added transient entities: when crossed, turns into wall tile

assets/levels/00-deadly-obstacles.lvl

@@ -4,8 +4,8 @@ Background: background-lila
                     
                     
    #######          
-   #  v  #          
    #     #          
+   #  t  #          
    #> S <#          
    #     #          
    #  ^  #          

assets/levels/01-friendly-obstacles.lvl

@@ -3,13 +3,13 @@ Background: background-lila
                     
                     
                     
-   #######          
-   #  ^  #          
+   #############    
    #           #    
-   #< S >#          
-   #     #          
-   #^   v#          
-   #######          
+   #>  S #     #    
+   #    >#    >#    
+   #<    #     #    
+   #    v#    <#    
+   #############    
                     
                     
                     

assets/loader-levels.go

@@ -30,8 +30,10 @@ type Tile struct {
 	Renderable  bool
 	Velocity    bool
 	Collectible bool
+	Transient   bool
 	Particle    int // -1=unused, 0-3 = show image of slice
-	Particles   []*ebiten.Image
+	Tiles       []*ebiten.Image
+	TileNames   []string // same thing, only the names
 	Obstacle    bool
 	Direction   int // obstacles
 }
@@ -77,6 +79,7 @@ func NewTileObstacle(class string, direction int) *Tile {
 		Renderable: true,
 		Obstacle:   true,
 		Direction:  direction,
+		Velocity:   true,
 	}
 }
 
@@ -93,7 +96,28 @@ func NewTileParticle(class []string) *Tile {
 		Solid:      false,
 		Renderable: false,
 		Particle:   0,
-		Particles:  sprites,
+		Tiles:      sprites,
+	}
+}
+
+func NewTileTranswall(class []string) *Tile {
+	sprites := []*ebiten.Image{}
+	names := []string{}
+
+	for _, sprite := range class {
+		sprites = append(sprites, Assets[sprite])
+		names = append(names, sprite)
+	}
+
+	return &Tile{
+		Id:         '*',
+		Class:      "transwall",
+		Solid:      false,
+		Renderable: true,
+		Transient:  true,
+		Tiles:      sprites,
+		Sprite:     sprites[0], // initially use the first
+		TileNames:  names,
 	}
 }
 
@@ -123,6 +147,7 @@ func InitTiles() TileRegistry {
 	return TileRegistry{
 		' ': {Id: ' ', Class: "floor", Renderable: false},
 		'#': NewTileBlock("block-grey32"),
+		'B': NewTileBlock("block-orange-32"),
 		'S': NewTilePlayer(),
 		'o': NewTileCollectible("collectible-orange"),
 		'+': NewTileObstacle("obstacle-star", config.All),
@@ -138,6 +163,7 @@ func InitTiles() TileRegistry {
 			"particle-ring-5",
 			"particle-ring-6",
 		}),
+		't': NewTileTranswall([]string{"transwall", "block-orange-32"}),
 	}
 }
 

components/components.go

@@ -13,7 +13,7 @@ type Renderable struct {
 type Particle struct {
 	Show  bool
 	Index int
-	Particles []*ebiten.Image
+	Tiles []*ebiten.Image
 }
 
 type Speed struct {
@@ -26,4 +26,7 @@ type Solid struct{}
 type Floor struct{}
 type Player struct{}
 type Collectible struct{}
-type Obstacle struct{}
+
+type Obstacle struct {
+	Direction int
+}

components/position.go

@@ -3,7 +3,6 @@ package components
 import (
 	"fmt"
 	"image"
-	"log/slog"
 	. "openquell/config"
 )
 
@@ -90,13 +89,14 @@ func (tile *Position) Intersects(moving *Position, velocity *Velocity) (bool, *P
 
 	is := tile.Rect.Bounds().Intersect(object.Rect.Bounds())
 	if is != image.ZR {
+		/*
 			slog.Debug("Intersect",
 				"velocity", velocity.Data,
 				"player", moving.Rect,
 				"moved player", object.Rect,
 				"collision at", is,
 			)
-
+		*/
 		// collision, snap into neighbouring tile depending on the direction
 		switch velocity.Direction {
 		case West:

components/transient.go

@@ -0,0 +1,24 @@
+package components
+
+import (
+	"log"
+)
+
+type Transient struct {
+	Activated bool
+	Sprites   []string
+	Current   int // sprite index
+}
+
+func (trans *Transient) GetNext() string {
+	if len(trans.Sprites) > trans.Current {
+		trans.Current++
+		return trans.Sprites[trans.Current]
+	}
+
+	log.Fatalf("not enough sprites in transient tile, have %d sprites, index requested: %d",
+		len(trans.Sprites), trans.Current+1,
+	)
+
+	return ""
+}

components/velocity.go

@@ -8,6 +8,18 @@ import (
 type Velocity struct {
 	Data       Position
 	Direction  int
+	PointingAt int
+}
+
+func (velocity *Velocity) Set(new *Velocity) {
+	velocity.Direction = new.Direction
+	velocity.Data.Set(&new.Data)
+}
+
+func (velocity *Velocity) ResetDirectionAndStop() {
+	velocity.Data.X = 0
+	velocity.Data.Y = 0
+	velocity.Direction = velocity.PointingAt
 }
 
 func (velocity *Velocity) Change(direction int) {

game/game.go

@@ -33,6 +33,7 @@ func NewGame(width, height, cellsize, startlevel int, startscene SceneName) *Gam
 
 	observers.NewPlayerObserver(&world)
 	observers.NewParticleObserver(&world)
+	observers.NewObstacleObserver(&world)
 	game.Observer = observers.NewGameObserver(&world, startlevel, width, height, cellsize)
 
 	game.Scenes[Welcome] = NewWelcomeScene(game)

game/level_scene.go

@@ -70,7 +70,8 @@ func (scene *LevelScene) Update() error {
 
 func (scene *LevelScene) Draw(screen *ebiten.Image) {
 	if scene.CurrentLevel != scene.Game.Observer.CurrentLevel {
-		slog.Debug("level", "current", scene.CurrentLevel, "next", scene.Game.Observer.CurrentLevel)
+		slog.Debug("level", "current", scene.CurrentLevel,
+			"next", scene.Game.Observer.CurrentLevel)
 		scene.CurrentLevel = scene.Game.Observer.CurrentLevel
 		scene.Levels[scene.CurrentLevel].SetupGrid(scene.Game)
 	}

game/levels.go

@@ -8,6 +8,7 @@ import (
 	"openquell/grid"
 	"openquell/observers"
 	"openquell/systems"
+	"openquell/util"
 	"strings"
 
 	"github.com/hajimehoshi/ebiten/v2"
@@ -79,9 +80,11 @@ func (level *Level) SetupGrid(game *Game) {
 	playerobserver.RemoveEntities()
 
 	// setup world
-	level.GridSystem.SetGrid(grid.NewGrid(game.World, level.Cellsize, level.Width, level.Height, level.Mapslice))
+	level.GridSystem.SetGrid(
+		grid.NewGrid(game.World, level.Cellsize, level.Width, level.Height, level.Mapslice))
 }
 
+// parses a RawLevel and generates a mapslice from it, which is being used as grid
 func LevelToSlice(game *Game, level *assets.RawLevel, tilesize int) map[image.Point]*assets.Tile {
 	size := game.ScreenWidth * game.ScreenHeight
 	mapslice := make(map[image.Point]*assets.Tile, size)
@@ -93,6 +96,10 @@ func LevelToSlice(game *Game, level *assets.RawLevel, tilesize int) map[image.Po
 		}
 
 		for x, char := range line {
+			if !util.Exists(assets.Tiles, byte(char)) {
+				log.Fatalf("unregistered tile type %c encountered", char)
+			}
+
 			mapslice[image.Point{x, y}] = assets.Tiles[byte(char)]
 		}
 	}

grid/grid.go

@@ -3,7 +3,6 @@ package grid
 import (
 	"image"
 	"log"
-	"log/slog"
 	"openquell/assets"
 	"openquell/components"
 	"openquell/config"
@@ -14,6 +13,7 @@ import (
 )
 
 type Grid struct {
+	World    *ecs.World
 	Width    int
 	Height   int
 	Size     int
@@ -48,18 +48,26 @@ func NewGrid(world *ecs.World,
 		components.Renderable,
 		components.Collectible](world)
 
-	obsmapper := generic.NewMap4[
+	obsmapper := generic.NewMap5[
 		components.Position,
 		components.Velocity,
 		components.Renderable,
+		components.Speed,
 		components.Obstacle](world)
 
+	transmapper := generic.NewMap3[
+		components.Position,
+		components.Renderable,
+		components.Transient](world)
+
 	var pos *components.Position
 	var vel *components.Velocity
 	var render *components.Renderable
 	var speed *components.Speed
+	var transient *components.Transient
 
 	playerobserver := observers.GetPlayerObserver(world)
+	obstacleobserver := observers.GetObstacleObserver(world)
 
 	for point, tile := range mapslice {
 		switch tile.Renderable {
@@ -73,15 +81,20 @@ func NewGrid(world *ecs.World,
 				pos, _, render, speed, _ = playermapper.Get(entity)
 				playerobserver.AddEntity(entity)
 				speed.Value = config.PLAYERSPEED
-				slog.Debug("player start pos", "X", point.X*tilesize,
-					"Y", point.Y*tilesize, "Z", 191)
 			case tile.Collectible:
 				entity := colmapper.New()
 				pos, render, _ = colmapper.Get(entity)
 			case tile.Obstacle:
 				entity := obsmapper.New()
-				pos, vel, render, _ = obsmapper.Get(entity)
+				pos, vel, render, speed, _ = obsmapper.Get(entity)
 				vel.Direction = tile.Direction
+				vel.PointingAt = tile.Direction
+				speed.Value = config.PLAYERSPEED
+				obstacleobserver.AddEntity(entity)
+			case tile.Transient:
+				entity := transmapper.New()
+				pos, render, transient = transmapper.Get(entity)
+				transient.Sprites = tile.TileNames
 			default:
 				log.Fatalln("unsupported tile type encountered")
 			}
@@ -106,6 +119,7 @@ func NewGrid(world *ecs.World,
 		Width:    width,
 		Height:   height,
 		Map:      mapslice,
+		World:    world,
 	}
 }
 
@@ -122,3 +136,19 @@ func (grid *Grid) GetTile(
 	tile := grid.Map[newpoint]
 	return tile
 }
+
+func (grid *Grid) SetTile(tile *assets.Tile, point image.Point) {
+	solidmapper := generic.NewMap4[
+		components.Position,
+		components.Renderable,
+		components.Tilish,
+		components.Solid](grid.World)
+
+	grid.Map[point] = tile
+
+	entity := solidmapper.New()
+	pos, render, _, _ := solidmapper.Get(entity)
+
+	render.Image = tile.Sprite
+	pos.Update(point.X*grid.Tilesize, point.Y*grid.Tilesize, grid.Tilesize)
+}

observers/obstacle_observer.go

@@ -0,0 +1,54 @@
+package observers
+
+import (
+	"github.com/mlange-42/arche/ecs"
+	"github.com/mlange-42/arche/ecs/event"
+	"github.com/mlange-42/arche/generic"
+	"github.com/mlange-42/arche/listener"
+)
+
+// will be added as  an ecs.Resource to the world so we  can use it in
+// CollisionSystem to dynamically make it appear or disappear
+type ObstacleObserver struct {
+	// we only have one obstacle so far, if we use multiple ones, turn
+	// this in to a map, see player_observer.go for an example
+	Entities map[ecs.Entity]int
+}
+
+// Create a  new resource of  type PlayerObserver which will  hold all
+// obstacle  entities.   We  also  add  a  Listener  which  looks  for
+// EntityRemoved events and  if a player gets removed,  we also remove
+// it from the Entity map in our observer.
+func NewObstacleObserver(world *ecs.World) {
+	observer := &ObstacleObserver{}
+	observer.Entities = make(map[ecs.Entity]int)
+
+	resmanger := generic.NewResource[ObstacleObserver](world)
+	resmanger.Add(observer)
+
+	listen := listener.NewCallback(
+		func(world *ecs.World, event ecs.EntityEvent) {
+			observerID := ecs.ResourceID[ObstacleObserver](world)
+			observer := world.Resources().Get(observerID).(*ObstacleObserver)
+			observer.RemoveEntity(event.Entity)
+		},
+
+		event.EntityRemoved,
+	)
+
+	world.SetListener(&listen)
+}
+
+func GetObstacleObserver(world *ecs.World) *ObstacleObserver {
+	observerID := ecs.ResourceID[ObstacleObserver](world)
+	observer := world.Resources().Get(observerID).(*ObstacleObserver)
+	return observer
+}
+
+func (observer *ObstacleObserver) AddEntity(entity ecs.Entity) {
+	observer.Entities[entity] = 1
+}
+
+func (observer *ObstacleObserver) RemoveEntity(entity ecs.Entity) {
+	observer.Entities = make(map[ecs.Entity]int)
+}

systems/collectible_system.go

@@ -109,7 +109,7 @@ func (system *CollectibleSystem) AddParticle(position *components.Position) {
 	particleobserver.AddEntity(entity)
 
 	particle.Index = assets.Tiles['*'].Particle
-	particle.Particles = assets.Tiles['*'].Particles
+	particle.Tiles = assets.Tiles['*'].Tiles
 
 	pos.Update(
 		position.X-(16), // FIXME: use global tilesize!

systems/grid_system.go

@@ -18,6 +18,7 @@ type GridSystem struct {
 	Selector                                *generic.Filter3[Renderable, Position, Solid]
 	UseCache                                bool
 	Cache                                   *ebiten.Image
+	Count                                   int // register tile count, invalidates cache
 	Background                              *ebiten.Image
 	Width, Height, TilesX, TilesY, Tilesize int
 	Grid                                    *grid.Grid
@@ -52,12 +53,13 @@ func (system *GridSystem) Update() {}
 
 func (system *GridSystem) Draw(screen *ebiten.Image) {
 	op := &ebiten.DrawImageOptions{}
+	query := system.Selector.Query(system.World)
 
-	if !system.UseCache {
+	if !system.UseCache || query.Count() != system.Count {
 		// map not cached or cacheable, write it to the cache
 		draw.Draw(system.Cache, system.Background.Bounds(), system.Background, image.ZP, draw.Src)
 
-		query := system.Selector.Query(system.World)
+		system.Count = query.Count()
 
 		for query.Next() {
 			sprite, pos, _ := query.Get()
@@ -76,6 +78,7 @@ func (system *GridSystem) Draw(screen *ebiten.Image) {
 		// use the cached map
 		op.GeoM.Reset()
 		screen.DrawImage(system.Cache, op)
+		query.Close()
 	}
 }
 
@@ -88,7 +91,7 @@ func (system *GridSystem) GetSolidNeighborPosition(
 		return false, nil
 	}
 
-	// set to true,  ifwe are already on the last  tile in the current
+	// set to true,  if we are already on the last  tile in the current
 	// direction, i.e. on the edge of the grid
 	edge := true
 	neighborpos := position.Point()

systems/obstacle_system.go

@@ -16,14 +16,15 @@ import (
 
 type ObstacleSystem struct {
 	World    *ecs.World
-	Selector        *generic.Filter4[Position, Velocity, Obstacle, Renderable]
-	PreviousFreePos *components.Position
+	Selector *generic.Filter5[Position, Velocity, Obstacle, Renderable, Speed]
+	Grid     *GridSystem
 }
 
-func NewObstacleSystem(world *ecs.World) *ObstacleSystem {
+func NewObstacleSystem(world *ecs.World, grid *GridSystem) *ObstacleSystem {
 	system := &ObstacleSystem{
-		Selector: generic.NewFilter4[Position, Velocity, Obstacle, Renderable](),
+		Selector: generic.NewFilter5[Position, Velocity, Obstacle, Renderable, Speed](),
 		World:    world,
+		Grid:     grid,
 	}
 
 	return system
@@ -32,6 +33,7 @@ func NewObstacleSystem(world *ecs.World) *ObstacleSystem {
 func (system *ObstacleSystem) Update() {
 	playerobserver := observers.GetPlayerObserver(system.World)
 	gameobserver := observers.GetGameObserver(system.World)
+	obstacleobserver := observers.GetObstacleObserver(system.World)
 
 	if gameobserver.Lost {
 		return
@@ -46,27 +48,67 @@ func (system *ObstacleSystem) Update() {
 	gameover := false
 
 	for query.Next() {
-		obsposition, obsvelocity, obstacle, _ := query.Get()
+		obsposition, obsvelocity, _, _, speed := query.Get()
 
+		// check if one player has bumped into current obstacle
 		for player := range playerobserver.Entities {
 			playerposition := (*Position)(system.World.Get(player, posID))
 			playervelocity := (*Velocity)(system.World.Get(player, veloID))
 
 			ok, newpos := obsposition.Intersects(playerposition, playervelocity)
 			if ok {
-				slog.Debug("bumped into obstacle", "obstacle", obstacle)
+				// slog.Debug("bumped into obstacle", "obstacle", obstacle)
 
 				if CheckObstacleSide(playervelocity, obsvelocity.Direction) {
+					// player died
 					EntitiesToRemove = append(EntitiesToRemove, player)
 					gameover = true
 				} else {
-					playervelocity.Change(Stop)
+					// bumped  into nonlethal obstacle side,  stop the
+					// player, set the obstacle in motion, if possible
 					slog.Debug("bump not die", "originalpos", playerposition)
+					obsvelocity.Set(playervelocity)
+					playervelocity.Change(Stop)
 					playerposition.Set(newpos)
-					slog.Debug("bump not die", "newpos", newpos)
 				}
 			}
 		}
+
+		// check if current obstacle bumped into another obstacle
+		for foreign_obstacle := range obstacleobserver.Entities {
+			if foreign_obstacle == query.Entity() {
+				// don't check obstacle against itself
+				continue
+			}
+
+			foreign_obstacle_position := (*Position)(system.World.Get(foreign_obstacle, posID))
+
+			ok, newpos := foreign_obstacle_position.Intersects(obsposition, obsvelocity)
+			if ok {
+				//slog.Debug("bumped into foreign obstacle", "obstacle", foreign_obstacle)
+				obsposition.Set(newpos)
+				obsvelocity.ResetDirectionAndStop()
+			}
+		}
+
+		// FIXME: this is the same loop as in player_system, unite the
+		// two, just iterate over all entities with pos,vel,render, dammit
+		if obsvelocity.Moving() {
+			ok, tilepos := system.Grid.GetSolidNeighborPosition(obsposition, obsvelocity, true)
+			if ok {
+				intersects, newpos := tilepos.Intersects(obsposition, obsvelocity)
+				if intersects {
+					// slog.Debug("collision with foreign obstacle detected", "tile",
+					// 	tilepos, "obs", obsposition, "new", newpos)
+
+					obsposition.Set(newpos)
+					obsvelocity.ResetDirectionAndStop()
+				}
+			}
+
+			obsposition.Move(obsvelocity, speed)
+		}
+
 	}
 
 	for _, entity := range EntitiesToRemove {
@@ -91,7 +133,7 @@ func (system *ObstacleSystem) Draw(screen *ebiten.Image) {
 	query := system.Selector.Query(system.World)
 
 	for query.Next() {
-		pos, _, _, sprite := query.Get()
+		pos, _, _, sprite, _ := query.Get()
 
 		op.GeoM.Reset()
 		op.GeoM.Translate(float64(pos.X), float64(pos.Y))
@@ -114,7 +156,7 @@ func (system *ObstacleSystem) AddParticle(position *components.Position) {
 	particleobserver.AddEntity(entity)
 
 	particle.Index = assets.Tiles['*'].Particle
-	particle.Particles = assets.Tiles['*'].Particles
+	particle.Tiles = assets.Tiles['*'].Tiles
 
 	pos.Update(
 		position.X-(16), // FIXME: use global tilesize!

systems/particle_system.go

@@ -40,7 +40,7 @@ func (system *ParticleSystem) Update() {
 		if timer.IsReady() {
 			switch {
 			// particle shows from earlier tick, animate
-			case particle.Index > -1 && particle.Index < len(particle.Particles)-1:
+			case particle.Index > -1 && particle.Index < len(particle.Tiles)-1:
 				particle.Index++
 				timer.Start(config.PARTICLE_LOOPWAIT)
 			default:
@@ -69,7 +69,7 @@ func (system *ParticleSystem) Draw(screen *ebiten.Image) {
 		if particle.Show {
 			op.GeoM.Reset()
 			op.GeoM.Translate(float64(pos.X), float64(pos.Y))
-			screen.DrawImage(particle.Particles[particle.Index], op)
+			screen.DrawImage(particle.Tiles[particle.Index], op)
 		}
 	}
 }

systems/player_system.go

@@ -1,7 +1,6 @@
 package systems
 
 import (
-	"log/slog"
 	. "openquell/components"
 	. "openquell/config"
 
@@ -16,6 +15,7 @@ type PlayerSystem struct {
 	Particle    *ParticleSystem
 	Collectible *CollectibleSystem
 	Obstacle    *ObstacleSystem
+	Transient   *TransientSystem
 	Grid        *GridSystem
 }
 
@@ -24,7 +24,8 @@ func NewPlayerSystem(world *ecs.World, grid *GridSystem) *PlayerSystem {
 		Selector:    generic.NewFilter5[Position, Velocity, Player, Renderable, Speed](),
 		Particle:    NewParticleSystem(world, grid.Tilesize),
 		Collectible: NewCollectibleSystem(world),
-		Obstacle:    NewObstacleSystem(world),
+		Obstacle:    NewObstacleSystem(world, grid),
+		Transient:   NewTransientSystem(world, grid),
 		Grid:        grid,
 		World:       world,
 	}
@@ -55,15 +56,15 @@ func (system PlayerSystem) Update() error {
 		if velocity.Moving() {
 			ok, newpos := system.Grid.BumpEdge(playerposition, velocity)
 			if ok {
-				slog.Debug("falling off the edge", "newpos", newpos)
+				//slog.Debug("falling off the edge", "newpos", newpos)
 				playerposition.Set(newpos)
 			} else {
 				ok, tilepos := system.Grid.GetSolidNeighborPosition(playerposition, velocity, true)
 				if ok {
 					intersects, newpos := tilepos.Intersects(playerposition, velocity)
 					if intersects {
-						slog.Debug("collision detected", "tile",
-							tilepos, "player", playerposition, "new", newpos)
+						// slog.Debug("collision detected", "tile",
+						// 	tilepos, "player", playerposition, "new", newpos)
 
 						playerposition.Set(newpos)
 						velocity.Change(Stop)
@@ -76,6 +77,7 @@ func (system PlayerSystem) Update() error {
 	system.Particle.Update() // may set player position
 	system.Obstacle.Update()
 	system.Collectible.Update()
+	system.Transient.Update()
 
 	query = system.Selector.Query(system.World)
 	for query.Next() {
@@ -104,4 +106,5 @@ func (system *PlayerSystem) Draw(screen *ebiten.Image) {
 	system.Collectible.Draw(screen)
 	system.Particle.Draw(screen)
 	system.Obstacle.Draw(screen)
+	system.Transient.Draw(screen)
 }

systems/transient_system.go

@@ -0,0 +1,116 @@
+package systems
+
+import (
+	"log/slog"
+	"openquell/assets"
+	"openquell/components"
+	. "openquell/components"
+	"openquell/observers"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/mlange-42/arche/ecs"
+	"github.com/mlange-42/arche/generic"
+)
+
+type TransientSystem struct {
+	World       *ecs.World
+	Selector    *generic.Filter3[Position, Renderable, Transient]
+	Grid        *GridSystem
+	SolidMapper generic.Map4[ // needed for replacement
+		components.Position,
+		components.Renderable,
+		components.Tilish,
+		components.Solid]
+}
+
+type TransientToWall struct {
+	Entity    ecs.Entity
+	NewSprite string
+	Position  components.Position
+}
+
+func NewTransientSystem(world *ecs.World, grid *GridSystem) *TransientSystem {
+	solidmapper := generic.NewMap4[
+		components.Position,
+		components.Renderable,
+		components.Tilish,
+		components.Solid](world)
+
+	system := &TransientSystem{
+		Selector:    generic.NewFilter3[Position, Renderable, Transient](),
+		World:       world,
+		Grid:        grid,
+		SolidMapper: solidmapper,
+	}
+
+	return system
+}
+
+func (system *TransientSystem) Update() {
+	playerobserver := observers.GetPlayerObserver(system.World)
+	posID := ecs.ComponentID[components.Position](system.World)
+	veloID := ecs.ComponentID[components.Velocity](system.World)
+
+	query := system.Selector.Query(system.World)
+
+	EntitiestoMakeSolid := []TransientToWall{}
+
+	for query.Next() {
+		transientposition, _, transient := query.Get()
+
+		for player := range playerobserver.Entities {
+			playerposition := (*Position)(system.World.Get(player, posID))
+			playervelocity := (*Velocity)(system.World.Get(player, veloID))
+
+			ok, _ := transientposition.Intersects(playerposition, playervelocity)
+			if ok {
+				// display the transient sprite as long as the player crosses it
+				transient.Activated = true
+			} else {
+				// the player crossed the transient wall completely
+				if transient.Activated {
+					EntitiestoMakeSolid = append(EntitiestoMakeSolid, TransientToWall{
+						Entity:    query.Entity(),
+						Position:  *transientposition,
+						NewSprite: transient.GetNext(),
+					})
+					slog.Debug("done transient", "transient", transientposition)
+				}
+			}
+		}
+	}
+
+	for _, convertible := range EntitiestoMakeSolid {
+		// remove transient entity
+		system.World.RemoveEntity(convertible.Entity)
+
+		// replace with solid entity
+		entity := system.SolidMapper.New()
+		pos, render, _, _ := system.SolidMapper.Get(entity)
+
+		// set it up apropriately
+		pos.Set(&convertible.Position)
+		render.Image = assets.Assets[convertible.NewSprite]
+
+		//  also  setup the  grid  tile  with  a  new solid,  so  that
+		// collision detection works
+		system.Grid.Grid.SetTile(
+			assets.NewTileBlock(convertible.NewSprite),
+			convertible.Position.Point(),
+		)
+	}
+}
+
+func (system *TransientSystem) Draw(screen *ebiten.Image) {
+	// write transients (these are no tiles!)
+	op := &ebiten.DrawImageOptions{}
+	query := system.Selector.Query(system.World)
+
+	for query.Next() {
+		pos, render, _ := query.Get()
+
+		op.GeoM.Reset()
+		op.GeoM.Translate(float64(pos.X), float64(pos.Y))
+		screen.DrawImage(render.Image, op)
+	}
+}