[image] Check the rotated benchmark frame against the plain one

The benchmark now draws once at each rotation and requires the rotated
frame to be the transpose of the plain one before timing, so a wrong
traversal fails the run instead of passing silently.

esphome/components/image/image.cpp

@@ -6,9 +6,9 @@
 namespace esphome::image {
 
 // Walks the clipped image range with rows (ROWS_OUTER) or columns as the outer
-// loop. The selects fold at compile time and the pixel body must be
-// ESPHOME_ALWAYS_INLINE: at -Os GCC otherwise leaves it out of line and calls it
-// once per pixel.
+// loop. The coordinate selection folds at compile time, and the pixel body must
+// be ESPHOME_ALWAYS_INLINE: at -Os GCC otherwise leaves it out of line and calls
+// it once per pixel.
 template<bool ROWS_OUTER, typename F>
 static void for_each_pixel(int x_start, int x_end, int y_start, int y_end, F &&f) {
   const int outer_start = ROWS_OUTER ? y_start : x_start;

tests/benchmarks/components/image/bench_image_draw.cpp

@@ -29,8 +29,29 @@ class BenchDisplay : public display::DisplayBuffer {
   void draw_absolute_pixel_internal(int x, int y, Color color) override { this->frame_[y * kStride + x] = color.r; }
 
   std::unique_ptr<uint8_t[]> frame_;
+
+ public:
+  uint8_t pixel(int x, int y) const { return this->frame_[y * kStride + x]; }
 };
 
+// Draws the image at rotation 0 and 90 and checks the second frame is the
+// transpose of the first, which is what a wrong traversal order would break.
+static bool frames_match(const Image &image) {
+  BenchDisplay plain;
+  BenchDisplay rotated;
+  rotated.set_rotation(display::DISPLAY_ROTATION_90_DEGREES);
+  const_cast<Image &>(image).draw(0, 0, &plain, display::COLOR_ON, display::COLOR_OFF);
+  const_cast<Image &>(image).draw(0, 0, &rotated, display::COLOR_ON, display::COLOR_OFF);
+  for (int y = 0; y < kHeight; y++) {
+    for (int x = 0; x < kWidth; x++) {
+      // DisplayBuffer maps (x, y) to (width - y - 1, x) at 90 degrees
+      if (plain.pixel(x, y) != rotated.pixel(kStride - y - 1, x))
+        return false;
+    }
+  }
+  return true;
+}
+
 // Deterministic pseudo random pixel bytes so the alpha branch sees a mix of
 // pixels. Sized for the widest format, the image reads only what it needs.
 static std::unique_ptr<uint8_t[]> make_pixels() {
@@ -50,6 +71,10 @@ static void draw_image(benchmark::State &state, ImageType type, Transparency tra
   Image image(data.get(), kWidth, kHeight, type, transparency);
   BenchDisplay display;
   display.set_rotation(rotation);
+  if (!frames_match(image)) {
+    state.SkipWithError("rotated draw is not the transpose of the plain draw");
+    return;
+  }
 
   for (auto _ : state) {
     image.draw(0, 0, &display, display::COLOR_ON, display::COLOR_OFF);