Prevent Gradients from creating textures of size 0 when stops overlap (#36218)

Author: Jonah Williams

impeller/geometry/geometry_unittests.cc

@@ -1451,6 +1451,17 @@ TEST(GeometryTest, Gradient) {
     ASSERT_EQ(texture_size, 2u);
   }
 
+  {
+    // Gradient with duplicate stops does not create an empty texture.
+    std::vector<Color> colors = {Color::Red(), Color::Yellow(), Color::Black(),
+                                 Color::Blue()};
+    std::vector<Scalar> stops = {0.0, 0.25, 0.25, 1.0};
+    uint32_t texture_size;
+
+    auto gradient = CreateGradientBuffer(colors, stops, &texture_size);
+    ASSERT_EQ(texture_size, 5u);
+  }
+
   {
     // Simple N color gradient produces color buffer containing exactly those
     // values.
@@ -1488,9 +1499,9 @@ TEST(GeometryTest, Gradient) {
     // Gradient size is capped at 1024.
     std::vector<Color> colors = {};
     std::vector<Scalar> stops = {};
-    for (auto i = 0u; i < 2000; i++) {
+    for (auto i = 0u; i < 1025; i++) {
       colors.push_back(Color::Blue());
-      stops.push_back(i / 2000.0);
+      stops.push_back(i / 1025.0);
     }
     stops[1999] = 1.0;
 

impeller/geometry/gradient.cc

@@ -22,14 +22,16 @@ std::vector<uint8_t> CreateGradientBuffer(const std::vector<Color>& colors,
   uint32_t texture_size;
   // TODO(jonahwilliams): we should add a display list flag to check if the
   // stops were provided or not, then we can skip this step.
-  // TODO(jonahwilliams): Skia has a check for stop sizes below a certain
-  // threshold, we should make sure that we behave reasonably with them.
   if (stops.size() == 2) {
     texture_size = 2;
   } else {
     auto minimum_delta = 1.0;
     for (size_t i = 1; i < stops.size(); i++) {
       auto value = stops[i] - stops[i - 1];
+      // Smaller than kEhCloseEnough
+      if (value < 0.0001) {
+        continue;
+      }
       if (value < minimum_delta) {
         minimum_delta = value;
       }
@@ -41,7 +43,6 @@ std::vector<uint8_t> CreateGradientBuffer(const std::vector<Color>& colors,
     texture_size =
         std::min((uint32_t)std::round(1.0 / minimum_delta) + 1, 1024u);
   }
-
   *out_texture_size = texture_size;
   std::vector<uint8_t> color_stop_channels;
   color_stop_channels.reserve(texture_size * 4);