1. 1
A Reactive 3D Game Engine in Scala
Aleksandar Prokopec
@_axel22_

2. 2
What’s a game engine?

3. 3
Simulation

4. 4
Real-time simulation

5. 5
15 ms
Real-time simulation

6. 6
Demo first!
http://youtu.be/pRCzSRhifLs

7. 7
Input Simulator
Interaction
Renderer

8. Reactive Collections
http://reactive-collections.com
8

9. 9
Reactive values

10. Reactive[T]
10

11. val ticks: Reactive[Long]
11
ticks 1
1
2
2
3
3
4
4
60
60
61
61

12. ticks onEvent { x =>
log.debug(s”tick no.$x”)
}
12
1 2 3 4 60 61
tick no.1
tick no.2
tick no.3
tick no.4
tick no.60
tick no.61
...

13. ticks foreach { x =>
log.debug(s”tick no.$x”)
}
13
1 2 3 4 60 61

14. 14
for (x <- ticks) {
log.debug(s”tick no.$x”)
}

15. 15
Reactive combinators

16. for (x <- ticks) yield {
x / 60
}
16

17. val seconds: Reactive[Long] =
for (x <- ticks) yield {
x / 60
}
17

18. 6061
val seconds: Reactive[Long] =
for (x <- ticks) yield {
x / 60
}
18
ticks 1
1
2
2
3
3 60 61
seconds
0 0 0 1 1
ticks
seconds
00011

19. val days: Reactive[Long] =
seconds.map(_ / 86400)
19

20. val days: Reactive[Long] =
seconds.map(_ / 86400)
val secondsToday =
20

21. val days: Reactive[Long] =
seconds.map(_ / 86400)
val secondsToday =
(seconds zip days) {
(s, d) =>
s – d * 86400
} 21

22. val days: Reactive[Long] =
seconds.map(_ / 86400)
val secondsToday =
(seconds zip days) {
_ – _ * 86400
}
22
seconds days
secondsToday

23. val angle =
secondsInDay.map(angleFunc)
23

24. val angle =
secondsInDay.map(angleFunc)
val light =
secondsInDay.map(lightFunc)
24

25. 25
https://www.youtube.com/watch?v=5g7DvNEs6K8&feature=youtu.be
Preview

26. 26
val rotate =
keys
a ↓shift ↓ a ↑ shift ↑pgup ↓ pgup ↑keys

27. 27
val rotate =
keys.filter(_ == PAGEUP)
a ↓shift ↓ a ↑ shift ↑pgup ↓ pgup ↑keys
pgup ↓ pgup ↑filter

28. 28
val rotate =
keys.filter(_ == PAGEUP)
.map(_.down)
a ↓shift ↓ a ↑ shift ↑pgup ↓ pgup ↑keys
pgup ↓ pgup ↑filter
true falsemap

29. 29
if (rotate()) viewAngle += 1
true falsemap

30. 30
Signals

31. 31
Reactives are
discrete

32. 32
Signals are
continuous

33. 33
trait Signal[T]
extends Reactive[T] {
def apply(): T
}

34. 34
val rotate =
keys.filter(_ == PAGEUP)
.map(_.down)
.signal(false)
true falsemap
signal

35. 35
val rotate: Signal[Boolean] =
keys.filter(_ == PAGEUP)
.map(_.down)
.signal(false)
true falsemap
signal

36. 36
val rotate: Signal[Boolean]
val ticks: Reactive[Long]
ticks

37. 37
val rotate: Signal[Boolean]
val ticks: Reactive[Long]
ticks
rotate

38. 38
val rotate: Signal[Boolean]
val ticks: Reactive[Long]
val viewAngle: Signal[Double] =
ticks
rotate
viewAngle

39. 39
List(1, 2, 3).scanLeft(0)(_ + _)

40. 40
List(1, 2, 3).scanLeft(0)(_ + _)
→ List(0, 1, 3, 6)

41. 41
def scanLeft[S](z: S)(f: (S, T) => S)
: List[S]

42. 42
def scanLeft[S](z: S)(f: (S, T) => S)
: List[S]
def scanPast[S](z: S)(f: (S, T) => S)
: Signal[S]

43. 43
val rotate: Signal[Boolean]
val ticks: Reactive[Long]
val viewAngle: Signal[Double] =
ticks.scanPast(0.0)
ticks
rotate
viewAngle

44. 44
val rotate: Signal[Boolean]
val ticks: Reactive[Long]
val viewAngle: Signal[Double] =
ticks.scanPast(0.0) {
(a, _) =>
if (rotate()) a + 1 else a
}
ticks
rotate
viewAngle

45. 45
http://youtu.be/blG95W5uWQ8
Preview

46. 46
val velocity =
ticks.scanPast(0.0) {
(v, _) =>
}
val viewAngle =

47. 47
val velocity =
ticks.scanPast(0.0) {
(v, _) =>
if (rotate()) v + 1
}
val viewAngle =

48. 48
val velocity =
ticks.scanPast(0.0) {
(v, _) =>
if (rotate()) v + 1
else v – 0.5
}
val viewAngle =

49. 49
val velocity =
ticks.scanPast(0.0) {
(v, _) =>
if (rotate()) v + 1
else v – 0.5
}
val viewAngle =
velocity.scanPast(0.0)(_ + _)

50. 50
http://youtu.be/NMVhirZLWmA
Preview

51. 51
Higher-order
reactive values

52. 52
(T => S) => (List[S] => List[T])

53. 53
(T => S) => (List[S] => List[T])
Reactive[Reactive[S]]

54. 54
val mids = mouse
.filter(_.button == MIDDLE)
mids ↓ ↓ ↓↑ ↑ ↑

55. 55
val mids = mouse
.filter(_.button == MIDDLE)
val up = mids.filter(!_.down)
val down = mids.filter(_.down)
mids ↓ ↓ ↓
up
down ↓ ↓ ↓
↑ ↑ ↑
↑ ↑ ↑

56. 56
val mids = mouse
.filter(_.button == MIDDLE)
val up = mids.filter(!_.down)
val down = mids.filter(_.down)
val drags = down
.map(_ => mouse)
up
down ↓ ↓ ↓
↑ ↑ ↑

57. 57
val mids = mouse
.filter(_.button == MIDDLE)
val up = mids.filter(!_.down)
val down = mids.filter(_.down)
val drags = down
.map(_ => mouse)
up
down ↓ ↓ ↓
↑ ↑ ↑
Reactive[Reactive[MouseEvent]]

58. 58
val mids = mouse
.filter(_.button == MIDDLE)
val up = mids.filter(!_.down)
val down = mids.filter(_.down)
val drags = down
.map(_ => mouse)
up
down ↓ ↓ ↓
↑ ↑ ↑
drags

59. drags
up ↑ ↑ ↑
59
val mids = mouse
.filter(_.button == MIDDLE)
val up = mids.filter(!_.down)
val down = mids.filter(_.down)
val drags = down
.map(_ => mouse.until(up))
down ↓ ↓ ↓
mouse.until(up)

60. 60
val mids = mouse
.filter(_.button == MIDDLE)
val up = mids.filter(!_.down)
val down = mids.filter(_.down)
val drags = down
.map(_ => mouse.until(up))
down ↓ ↓ ↓
up ↑ ↑ ↑
drags

61. 61
val drags = down
.map(_ => mouse.until(up))
.map(_.map(_.xy))
drags
1, 1
2, 3
3, 5
4, 6
6, 9
9, 9

62. 62
val drags = down
.map(_ => mouse.until(up))
.map(_.map(_.xy))
.map(_.diffPast(_.xy - _.xy))
drags
0, 0
1, 2
1, 2
0, 0
2, 3
0, 0

63. 63
val drags = down
.map(_ => mouse.until(up))
.map(_.map(_.xy))
.map(_.diffPast(_.xy - _.xy))
.concat()
drags 0, 0 1, 2 1, 2 0, 0 2, 3 0, 0

64. 64
val drags = down
.map(_ => mouse.until(up))
.map(_.map(_.xy))
.map(_.diffPast(_.xy - _.xy))
.concat()
val pos =
drags.scanPast((0, 0))(_ + _)
drags 0, 0 1, 2 1, 2 0, 0 2, 3 0, 0
pos 0, 0 1, 2 2, 4 2, 4 4, 7 4, 7

65. 65
http://youtu.be/RsMSZ7OH2fo
Preview
However, a lot of object
allocations lead to GC issues.

66. 66
Reactive mutators

67. 67
class Matrix {
def apply(x: Int, y: Int): Double
def update(x: Int, y: Int, v: Double)
}

68. 68
val screenMat: Signal[Matrix] =
(projMat zip viewMat)(_ * _)
val invScreenMat =
screenMat.map(_.inverse)

69. 69
Reactive[immutable.Matrix[T]]

70. 70
val screenMat: Signal[Matrix] =
(projMat zip viewMat)(_ * _)
val invScreenMat =
screenMat.map(_.inverse)
(4*4*8 + 16 + 16)*4*100 = 64 kb/s

71. 71
val screenMat = Mutable(new Matrix)
(projMat, viewMat).mutate(screenMat) {
(p, v) => screenMat().assignMul(p, v)
}
val invScreenMat = Mutable(new Matrix)
screenMat.mutate(invScreenMat) {
m => invScreenMat().assignInv(m)
}

72. 72
Reactive collections

73. 73
http://youtu.be/ebbrAHNsexc
Preview
How do we model that a
character is selected?

74. 74
val selected: Reactive[Set[Character]]

75. 75
val selected: ReactSet[Character]

76. 76
trait ReactSet[T]
extends ReactContainer[T] {
def apply(x: T): Boolean
}

77. 77
trait ReactContainer[T] {
def inserts: Reactive[T]
def removes: Reactive[T]
}

78. 78
A reactive collection
is a pair
of reactive values

79. 79
val selected =
new ReactHashSet[Character]

80. 80

81. 81
val selected =
new ReactHashSet[Character]
val decorations = selected
.map(c => (c, decoFor(c)))

82. 82

83. 83
class ReactContainer[T] { self =>
def inserts: Reactive[T]
def removes: Reactive[T]
def map[S](f: T => S) =
new ReactContainer[S] {
def inserts: Reactive[T] =
self.inserts.map(f)
def removes: Reactive[T] =
self.removes.map(f)
}
}

84. 84
val selected =
new ReactHashSet[Character]
val decorations = selected
.map(c => (c, decoFor(c)))
.to[ReactHashMap]

85. 85
val selected =
new ReactHashSet[Character]
val decorations = selected
.map(c => (c, decoFor(c)))
.to[ReactHashMap]

86. 86
val selected =
new ReactHashSet[Character]
val decorations = selected
.map(c => (c, decoFor(c)))
.to[ReactHashMap]

87. 87
val selected =
new ReactHashSet[Character]
val decorations = selected
.map(c => (c, decoFor(c)))
.to[ReactHashMap]

88. 88
val selected =
new ReactHashSet[Character]
val decorations = selected
.map(c => (c, decoFor(c)))
.react.to[ReactHashMap]

89. 89
val selected =
new ReactHashSet[Character]
val decorations = selected
.map(c => (c, decoFor(c)))
.react.to[ReactHashMap]

90. 90
val selected =
new ReactHashSet[Character]
val decorations = selected
.map(c => (c, decoFor(c)))
.react.to[ReactHashMap]

91. 91
val selected =
new ReactHashSet[Character]
val decorations = selected
.map(c => (c, decoFor(c)))
.react.to[ReactHashMap]

92. 92
• reactive mutators
• reactive collections
• @specialized
• Scala Macros
• shipping computations to the GPU

93. 93
http://storm-enroute.com/macrogl/
MacroGL

94. 94
https://www.youtube.com/watch?v=UHCeXdxkx70
GC Preview

95. 95
Is Scala Ready?

96. 96
YES!

97. 97
Thank you!