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Realistic and Fast

Cloud Rendering in
Computer Games

Niniane Wang
Software Engineer
Microsoft Flight Simulator
(now at Google Inc)

Intro Video

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 Agenda
• Previous Work

• 3- D Modeling + Art Pipeline

• Performance

• Shading model

• Animation: Formation and Dissipation

• Q & A

(All slide backgrounds are actual screenshots.)

Previous Research
• Harris, S k y W o r k s

– Use GPU to improve performance

– Impostor for each cloud

• Dobashi

– Metaballs

– Anisotropic scattering

• Ebert, Blinn, others

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 Previous Games
• Flight Simulator 2002
– Each cloud is a single billboard
• Combat Flight Simulator 3
– Each cloud is a few unique billboards
• I L- 2 S t u r m o v i k
– Each cloud is a large number of small particles

Our Enhancements Over Previous

Systems

• Many distinct cloud types (e.g. altocumulus,

cumulonimbus)
• Art pipeline allows fine -grained control
over model and shading
• Real-time performance (15 – 60 fps)
– Even for overcast scenes

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 Concept: Cluster of Sprites
Each cloud is composed of 5 – 50 textured sprites.

Cloud Creation

Each cloud is created by artists in 3D Studio Max.

• Use boxes to build cloud shape.

• Custom-written Max script to randomly fill boxes with

sprites.

• Immediate visual feedback

• Export final model to a file to load into game

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 Cloud Creation Video

Artist-specified Parameters

• # sprites to control cloud density

• Category (“stratus”, “solid cumulus”) to
detemine texture
• Range for width and height of sprite
• Range of rotation for each sprite to give
more variety

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 Cloud Sprite Generation

• Artist presses a button…

• 3 D S M a x p l u g- in creates a list of randomly
placed sprite centers
• It culls all sprites whose centers are within a
“cull radius” of each other
– Cull radius of 1/3 of cloud height is good for
typical clouds, 1/5 for dense clouds

Real- World Cloud Types

Real life cloud types have distinct looks

• Cumulus, stratus, cumulonimbus

• Sub -categories

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 Simulating Cloud Types

Mix and match 16 textures

• Solid puffs for cumulus, blurry
puffs for stratus,

• Less video memory than using

unique textures for each cloud

Cloud Types Video

7
 In
In-- Cloud Experience

• Sprite disappears as the camera passes through it

• Advantages of using cluster of sprites:

– Consistent with cloud as seen from the outside – wispy parts are
still wispy

– Each cloud has different in -cloud experience, unlike with

canned animation

In-cloud problems and solutions

• Initially, “parting of the Red Sea” problem

• Solve by locking the sprite within a distance

• This causes sprite edges to be visible

• Solution: Take dot product of lock angle and angle to

camera, and adjust sprite transparency

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 In
In-- Cloud Video

Performance

• Requirements:
– Flight Simulator must maintain 15 to 60 fps
– Overcast scenes are the biggest challenge
– Emulate real-world conditions (“Real- World
Weather” feature)
– Range of machines: 700 MHz to 3.0 GHz

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 Performance: Impostors

Main bottleneck is in overdraw

Reduce overdraw by rendering

multiple clouds into a single
billboard

Ring of Impostors

Octagonal ring around user eyepoint

• Clouds within ring are drawn in 3 - D

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 Impostors

• Render same billboard across many frames

• Dynamically update impostor upon position/time change

– Empirical results are 15% of impostor ring radius horizontally a nd

2% vertically

– Time change of 10 minutes

• User can set ring radius

– Smaller ring means better performance but more visual anomalies

Impostors: Visual Anomalies

• Parallax

• Interaction with terrain and objects

• Rendering to texture not supported across all video card

hardware

• Gray edges (the “silver lining”)

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 Fallback on older systems

• On old systems (< 450 MHz), even

rendering a single block of 3- d sprites is too
expensive
• Fall back to L o D scheme of single-billboard
clouds
– This is a degenerate case of our cluster of
sprites model

Performance Results

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 Impostor Video

Shading

• We chose artist -driven system rather than

simulating scattering of light
• Model lighting for different times of day
• Ambient and directional

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 Ambient Shading
Simulate the filtering of light from the sky
• Clouds have dark bottoms, esp. cumulus

• Artist specifies 5 “color levels”. Each level is a height

with associated RGBA color.

– Color also used to give cloud types their distinct look (e.g.
more transparency for stratus)

Illustration of Color Levels

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 Ambient Shading Computation

• Interpolate maximum ambient RGBA for given time of

day

• For each cloud vertex

– Interpolate its ambient percentage of maximum value, based on

vertex height within the cloud

– Multiply by maximum RGBA

Ambient Shading Video

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 Directional Shading

• Parts of the cloud facing the sun receive more

directional sunlight
• Artist specifies
– Shading groups (sections of 1 -30 sprites that are
shaded as a unit)
– Maximum directional color for a set of times
throughout the day

Directional Shading Computation

• Find maximum directional RGBA for a given time of

day
• For a vertex:
– Compute dot product of (vector from cloud center to
sun) with (vector from vertex to cloud center)
– Multiply by interpolated maximum color

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Illustration for Directional Light

Directional Shading Video

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 Animation

Adjust transparency values of sprites

• Form clouds from core first

– Multiply cloud vertex with transparency factor based on its

distance from cloud center

– Render all of core first before edges

• Dissipate from edges

Animation Video

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 Limitations and Future Work

• Not suited for flat clouds (i.e. cirrus)

• Extension to fog and smoke

• Form animations on meteorological data and fluid

simulation

• Pre-computed self-shadowing term

Contact info, Q& A

• E- mail: niniane@ofb.net

• http://ofb.net/~niniane/clouds

Acknowledgements to two extraordinarily talented artists --

John Smith, Adrian Woods -- and the Microsoft Flight
Simulator development team.

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