColumnCount(KitBox{}) == 0);
+    CHECK(waveformColumnCount(KitBox{0, 0, 0, 40}) == 0);
+}
+
 int main() {
     testHitTestBoxHalfOpen();
     testButtonBoxInset();
@@ -190,6 +206,7 @@ int main() {
     testListRowHitTest();
     testListRowHitTestBoundedByCount();
     testListRowLayoutHitAgreement();
+    testWaveformColumnCount();
 
     if (g_fail == 0) std::printf("component_geometry: all tests passed\n");
     else std::printf("component_geometry: %d CHECK(s) FAILED\n", g_fail);

tests/test_peaks.cpp

@@ -6,7 +6,10 @@
 // ramp envelope monotonic across bins; DC/silence -> min==max; multi-channel
 // independence (no fold); short buffer (fewer frames than bins) and non-divisible
 // length (remainder bin); binCount==1 whole-buffer envelope; zero frames / zero
-// channels / binCount==0 degenerate inputs.
+// channels / binCount==0 degenerate inputs. Plus columnMinMax (the display-side
+// per-pixel-column collapse, FA3): 1:1 passthrough, upsample fallback, downsample
+// merge, steep disjoint-span merge, no-bin-dropped spike sweep, no-column-empty
+// coverage, col clamp, degenerate inputs.
 
 #include "../src/peaks.h"
 
@@ -348,6 +351,106 @@ static void testLastFrameDegenerate() {
     CHECK(lastFrameAboveThreshold(buf, 2, 100, 0.1f) == 1);
 }
 
+// --- columnMinMax (display-side per-pixel-column collapse, FA3) ----------------
+
+// Build a ChannelEnvelope from parallel min/max arrays.
+static ChannelEnvelope makeEnvelope(const std::vector<float>& mins,
+                                    const std::vector<float>& maxs) {
+    ChannelEnvelope env(mins.size());
+    for (std::size_t i = 0; i < mins.size(); ++i) env[i] = MinMax{mins[i], maxs[i]};
+    return env;
+}
+
+static void testColumnOneToOne() {
+    // 4 bins, 4 columns: each column is a pure passthrough of its one bin.
+    ChannelEnvelope env = makeEnvelope({-1.0f, -0.5f, 0.0f, 0.5f},
+                                       { 0.5f,  0.0f, 0.5f, 1.0f});
+    CHECK(columnMinMax(env, 4, 0).min == -1.0f && columnMinMax(env, 4, 0).max == 0.5f);
+    CHECK(columnMinMax(env, 4, 1).min == -0.5f && columnMinMax(env, 4, 1).max == 0.0f);
+    CHECK(columnMinMax(env, 4, 2).min ==  0.0f && columnMinMax(env, 4, 2).max == 0.5f);
+    CHECK(columnMinMax(env, 4, 3).min ==  0.5f && columnMinMax(env, 4, 3).max == 1.0f);
+}
+
+static void testColumnUpsampleFallback() {
+    // 2 bins, 4 columns: columns 0,1 fall back to enclosing bin 0; 2,3 to bin 1 —
+    // no column left empty when there are more columns than bins.
+    ChannelEnvelope env = makeEnvelope({-1.0f, 0.5f}, {0.0f, 1.0f});
+    CHECK(columnMinMax(env, 4, 0).min == -1.0f && columnMinMax(env, 4, 0).max == 0.0f);
+    CHECK(columnMinMax(env, 4, 1).min == -1.0f && columnMinMax(env, 4, 1).max == 0.0f);
+    CHECK(columnMinMax(env, 4, 2).min ==  0.5f && columnMinMax(env, 4, 2).max == 1.0f);
+    CHECK(columnMinMax(env, 4, 3).min ==  0.5f && columnMinMax(env, 4, 3).max == 1.0f);
+}
+
+static void testColumnDownsampleMerge() {
+    // 4 bins, 2 columns: each column is the true min/max union of its 2 bins.
+    ChannelEnvelope env = makeEnvelope({-1.0f, -0.5f, 0.0f, 0.5f},
+                                       { 0.5f,  0.0f, 0.5f, 1.0f});
+    CHECK(columnMinMax(env, 2, 0).min == -1.0f && columnMinMax(env, 2, 0).max == 0.5f);
+    CHECK(columnMinMax(env, 2, 1).min ==  0.0f && columnMinMax(env, 2, 1).max == 1.0f);
+}
+
+static void testColumnSteepDisjointSpans() {
+    // THE anti-alias case: two adjacent bins holding DISJOINT spans (a steep edge —
+    // all-positive then all-negative). Merged into one column the result must bridge
+    // both extremes as one full span {-1.0, 1.0}. The old per-bin overdraw could never
+    // produce a wrong value here — only the merge path exercises this.
+    ChannelEnvelope env = makeEnvelope({0.9f, -1.0f}, {1.0f, -0.9f});
+    const MinMax mm = columnMinMax(env, 1, 0);
+    CHECK(mm.min == -1.0f && mm.max == 1.0f);
+}
+
+static void testColumnNoBinDropped() {
+    // Downsample coverage: EVERY bin must land in some column (union of columns ==
+    // union of bins). For several non-integer ratios, plant a lone {-1,+1} spike in
+    // bin j (all other bins {0,0}) and assert some column reports it — a partition
+    // that skipped bin j would lose the spike entirely.
+    const struct { int nbins; int cols; } cases[] = {{7, 3}, {10, 4}, {9, 2}, {6, 10}};
+    for (const auto& c : cases) {
+        for (int j = 0; j < c.nbins; ++j) {
+            ChannelEnvelope env(static_cast<std::size_t>(c.nbins));  // all {0,0}
+            env[static_cast<std::size_t>(j)] = MinMax{-1.0f, 1.0f};
+            bool found = false;
+            for (int col = 0; col < c.cols; ++col) {
+                const MinMax mm = columnMinMax(env, c.cols, col);
+                if (mm.min == -1.0f && mm.max == 1.0f) { found = true; break; }
+            }
+            CHECK(found);
+        }
+    }
+}
+
+static void testColumnNoColumnEmpty() {
+    // Gap-free coverage: 6 bins over 10 columns (non-integer ratio) — every column
+    // must carry a real bin's values (all bins strictly positive, so a default {0,0}
+    // would expose a skipped column).
+    ChannelEnvelope env = makeEnvelope({0.1f, 0.2f, 0.3f, 0.4f, 0.5f, 0.6f},
+                                       {0.2f, 0.3f, 0.4f, 0.5f, 0.6f, 0.7f});
+    for (int col = 0; col < 10; ++col) {
+        const MinMax mm = columnMinMax(env, 10, col);
+        CHECK(mm.min >= 0.1f && mm.max <= 0.7f);
+        CHECK(mm.min <= mm.max);
+    }
+}
+
+static void testColumnColClamp() {
+    // col outside [0, columnCount-1] clamps: negative to the first, large to the last.
+    ChannelEnvelope env = makeEnvelope({-0.5f, 0.5f}, {-0.1f, 0.9f});
+    CHECK(columnMinMax(env, 2, -5).min == -0.5f);
+    CHECK(columnMinMax(env, 2, 999).max == 0.9f);
+}
+
+static void testColumnDegenerate() {
+    // Empty envelope, columnCount <= 0 -> {0, 0}.
+    ChannelEnvelope empty;
+    const MinMax z = columnMinMax(empty, 4, 0);
+    CHECK(z.min == 0.0f && z.max == 0.0f);
+    ChannelEnvelope env = makeEnvelope({0.3f}, {0.7f});
+    const MinMax z2 = columnMinMax(env, 0, 0);
+    CHECK(z2.min == 0.0f && z2.max == 0.0f);
+    const MinMax z3 = columnMinMax(env, -1, 0);
+    CHECK(z3.min == 0.0f && z3.max == 0.0f);
+}
+
 int main() {
     testSineEnvelope();
     testRampMonotonic();
@@ -364,6 +467,14 @@ int main() {
     testLastFrameAllAbove();
     testLastFramePerChannelMaxAbs();
     testLastFrameDegenerate();
+    testColumnOneToOne();
+    testColumnUpsampleFallback();
+    testColumnDownsampleMerge();
+    testColumnSteepDisjointSpans();
+    testColumnNoBinDropped();
+    testColumnNoColumnEmpty();
+    testColumnColClamp();
+    testColumnDegenerate();
 
     if (g_fail == 0) std::printf("All tests passed.\n");
     return g_fail ? 1 : 0;