(svn r13667) -Documentation: Comment 'tgp.cpp' a bit. Based on patch by CommanderZ.

src/tgp.cpp

@@ -174,6 +174,12 @@ struct HeightMap
 	uint     size_x;     //< MapSizeX()
 	uint     size_y;     //< MapSizeY()
 
+	/**
+	 * Height map accessor
+	 * @param x X position
+	 * @param y Y position
+	 * @return height as fixed point number
+	 */
 	inline height_t &height(uint x, uint y) {
 		return h[x + y * dim_x];
 	}
@@ -234,7 +240,10 @@ static inline bool IsValidXY(uint x, uint y)
 }
 
 
-/** Allocate array of (MapSizeX()+1)*(MapSizeY()+1) heights and init the _height_map structure members */
+/**
+ * Allocate array of (MapSizeX()+1)*(MapSizeY()+1) heights and init the _height_map structure members
+ * @return true on success
+ */
 static inline bool AllocHeightMap()
 {
 	height_t *h;
@@ -262,7 +271,11 @@ static inline void FreeHeightMap()
 	_height_map.h = NULL;
 }
 
-/** RandomHeight() generator */
+/**
+ * Generates new random height in given amplitude (generated numbers will range from - amplitude to + amplitude)
+ * @param rMax Limit of result
+ * @return generated height
+ */
 static inline height_t RandomHeight(amplitude_t rMax)
 {
 	amplitude_t ra = (Random() << 16) | (Random() & 0x0000FFFF);
@@ -272,13 +285,31 @@ static inline height_t RandomHeight(amplitude_t rMax)
 	return rh;
 }
 
-/** One interpolation and noise round */
+/**
+ * One interpolation and noise round
+ *
+ * The heights on the map are generated in an iterative process.
+ * We start off with a frequency of 1 (log_frequency == 0), and generate heights only for corners on the most coarsly mesh
+ * (i.e. only for x/y coordinates which are multiples of the minimum edge length).
+ *
+ * After this initial step the frequency is doubled (log_frequency incremented) each iteration to generate corners on the next finer mesh.
+ * The heights of the newly added corners are first set by interpolating the heights from the previous iteration.
+ * Finally noise with the given amplitude is applied to all corners of the new mesh.
+ *
+ * Generation terminates, when the frequency has reached the map size. I.e. the mesh is as fine as the map, and every corner height
+ * has been set.
+ *
+ * @param log_frequency frequency (logarithmic) to apply noise for
+ * @param amplitude Amplitude for the noise
+ * @return false if we are finished (reached the minimal step size / highest frequency)
+ */
 static bool ApplyNoise(uint log_frequency, amplitude_t amplitude)
 {
 	uint size_min = min(_height_map.size_x, _height_map.size_y);
 	uint step = size_min >> log_frequency;
 	uint x, y;
 
+	/* Trying to apply noise to uninitialized height map */
 	assert(_height_map.h != NULL);
 
 	/* Are we finished? */
@@ -316,11 +347,13 @@ static bool ApplyNoise(uint log_frequency, amplitude_t amplitude)
 		}
 	}
 
+	/* Add noise for next higher frequency (smaller steps) */
 	for (y = 0; y <= _height_map.size_y; y += step) {
 		for (x = 0; x <= _height_map.size_x; x += step) {
 			_height_map.height(x, y) += RandomHeight(amplitude);
 		}
 	}
+
 	return (step > 1);
 }
 
@@ -338,16 +371,19 @@ static void HeightMapGenerate()
 	for (log_size_min = 6; (1U << log_size_min) < size_min; log_size_min++) { }
 	log_frequency_min = log_size_min - 6;
 
+	/* Keep increasing the frequency until we reach the step size equal to one tile */
 	do {
 		log_frequency = iteration_round - log_frequency_min;
 		if (log_frequency >= 0) {
+			/* Apply noise for the next frequency */
 			amplitude = _amplitudes_by_smoothness_and_frequency[_settings_game.game_creation.tgen_smoothness][log_frequency];
 		} else {
+			/* Amplitude for the low frequencies on big maps is 0, i.e. initialise with zero height */
 			amplitude = 0;
 		}
 		continue_iteration = ApplyNoise(iteration_round, amplitude);
 		iteration_round++;
-	} while(continue_iteration);
+	} while (continue_iteration);
 }
 
 /** Returns min, max and average height from height map */
@@ -379,7 +415,7 @@ static int *HeightMapMakeHistogram(height_t h_min, height_t h_max, int *hist_buf
 	int *hist = hist_buf - h_min;
 	height_t *h;
 
-	/* Fill histogram */
+	/* Count the heights and fill the histogram */
 	FOR_ALL_TILES_IN_HEIGHT(h) {
 		assert(*h >= h_min);
 		assert(*h <= h_max);
@@ -769,7 +805,7 @@ static double perlin_coast_noise_2D(const double x, const double y, const double
 }
 
 
-/** A small helper function */
+/** A small helper function to initialize the terrain */
 static void TgenSetTileHeight(TileIndex tile, int height)
 {
 	SetTileHeight(tile, height);