implement heightmap grid

engine/image.cpp

@@ -46,3 +46,36 @@ void Image::flip()
     }
     std::swap(m_data, flipped);
 }
+
+ui::Image ui::Image::resize(int w, int h)
+{
+    Image ret;
+    ret.create(w, h);
+    auto temp = (glm::u8vec4*)ret.data();
+    auto pixels = (glm::u8vec4*)data();
+    float x_ratio = ((float)(width - 1)) / w;
+    float y_ratio = ((float)(height - 1)) / h;
+    float x_diff, y_diff, ya, yb;
+    int offset = 0;
+    for (int i = 0; i < h; i++) {
+        for (int j = 0; j < w; j++) {
+            int x = (int)(x_ratio * j);
+            int y = (int)(y_ratio * i);
+            x_diff = (x_ratio * j) - x;
+            y_diff = (y_ratio * i) - y;
+            int index = y * width + x;
+
+            // range is 0 to 255 thus bitwise AND with 0xff
+            glm::vec4 A = pixels[index];
+            glm::vec4 B = pixels[index + 1];
+            glm::vec4 C = pixels[index + width];
+            glm::vec4 D = pixels[index + width + 1];
+
+            // Y = A(1-w)(1-h) + B(w)(1-h) + C(h)(1-w) + Dwh
+            glm::vec4 gray = A*(1 - x_diff)*(1 - y_diff) + B * (x_diff)*(1 - y_diff) +
+                C * (y_diff)*(1 - x_diff) + D * (x_diff*y_diff);
+            temp[offset++] = glm::clamp(gray, glm::vec4(0), glm::vec4(255));
+        }
+    }
+    return ret;
+}