[light] Fix gamma table dead zone collapsing to 0 (#18845)

tests/components/light/test_gamma_correction.cpp

@@ -0,0 +1,92 @@
+#include <gtest/gtest.h>
+
+#include <algorithm>
+#include <array>
+#include <cmath>
+#include <cstdint>
+
+#include "esphome/components/light/esp_color_correction.h"
+
+namespace esphome::light::testing {
+
+namespace {
+
+// A representative fixture for ESPColorCorrection/gamma_table_reverse_search tests below --
+// not a spec for generate_gamma_table() itself, which the Python tests own.
+std::array<uint16_t, 256> build_gamma_table(double gamma) {
+  std::array<uint16_t, 256> table{};
+  table[0] = 0;
+  for (int i = 1; i < 256; i++) {
+    double raw = std::round(std::pow(i / 255.0, gamma) * 65535.0);
+    table[i] = static_cast<uint16_t>(std::max(1.0, std::min(65535.0, raw)));
+  }
+  return table;
+}
+
+// Bundles a table with an ESPColorCorrection pointing at it, since the correction only holds
+// a raw pointer into the table and doesn't own it.
+struct GammaFixture {
+  explicit GammaFixture(double gamma) : table(build_gamma_table(gamma)) { correction.set_gamma_table(table.data()); }
+  std::array<uint16_t, 256> table;
+  ESPColorCorrection correction;
+};
+
+}  // namespace
+
+// Regression test for esphome/esphome#18842: ESPColorCorrection's own 16-bit -> 8-bit
+// conversion must never round a non-zero table entry down to a zero 8-bit output.
+TEST(GammaCorrection, NonZeroInputsSurviveConversion) {
+  for (double gamma : {1.0, 1.8, 2.0, 2.2, 2.8, 3.0, 4.0}) {
+    GammaFixture fixture(gamma);
+    for (int i = 1; i < 256; i++) {
+      EXPECT_GE(fixture.correction.color_correct_red(i), 1) << "gamma=" << gamma << " index=" << i;
+    }
+  }
+}
+
+TEST(GammaCorrection, ZeroInputStaysZero) {
+  for (double gamma : {1.0, 2.2, 2.8, 4.0}) {
+    GammaFixture fixture(gamma);
+    EXPECT_EQ(fixture.correction.color_correct_red(0), 0) << "gamma=" << gamma;
+  }
+}
+
+TEST(GammaCorrection, FullBrightnessStaysFull) {
+  for (double gamma : {1.0, 2.2, 2.8, 4.0}) {
+    GammaFixture fixture(gamma);
+    EXPECT_EQ(fixture.correction.color_correct_red(255), 255) << "gamma=" << gamma;
+  }
+}
+
+// Reproduces the reporter's own numbers from esphome/esphome#18842 at gamma=2.8: codes
+// 1-27 previously collapsed to an 8-bit output of 0 and must now be non-zero.
+TEST(GammaCorrection, DeadZoneFixedAtGamma28) {
+  GammaFixture fixture(2.8);
+  for (int i = 1; i < 28; i++) {
+    EXPECT_GE(fixture.correction.color_correct_red(i), 1) << "index=" << i << " still collapses to 0";
+  }
+}
+
+TEST(GammaCorrection, ReverseSearchFindsLargestIndexLessEqualTarget) {
+  auto table = build_gamma_table(2.8);
+  for (uint16_t target : {0, 128, 129, 135, 1000, 32768, 65535}) {
+    uint8_t lo = gamma_table_reverse_search(table.data(), target);
+    EXPECT_LE(table[lo], target) << "target=" << target;
+    if (lo < 255) {
+      EXPECT_GT(table[lo + 1], target) << "target=" << target;
+    }
+  }
+}
+
+// color_uncorrect_* binary-searches the table via gamma_table_reverse_search().
+TEST(GammaCorrection, UncorrectStaysMonotonic) {
+  GammaFixture fixture(2.8);
+  uint8_t prev = 0;
+  for (int i = 1; i < 256; i++) {
+    uint8_t result = fixture.correction.color_uncorrect_red(i);
+    EXPECT_GE(result, prev) << "index=" << i;
+    prev = result;
+  }
+}
+
+}  // namespace esphome::light::testing