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

esphome/components/light/esp_color_correction.cpp

@@ -5,7 +5,11 @@ namespace esphome::light {
 uint8_t ESPColorCorrection::gamma_correct_(uint8_t value) const {
   if (this->gamma_table_ == nullptr)
     return value;
-  return static_cast<uint8_t>((progmem_read_uint16(&this->gamma_table_[value]) + 128) / 257);
+  uint16_t table_value = progmem_read_uint16(&this->gamma_table_[value]);
+  uint8_t result = (table_value + 128) / 257;
+  if (result == 0 && table_value != 0)
+    return 1;
+  return result;
 }
 
 uint8_t ESPColorCorrection::gamma_uncorrect_(uint8_t value) const {

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

tests/unit_tests/components/light/test_gamma_table.py

@@ -53,9 +53,12 @@ def test_nonzero_indices_are_nonzero(gamma: float) -> None:
         assert table[i] >= 1, f"gamma={gamma}, index {i}: got {table[i]}"
 
 
-@pytest.mark.parametrize("gamma", [1.0, 2.0, 2.2, 2.8, 3.0])
+@pytest.mark.parametrize("gamma", [1.0, 1.8, 2.0, 2.2, 2.8, 3.0, 4.0])
 def test_table_monotonically_nondecreasing(gamma: float) -> None:
-    """The gamma table must be monotonically non-decreasing."""
+    """The gamma table must be monotonically non-decreasing.
+
+    gamma_table_reverse_search()'s binary search depends on this.
+    """
     table = generate_gamma_table(gamma)
     for i in range(1, 256):
         assert table[i] >= table[i - 1], (
@@ -115,3 +118,13 @@ def test_lut_output_monotonically_nondecreasing() -> None:
         result = _simulate_gamma_correct_lut(table, value)
         assert result >= prev, f"value={value}: result {result} < previous {prev}"
         prev = result
+
+
+def test_table_matches_raw_power_curve() -> None:
+    """Check the gamma table against known good values for gamma=2.8."""
+    table = generate_gamma_table(2.8)
+    golden = {1: 1, 5: 1, 15: 24, 27: 122, 28: 135, 100: 4766, 200: 33193, 254: 64818}
+    for i, expected in golden.items():
+        assert table[i] == expected, (
+            f"index {i}: table[{i}]={table[i]} expected {expected}"
+        )