Implement screen shaking on the battle screen

Moving the background was done my modifying Background directly, and
the sprites were moved by mimicing the method used for moving
characters on the map screen, with similar GetDisplayX functions.

Other objects on the battle screen don't seem to shake.

src/background.cpp

@@ -24,6 +24,7 @@
 #include "cache.h"
 #include "background.h"
 #include "bitmap.h"
+#include "main_data.h"
 
 Background::Background(const std::string& name) :
 	visible(true),
@@ -61,7 +62,7 @@ Background::Background(int terrain_id) :
 		FileRequestAsync* request = AsyncHandler::RequestFile("Frame", terrain.background_a_name);
 		request_id = request->Bind(&Background::OnBackgroundGraphicReady, this);
 		request->Start();
-	
+
 		bg_hscroll = terrain.background_a_scrollh ? terrain.background_a_scrollh_speed : 0;
 		bg_vscroll = terrain.background_a_scrollv ? terrain.background_a_scrollv_speed : 0;
 	}
@@ -125,10 +126,14 @@ void Background::Draw() {
 		return;
 
 	BitmapRef dst = DisplayUi->GetDisplaySurface();
+	Rect dst_rect = dst->GetRect();
+
+	int shake_pos = Main_Data::game_data.screen.shake_position;
+	dst_rect.x += shake_pos;
 
 	if (bg_bitmap)
-		dst->TiledBlit(-Scale(bg_x), -Scale(bg_y), bg_bitmap->GetRect(), *bg_bitmap, dst->GetRect(), 255);
+		dst->TiledBlit(-Scale(bg_x), -Scale(bg_y), bg_bitmap->GetRect(), *bg_bitmap, dst_rect, 255);
 
 	if (fg_bitmap)
-		dst->TiledBlit(-Scale(fg_x), -Scale(fg_y), fg_bitmap->GetRect(), *fg_bitmap, dst->GetRect(), 255);
+		dst->TiledBlit(-Scale(fg_x), -Scale(fg_y), fg_bitmap->GetRect(), *fg_bitmap, dst_rect, 255);
 }

src/game_battler.cpp

@@ -157,7 +157,7 @@ bool Game_Battler::IsSkillUsable(int skill_id) const {
 	if (CalculateSkillCost(skill_id) > GetSp()) {
 		return false;
 	}
-	
+
 	// > 10 makes any skill usable
 	int smallest_physical_rate = 11;
 	int smallest_magical_rate = 11;
@@ -230,11 +230,11 @@ bool Game_Battler::UseItem(int item_id) {
 
 		return was_used;
 	}
-	
+
 	if (item.type == RPG::Item::Type_switch) {
 		return true;
 	}
-	
+
 	switch (item.type) {
 		case RPG::Item::Type_weapon:
 		case RPG::Item::Type_shield:
@@ -557,6 +557,15 @@ int Game_Battler::GetAgi() const {
 	return n;
 }
 
+int Game_Battler::GetDisplayX() const {
+	int shake_pos = Main_Data::game_data.screen.shake_position;
+	return GetBattleX() + shake_pos;
+}
+
+int Game_Battler::GetDisplayY() const {
+	return GetBattleY();
+}
+
 int Game_Battler::GetHue() const {
 	return 0;
 }

src/game_battler.h

@@ -60,7 +60,7 @@ class Game_Battler {
 
 	/**
 	 * Apply effects of Conditions to Battler
-	 * 
+	 *
 	 * @return Damage taken to Battler from conditions
 	 */
 	int ApplyConditions();
@@ -93,7 +93,7 @@ class Game_Battler {
 	/**
 	 * Tests if the battler has a "No Action" condition like sleep.
 	 *
-	 * @return can act 
+	 * @return can act
 	 */
 	bool CanAct();
 
@@ -138,7 +138,7 @@ class Game_Battler {
 
 	/**
 	 * Gets probability that a state can be inflicted on this actor.
-	 * 
+	 *
 	 * @param state_id State to test
 	 * @return Probability of state infliction
 	 */
@@ -410,21 +410,34 @@ class Game_Battler {
 	 * @return Reflect is enabled.
 	 */
 	bool HasReflectState() const;
-	
+
 	/**
-	 * Gets X position on battlefield
+	 * Gets X position against the battle background.
 	 *
 	 * @return X position in battle scene
 	 */
 	virtual int GetBattleX() const = 0;
 
 	/**
-	 * Gets Y position on battlefield
+	 * Gets Y position against the battle background.
 	 *
 	 * @return Y position in battle scene
 	 */
 	virtual int GetBattleY() const = 0;
 
+	/**
+	 * Gets X position on the screen.
+	 *
+	 * This is equal to GetBattleX, plus a displacement for
+	 * any screen shaking.
+	 */
+	int GetDisplayX() const;
+
+	/**
+	 * Gets Y position on the screen.
+	 */
+	int GetDisplayY() const;
+
 	virtual int GetHue() const;
 
 	virtual int GetBattleAnimationId() const = 0;
@@ -449,7 +462,7 @@ class Game_Battler {
 	 * @return If battler is defending (next turn, defense is doubled)
 	 */
 	bool IsDefending() const;
-	
+
 	/**
 	 * @return If battler has strong defense (defense is tripled when defending)
 	 */
@@ -480,7 +493,7 @@ class Game_Battler {
 	 * Convenience function to access the party based on the type of this
 	 * battler. This function does not ensure that the battler is in the
 	 * party.
-	 * @return Party this member probably belongs to. 
+	 * @return Party this member probably belongs to.
 	 */
 	Game_Party_Base& GetParty() const;
 
@@ -538,7 +551,7 @@ class Game_Battler {
 	 */
 	void SetBattleAlgorithm(const BattleAlgorithmRef battle_algorithm);
 
-	/** 
+	/**
 	 * @return Current turn in battle
 	 */
 	int GetBattleTurn() const;

src/sprite_battler.cpp

@@ -152,6 +152,10 @@ void Sprite_Battler::Update() {
 			}
 		}
 	}
+
+	SetX(battler->GetDisplayX());
+	SetY(battler->GetDisplayY());
+	SetZ(battler->GetBattleY());
 }
 
 void Sprite_Battler::SetAnimationState(int state, LoopState loop) {
@@ -263,8 +267,8 @@ void Sprite_Battler::CreateSprite() {
 	sprite_name = battler->GetSpriteName();
 	hue = battler->GetHue();
 
-	SetX(battler->GetBattleX());
-	SetY(battler->GetBattleY());
+	SetX(battler->GetDisplayX());
+	SetY(battler->GetDisplayY());
 	SetZ(battler->GetBattleY()); // Not a typo
 
 	// Not animated -> Monster