Added bi-linear interpolation for getting Z coordinate at any location in the grid.

heightmap_test.cpp

@@ -0,0 +1,26 @@
+#include <gtest/gtest.h>
+
+#include "heightmap.h"
+
+TEST(heightmap, get_z_at) {
+    auto grid = grbl::heightmap::from_params(0, 0, 1, 1, 1);
+    grid.vertices[0].z = 0;
+    grid.vertices[1].z = 1;
+    grid.vertices[2].z = 2;
+    grid.vertices[3].z = 3;
+
+    auto z = grid.get_z_at(0.5f, 0.5f);
+    EXPECT_EQ(1.5, z);
+
+    z = grid.get_z_at(0.5f, 0);
+    EXPECT_EQ(0.5, z);
+
+    z = grid.get_z_at(0.5f, 1);
+    EXPECT_EQ(2.5, z);
+
+    z = grid.get_z_at(0, 0.5f);
+    EXPECT_EQ(1, z);
+
+    z = grid.get_z_at(1, 0.5f);
+    EXPECT_EQ(2, z);
+}