1. Leszek Godlewski
Programmer, Nordic Games
Gamedev-grade debugging
Source: http://igetyourfail.blogspot.com/2009/01/reaching-out-tale-of-failed-skinning.html

2. Nordic Games GmbH
● Started in 2011 as a sister company to Nordic Games
Publishing (We Sing)
● Base IP acquired from JoWooD and DreamCatcher
(SpellForce, The Guild, Aquanox, Painkiller)
● Initially focusing on smaller, niche games
● Acquired THQ IPs in 2013 (Darksiders, Titan Quest, Red
Faction, MX vs. ATV)
● Now shifting towards being a production company with
internal devs
● Since fall 2013: internal studio in Munich, Germany
(Grimlore Games)

3. Who is this guy?
Leszek Godlewski
Programmer, Nordic Games (early 2014 – Nov 2014)
– Linux port of Darksiders
Freelance Programmer (Sep 2013 – early 2014)
– Linux port of Painkiller Hell & Damnation
– Linux port of Deadfall Adventures
Generalist Programmer, The Farm 51 (Mar 2010 – Aug 2013)
– Painkiller Hell & Damnation, Deadfall Adventures

4. Agenda
How is gamedev different?
Bug species
Case studies
Conclusions

5. How is gamedev different?
StartStart Exit?Exit?
EndEnd
Yes
No
UpdateUpdate DrawDraw

6. 33 milliseconds
How much time you have to get shit done™
– 30 Hz → 33⅓ ms per frame
– 60 Hz → 16⅔ ms per frame
EditorEditor
Level toolsLevel tools
Asset toolsAsset tools
EngineEngine
PhysicsPhysics
RenderingRendering AudioAudio
NetworkNetwork
PlatformPlatform
InputInput
Network
back-end
Network
back-end
GameGame
UIUI LogicLogic AIAI

7. Interdisciplinary working environment
Designers
– Game, Level, Quest, Audio…
Artists
– Environment, Character, 2D, UI, Concept…
Programmers
– Gameplay, Engine, Tools, UI, Audio…
Writers
Composers
Actors
Producers
PR & Marketing Specialists
…
}Tightly
woven
teams

8. Severe, fixed hardware constraints
Main reason for extensive use of native code

9. Different trade-offs
Robustness
Cost
Performance
Fun
/Coolness
Enterprise/B2B/webdev Gamedev

10. Indeterminism & complexity
Leads to poor testability
– Parts make no sense in isolation
– What exactly is correct?
– Performance regressions?
Source: https://github.com/memononen/recastnavigation

11. Aversion to general software engineering
Modelling
Object-Oriented Programming
Design patterns
C++ STL
Templates in general
…

12. Agenda
How is gamedev different?
Bug species
Case studies
Conclusions

13. Bug species
Source: http://benigoat.tumblr.com/post/100306422911/press-b-to-crouch

14. General programming bugs
Memory access violations
Memory stomping/buffer overflows
Infinite loops
Uninitialized variables
Reference cycles
Floating point precision errors
Out-Of-Memory/memory fragmentation
Memory leaks
Threading errors

15. Bad maths
Incorrect transform order
– Matrix multiplication not commutative
– AB ≠ BA
Incorrect transform space
Source: http://leadwerks.com/wiki/index.php?title=TFormQuat

16. Temporal bugs
Incorrect update order
– for (int i = 0; i < entities.size(); ++i)
entities[i].update();
Incorrect interpolation/blending
– Bad alpha term
– Bad blending mode (additive/modulate)
Deferred effects
– After n frames
– After n times an action happens
– n may be random, indeterministic

17. Graphical glitches
Incorrect render state
Shader code bugs
Precision
Source: http://igetyourfail.blogspot.com/2009/01/visit-lake-fail-this-weekend.html

18. Content bugs
Incorrect scripts
Buggy assets
Source: http://www.polycount.com/forum/showpost.php?p=1263124&postcount=10466

19. Worst part?
Most cases are two or more of the aforementioned,
intertwined

20. Agenda
How is gamedev different?
Bug species
Case studies
Conclusions

21. Case studies
Most material captured by

22. Video settings not updating

23. Incorrect weapon after demon
mode foreshadowing

24. Post-death sprint camera anim

25. Corpses teleported on death

26. Corpses teleported on death
In normal gameplay, pawns have simplified movement
– Sweep the actor's collision primitive through the world
– Slide along slopes, stop against walls
Source: http://udn.epicgames.com/Three/PhysicalAnimation.html

27. Corpses teleported on death
Upon death, pawns switch to physics-based movement
(ragdoll)
Source: http://udn.epicgames.com/Three/PhysicalAnimation.html

28. Corpses teleported on death (cont.)
Physics bodies have separate state from the game actor
– Actor does not drive physics bodies, unless requested
– If actor is driven by physics simulation, their location is synchronized to
the hips bone body's
Source: http://udn.epicgames.com/Three/PhysicalAnimation.html

29. Corpses teleported on death (cont.)
Idea: breakpoint in FarMove()?
– One function because world octree is updated
– Function gets called a gazillion times per frame �
– Terrible noise
Breakpoint condition?
– Teleport from arbitrary point A to arbitrary point B
– Distance?
Breakpoint sequence?
– Break on death instead
– When breakpoint hit, break in FarMove()

30. Corpses teleported on death (cont.)
Cause: physics body driving the actor with out-of-date
state
Fix: request physics body state synchronization to
animation before switching to ragdoll

31. Weapons floating away from the
player

32. Weapons floating away from the
player

33. Weapons floating away from the player
Extremely rare, only encountered on consoles
– Reproduction rate somewhere at 1 in 50 attempts
– And never on developer machines �
Player pawn in a special state for the rollercoaster ride
– Many things could go wrong
For the lack of repro, sprinkled the code with debug logs

34. Weapons floating away from the player (cont.)
Cause: incorrect update order
– for (int i = 0; i < entities.size(); ++i)
entities[i].update();
– Player pawn forced to update after rollercoaster car
– Possible for weapons to be updated before player pawns
Fix: enforce weapon update after player pawns

35. Characters with “rapiers”

36. Characters with “rapiers”
UE3 has ”content cooking” as part of game build pipeline
– Redistributable builds are ”cooked” builds
Artifact appears only in cooked builds

37. Characters with “rapiers” – cont.
Logs contained assertions for ”out-of-bounds vertices”
Mesh vertex compression scheme
– 32-bit float → 16-bit short int (~50% savings)
– Find bounding sphere for all vertices
– Normalize all vertices to said sphere radius
– Map [-1; 1] floats to [-32768; 32767] 16-bit integers
Assert condition
– for (int i = 0; i < 3; ++i)
assert(v[i] >= -1.f && v[i] <= 1.f,
”Out-of-bound vertex!”);

38. Characters with “rapiers” – cont.
v[i] was NaN
– Interesting property of NaN: all comparisons fail
– Even with itself
●
float f = nanf();
bool b = (f == f);
// b is false
How did it get there?!
Tracked the NaN all the way down to the raw engine
asset!

39. Characters with “rapiers” (cont.)
Cause: ???
Fix: re-export the mesh from 3D software
– Magic!

40. Meta-case: undeniable assertion

41. Undeniable assertion
Happened while debugging ”rapiers”
Texture compression library without sources
Flood of non-critical assertions
– For almost every texture
– Could not ignore in bulk �
– Terrible noise
Solution suggestion taken from [SINILO12]

42. Undeniable assertion (cont.)
Enter disassembly

43. Undeniable assertion (cont.)
Locate assert message function call instruction

44. Undeniable assertion (cont.)
Enter memory view and look up the adress
– 0xE8 is the CALL opcode
– 4-byte address argument

45. Undeniable assertion (cont.)
NOP it out!
– 0x90 is the NOP opcode

46. Undeniable assertion (cont.)

47. Incorrect player movement

48. Incorrect player movement

49. Incorrect player movement
Recreating player movement from one engine in another
(Pain Engine → Unreal Engine 3)
Different physics engines (Havok vs PhysX)
Many nuances
– Air control
– Jump and fall heights
– Slope & stair climbing & sliding down

50. Incorrect player movement (cont.)
Main nuance: capsule vs cylinder

51. Incorrect player movement (cont.)
Switching our pawn collision to capsule-based was not an
option
Emulate by sampling the ground under the cylinder
instead
No clever way to debug, just make it ”bug out” and break
in debugger

52. Incorrect player movement (cont.)
Situation when getting stuck
Cause: vanilla UE3 code sent a player locked between
non-walkable surfaces into the ”falling” state
Fix: keep the player ”walking”

53. Incorrect player movement (cont.)
Situation when moving without player intent
Added visualization of sampling, turned on collision
display
Cause: undersampling
Fix: increase radial sampling resolution
1) 2)

54. Blinking full-screen damage
effects

55. Blinking full-screen damage effects
Post-process effects are organized in one-way chains

56. Blinking full-screen damage effects (cont.)
No debugger available to observe the PP chain
Rolled my own overlay that walked and dumped the
chain contents
MaterialEffect 'Vignette'
Param 'Strength' 0.83 [IIIIIIII ]
MaterialEffect 'FilmGrain'
Param 'Strength' 0.00 [ ]
UberPostProcessEffect 'None'
SceneHighLights (X=0.80,Y=0.80,Z=0.80)
SceneMidTones (X=0.80,Y=0.80,Z=0.80)
…
MaterialEffect 'Blood'
Param 'Strength' 1.00 [IIIIIIIIII]

57. Blinking full-screen damage effects (cont.)
Cause: entire PP chain override
– Breakpoint in chain setting revealed the level script as the source
– Overeager level designer ticking one checkbox too many when setting
up thunderstorm effects
Fix: disable chain overriding altogether
– No use case for it in our game anyway

58. Incorrect animation states

59. Incorrect animation states

60. Incorrect animation states

61. Incorrect animation states
Animation in UE3 is done by evaluating a tree
– Branches are weight-blended (either replacement or additive blend)
– Sequences (raw animations) for whole-skeleton poses
– Skeletal controls for fine-tuning of individual bones
Source: http://udn.epicgames.com/Three/AnimTreeEditorUserGuide.html

62. Incorrect animation states (cont.)
Prominent case for domain-specific debuggers
No tools for that in UE3, rolled my own visualizer
– Allows inspection of animation state, but not the reasons for transitions
– Still requires conventional debugging, but narrows it down greatly
– Walks the animation tree and dumps active branches and its parameters

63. Incorrect animation states (cont.)
We have developed sort of an animation bug checklist
Inspect the animation state in slow motion
– Is the correct blending mode used?
Inspect the AI and cutscene state
– Capable of animation overrides
Inspect the assets (animation sequences)
– Is the root bone correctly oriented?
– Is the root bone motion correct?
– Are inverse kinematics targets present and correctly placed?
– Is the mesh skeleton complete and correct?

64. Incorrect animation states (cont.)
Incorrect blend of reload animation
– Cause: bad root bone orientation in animation sequence
Left hand off the weapon
– Cause: left hand inverse kinematics was off
– Fix: revise IK state control code
Left hand incorrectly oriented
– Cause: bad IK target marker orientation on weapon mesh

65. Viewport stretched when portals
are in view

66. Viewport stretched when portals are in view
Graphics debugging is:
– Tracing & recording graphics API (OpenGL/Direct3D) calls
– Replaying the trace
– Reviewing the renderer state and resources
Trace may be somewhat unreadable at first…

67. Viewport stretched when portals are… (cont.)
Traces may be annotated for clarity
– Direct3D: ID3DUserDefinedAnnotation
– OpenGL: GL_KHR_debug (more info: [GODLEWSKI01])

68. Viewport stretched when portals are… (cont.)
Quick renderer state inspection revealed that viewport
dimensions were off
– 1024x1024, 1:1 aspect ratio instead of 1280x720, 16:9
– Shadow map resolution?
Found the latest glViewport() call
– Shadow map indeed
Why wasn't the viewport updated for main scene
rendering?

69. Viewport stretched when portals are… (cont.)
Renderer state changes are expensive
– New state needs to be validated
– Modern graphics APIs are asynchronous
– State reading may requrie synchronization → stalls
Cache the current renderer state to avoid redundant calls
– Cache ↔ state divergence → bugs!

70. Viewport stretched when portals are… (cont.)
Cause: cache ↔ state divergence
– Difference between Direct3D and OpenGL: viewport dimensions as part
of render target state, or global state
Fix: tie viewport dimensions to render target in the cache

71. Black artifacts

72. Black artifacts

73. Black artifacts

74. Black artifacts

75. Black artifacts

76. Black artifacts
First thing to do is to inspect the state
Nothing suspicious found, turned to shaders
On OpenGL 4.2+, shaders could be debugged in NSight…
OpenGL 2.1, so had to resort to early returns from shader
with debug colours
– Shader equivalent of debug logs, a.k.a. ”Your Mum's Debugger”
”Shotgun debugging” with is*() functions
isnan() returned true!

77. Black artifacts (cont.)
Cause: undefined behaviour in NVIDIA's pow()
implementation
– Results are undefined if x < 0.
Results are undefined if x = 0 and y <= 0. [GLSL120]
– Undefined means the implementation is free to do whatever
●
NVIDIA returns QNaN the Barbarian (displayed as black, poisoning
all involved calculations)
●
Other vendors usually return 0
Fix: for all pow() calls, clamp either:
– Arguments to their proper ranges
– Output to [0; ∞)

78. Mysterious crash

79. Mysterious crash
Game in content lock (feature freeze) for a while
Playstation 3 port nearly done
Crash ~3-5 frames after entering a specific room
First report included a perfectly normal callstack but no
obvious reason
QA reassigned to another task, could not pursue more
Concluded it must've been an OOM crash

80. Mysterious crash (cont.)
Bug comes back, albeit with wildly different callstack
Asked QA to reproduce mutliple times, including other
platforms
– No crashes on X360 & Windows!
Totally different callstack each time
Confusion!
– OOM? Even in 512 MB developer mode (256 MB in retail units)?
– Bad content?
– Console OS bug?
– Audio thread?
– ???

81. Mysterious crash (cont.)
Reviewed a larger sample of callstacks
Most ended in dlmalloc's integrity checks
– Assertions triggered upon allocations and frees
Memory stomping…? Could it be…?

82. Mysterious crash (cont.)
Started researching memory debugging
No tools provided by Sony
Attempted to use debug allocators (dmalloc et al.)
– Most use the concept of memory fences
– Difficult to hook up to UE3
malloc
Regular allocation Fenced allocation
malloc

83. Mysterious crash (cont.)
Found and integrated a community-developed tool, Heap
Inspector [VANDERBEEK14]
– Memory analyzer
– Focused on consumption and usage patterns monitoring
– Records callstacks for allocations and frees
Several reproduction attempts revealed a correlation
– Crash adress
– Construction of a specific class
Gotcha!

84. Mysterious crash (cont.)
// class declaration
class Crasher extends ActorComponent;
var int DummyArray[1024];
// in ammo consumption code
Crash = new class'Crasher';
Comp = new class'ActorComponent'
(Crash);

85. Mysterious crash (cont.)
// class declaration
class Crasher extends ActorComponent;
var int DummyArray[1024];
// in ammo consumption code
Crash = new class'Crasher';
Comp = new class'ActorComponent'
(Crash);

86. Mysterious crash (cont.)
Cause: buffer overflow vulnerability in UnrealScript VM
– No manifestation on X360 & Windows due to larger allocation
alignment value (8 vs 16 bytes)
Fix: make copy-construction with subclassed object as
template fail
I wish I had Valgrind! [GODLEWSKI02]

87. Agenda
How is gamedev different?
Bug species
Case studies
Conclusions

88. Takeaway
Time is of the essence!
Always on a tight schedule
Constantly in motion
– Temporal visualization is key
– Custom, domain-specific tools
Complex and indeterministic
– Difficult to automate testing
– Wide knowledge required
Prone to bugs outside the code
– Custom, domain-specific tools, again

89. Takeaway (cont.)
Rendering is a whole separate beast
– Absolutely custom tools in isolation from the rest of the game
– Still far from ideal usability
Good to know your machine down to the metal
Good memory debugging tools make a world's difference
You are never safe, not even in managed languages!

90. @ l go d l ews k i @ n o rd i c ga m e s . at
t @ T h e I n e Q u ati o n
K w w w. i n e q u ati o n . o rg
Questions?

91. F u rt h e r N o rd i c G a m e s i nfo rm ati o n :
K w ww. n o rd i c ga m e s . at
Deve l o p me nt i nfo rmati o n :
K ww w. gr i m l o re ga m e s . co m
Thank you!

92. References
 SINILO12 – Sinilo, M. ”Coding in a debugger” [link]
 GODLEWSKI01 – Godlewski, L. ”OpenGL (ES) debugging” [link]
 GLSL120 – Kessenich, J. ”The OpenGL® Shading Language”, Language Version: 1.20, Document
Revision: 8, p. 57 [link]
 VANDERBEEK14 – van der Beek, J. ”Heap Inspector” [link]
 GODLEWSKI02 – Godlewski, L. ”Advanced Linux Game Programming” [link]