object collision done

enemy spawning done

galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm

@@ -1,7 +1,12 @@
+; ANCHOR: enemies-collision-start
+
 include "src/main/includes/constants.inc"
 SECTION "EnemiesCollision", ROM0
 
 CheckCollisionForCurrentEnemy::
+
+; ANCHOR_END: enemies-collision-start
+; ANCHOR: enemies-collision-player
     push hl
 
     ld a, 16
@@ -11,14 +16,41 @@ CheckCollisionForCurrentEnemy::
     call CheckCollisionWithObjectsInHL_andDE
 
     pop hl
+    
     jp nz, EnemyPlayerCollision
     jp UpdateEnemy_CheckAllBulletCollision
+; ANCHOR_END: enemies-collision-player
+
+; ANCHOR: enemies-collision-player2
+EnemyPlayerCollision:
+
+    push hl
+    push bc
+
+    call DamagePlayer
+	
+    ld hl, wLives
+    ld de, $9C13 ; The window tilemap starts at $9C00
+	ld b, 1
+	call DrawBDigitsHL_OnDE
+
+    pop bc
+    pop hl
 
+    ; Set a to be 1
+    ld a, ENEMY_COLLISION_END
+    ret
+; ANCHOR_END: enemies-collision-player2
+
+
+; ANCHOR: enemies-collision-bullets1
 UpdateEnemy_CheckAllBulletCollision:
 
     ld b,MAX_BULLET_COUNT
     ld de, wObjects+BULLETS_START
+; ANCHOR_END: enemies-collision-bullets1
 
+; ANCHOR: enemies-collision-bullets2
 UpdateEnemy_CheckBulletCollision:
 
     ; Save the start of our enemy's bytes
@@ -28,8 +60,9 @@ UpdateEnemy_CheckBulletCollision:
     push bc
     push de
 
-    ld a, 16
+    ld a, 12
     ld [wSizeX], a
+    ld a, 16
     ld [wSizeY], a
     call CheckCollisionWithObjectsInHL_andDE
 
@@ -42,7 +75,9 @@ UpdateEnemy_CheckBulletCollision:
 
     jp z, MoveToNextBullet
     jp EnemyBulletCollision
+; ANCHOR_END: enemies-collision-bullets2
 
+; ANCHOR: enemies-collision-bullets3
 MoveToNextBullet:
 
     ; Decrease b
@@ -51,23 +86,17 @@ MoveToNextBullet:
     dec a
     ld b, a
     and a
-    jp z, NoDamage
+    ret z
 
     ; Move to the next object
     ld a, e
     add a, PER_OBJECT_BYTES_COUNT
     ld e, a
 
     jp UpdateEnemy_CheckBulletCollision
+; ANCHOR_END: enemies-collision-bullets3
 
-
-NoDamage::
-
-    ; Set a to be 0
-    ld a, ENEMY_COLLISION_NOTHING
-    ret
-    
-
+; ANCHOR: enemies-collision-bullet4
 EnemyBulletCollision:
 
     push hl
@@ -86,22 +115,4 @@ EnemyBulletCollision:
     ld a, ENEMY_COLLISION_DAMAGED
 
     ret
-
-EnemyPlayerCollision:
-
-    push hl
-    push bc
-
-    call DamagePlayer
-	
-    ld hl, wLives
-    ld de, $9C13 ; The window tilemap starts at $9C00
-	ld b, 1
-	call DrawBDigitsHL_OnDE
-
-    pop bc
-    pop hl
-
-    ; Set a to be 1
-    ld a, ENEMY_COLLISION_END
-    ret
+; ANCHOR_END: enemies-collision-bullet4

galactic-armada/src/main/states/gameplay/objects/enemies-spawning.asm

@@ -2,15 +2,14 @@
 include "src/main/includes/hardware.inc"
 include "src/main/includes/constants.inc"
 
-; ANCHOR_END: enemies-start
 SECTION "EnemySpawningVariables",    WRAM0
 
 wSpawnCounter: db
 
 SECTION "EnemySpawning", ROM0
+; ANCHOR_END: enemies-start
 
-
-; ANCHOR: enemies-spawn
+; ANCHOR: enemies-spawn1
 TryToSpawnEnemies::
 
     ; Increase our spwncounter
@@ -32,7 +31,9 @@ TryToSpawnEnemies::
     ; if the zero flag is set, stop early
     call GetNextAvailableObject_InHL
     ret z
+; ANCHOR_END: enemies-spawn1
 
+; ANCHOR: enemies-spawn2
 .GetSpawnPosition
 
     push hl
@@ -42,15 +43,18 @@ TryToSpawnEnemies::
 
     pop hl
     
-    ; make sure it's not above 150
+    ; make sure it's not above 136
     ld a,b
-    cp a, 150
+    cp a, 136
     ret nc
 
     ; make sure it's not below 24
     ld a, b
     cp a, 24
     ret c
+; ANCHOR_END: enemies-spawn2
+
+; ANCHOR: enemies-spawn3
 
 .SpawnEnemy
 
@@ -109,4 +113,4 @@ TryToSpawnEnemies::
     ld [hli], a
 
     ret
-; ANCHOR_END: enemies-spawn
+; ANCHOR_END: enemies-spawn3

galactic-armada/src/main/states/gameplay/objects/object-collision.asm

@@ -13,13 +13,10 @@ wCheckByte: db
 SECTION "ObjectObjectCollision", ROM0
 
 ; ANCHOR_END: object-collision-start
-; ANCHOR: object-collision-x
-
+; ANCHOR: object-collision-function
 CheckCollisionWithObjectsInHL_andDE::
-
-    ; SAve original values for y axis
-    push de
-    push hl
+; ANCHOR_END: object-collision-function
+; ANCHOR: object-collision-x
 XAxis:
 
     ; Save which byte we are checking
@@ -30,6 +27,10 @@ XAxis:
     ld a, [wSizeX]
     ld [wSize], a
 
+    ; SAve original values for y axis
+    push de
+    push hl
+
     call CheckObjectBytesOfObjects_InDE_AndHL
 
     ; Restore original vaues just in case
@@ -115,6 +116,48 @@ CheckObjectBytesOfObjects_InDE_AndHL::
     ld a, c
     ld [wObject2Value], a
 
-    call CheckObjectPositionDifference
-    ret
+
+CheckObjectPositionDifference::
+
+    ; at this point in time; e = enemy.y, b =bullet.y
+
+    ld a, [wObject1Value]
+    ld e, a
+    ld a, [wObject2Value]
+    ld b, a
+
+    ld a, [wSize]
+    ld d, a
+
+    ; subtract  bullet.y, (aka b) - (enemy.y+8, aka e)
+    ; carry means e<b, means enemy.bottom is visually above bullet.y (no collision)
+
+    ld a, e
+    add a, d
+    cp a, b
+
+    ;  carry means  no collision
+    jp c, CheckObjectPositionDifference_Failure
+
+    ; subtract  enemy.y-8 (aka e) - bullet.y (aka b)
+    ; no carry means e>b, means enemy.top is visually below bullet.y (no collision)
+    ld a, e
+    sub a, d
+    cp a, b
+
+    ; no carry means no collision
+    jp nc, CheckObjectPositionDifference_Failure
+
+    
+CheckObjectPositionDifference_Intersection:
+
+    ld a,1
+    and a
+    ret;
+    
+CheckObjectPositionDifference_Failure:
+
+    ld a,0
+    and a
+    ret;
 ; ANCHOR_END: object-collision-check-bytes

src/SUMMARY.md

@@ -42,7 +42,6 @@
   - [Drawing Text](part3/drawing-text.md)
   - [Included Utilities](part3/utilities.md)
   - [Compilation](part3/compilation.md)
-  - [Collision Detection](part3/collision.md)
 - [Game State Management](part3/game-state-management.md)
 - [Title Screen](part3/title-screen.md)
 - [Story Screen](part3/story-screen.md)
@@ -54,8 +53,8 @@
   - [The Player](part3/the-player.md)
   - [Bullets](part3/bullets.md)
   - [Enemies](part3/enemies.md)
+  - [Enemy Collisions](part3/enemy-collisions.md)
   - [Spawning Enemies](part3/enemy-spawning.md)
-  - [Enemy Collisions](part3/enemy-collision.md)
 - [Conclusion](part3/conclusion.md)
 
 [Where to go next](next.md)

src/part3/collision.md

@@ -1,12 +1,5 @@
 # Collision Detection
 
-Collision Detection is cruical to games. It can be a very complicated topic. In Galactic Armada, things will be kept super simple. We're going to perform a basic implementation of "Axis-Aligned Bounding Box Collision Detection":
-
-> One of the simpler forms of collision detection is between two rectangles that are axis aligned — meaning no rotation. The algorithm works by ensuring there is no gap between any of the 4 sides of the rectangles. Any gap means a collision does not exist.[^mdn_source]
-
-The easiest way to check for overlap, is to check the difference bewteen their centers. If the absolute value of their x & y differences (I'll refer to as "the absolute difference") are BOTH smaller than the sum of their half widths, we have a collision. This collision detection is run for bullets against enemies, and enemies against the player. Here's a visualization with bullets and enemies.
-
-![CollisionDetectionVisualized.png](../assets/part3/img/CollisionDetectionVisualized.png)
 
 For this, we've created a basic function called "CheckObjectPositionDifference". This function will help us check for overlap on the x or y axis. When the (absolute) difference between the first two values passed is greater than the third value passed, it jump's to the label passed in the fourth parameter.
 

src/part3/enemy-collisions.md

@@ -6,9 +6,64 @@ There are two parts to enemy collision detection:
 - Collision Detection against the player
 - Collision Detection against bullets
 
-## Against the Player
+**Create a file called `enemies-collision.asm`, and define the `CheckCollisionForCurrentEnemy` function in it:**
 
-Checking agains the player is really simple
+```rgbasm,linenos,start={{#line_no_of "" ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-start}}
+{{#include ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-start}}
+```
+## Collision Detection Against the Player
 
+Firstly, enemies will check for collision against the player. To do this, we'll use the `CheckCollisionWithObjectsInHL_andDE` function previously created. Our player and enemies are both 16x16. The minimum allowed distances on both axes is 16.
 
-## Against bullets
+> **Note:** The player will always be the first object in `wObjects`, so we can use it for `de`.
+
+**Add to the `CheckCollisionForCurrentEnemy` function, the following logic:**
+
+```rgbasm,linenos,start={{#line_no_of "" ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-player}}
+{{#include ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-player}}
+```
+
+If no collision occurs (the zero flag is set), we'll check against each bullet. If a collision has occured, we'll destroy the enemy and damage the player:
+
+**Add the following below the `CheckCollisionForCurrentEnemy` function:**
+
+```rgbasm,linenos,start={{#line_no_of "" ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-player2}}
+{{#include ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-player2}}
+```
+## Collision Detection Against bullets
+
+Checking for collisions against bullets is essentially the same thing. The major difference is that we have multiple enemies, and thus must loop & check each.
+
+The starter has a constant in `constants.inc` called `BULLETS_START`. We'll use this with the `wObjects` array to specify the first possible enemy bullet.
+
+**Immediately below the `EnemyPlayerCollision` logic, Start the `UpdateEnemy_CheckAllBulletCollision` logic with the following code:**
+
+```rgbasm,linenos,start={{#line_no_of "" ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-bullets1}}
+{{#include ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-bullets1}}
+```
+
+During each iteration of this loop, we'll have a pointer to the current enemy in `hl`. In addition, we'll have a pointer to the bullet in `de`. With that setup, we can call `CheckCollisionWithObjectsInHL_andDE` without much effort.
+
+Our bullets are 8x16 and our enemies are 16x16. The minimum allowed distance on the x-axis is 12, and the minimum allowed distance on the y-axis is 16.
+
+**Add the `UpdateEnemy_CheckBulletCollision` logic below, to the `UpdateEnemy_CheckAllBulletCollision` function:** 
+
+```rgbasm,linenos,start={{#line_no_of "" ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-bullets2}}
+{{#include ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-bullets2}}
+```
+
+If there's no collision we'll jump to the `MoveToNextBullet` label. Here, we'll decrease our counter (in `b`). When it reaches 0, we've checked all bullets. If we've checked all bullets, we'll return. Otherwise, we'll increase our `de` pointer and loop back around.
+
+**Below the `UpdateEnemy_CheckBulletCollision` code, add the `MoveToNextBullet` logic below:**
+
+```rgbasm,linenos,start={{#line_no_of "" ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-bullets3}}
+{{#include ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-bullets3}}
+```
+
+In the case of a enemy-bullet collision, we'll deactivate the bullet. Before returning, we'll set our `a` register to `ENEMY_COLLISION_DAMAGED`.
+
+**Finish the `enemies-collision.asm` file with the `EnemyBulletCollision` logic below:**
+
+```rgbasm,linenos,start={{#line_no_of "" ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-bullet4}}
+{{#include ../../galactic-armada/src/main/states/gameplay/objects/enemies-collision.asm:enemies-collision-bullet4}}
+```