siggraph2010_vlachos_waterflow

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Water Flow in

Alex Vlachos, Valve

July 28, 2010


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Outline

Goals & Technical Constraints

How Artists Create Flow Maps

Flowing Normal Maps in Left 4 Dead 2

Flowing Color Maps in Portal 2


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Left 4 Dead 2


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Goals

Visual

Solve repeating texture artifacts

Flow around obstacles

Vary water speed and bump strength

Technical Work with existing reflective surfaces

Min hardware ps2.0b (6-year-old hardware) & Xbox

Gameplay


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Gameplay

Early Left 4 Dead 2 playtests showed players were con

got lost often in the swamps Soft non-directional lighting

Trees provided too much cover

My theory was that water flow would improve gamepl

highlighting the correct path We tested this theory through playtesting

In practice, we found testers took 17% fewer wrong tu

decreased the time it took to traverse the level!


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Technical Constraints

Already at perf limits on the Xbox 360 & low-e

Already at memory limits on the Xbox 360

Our water shader had limited instructions left low end hardware ps2.0b


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Algorithm Overview

Pixel shader flow, not geometric flow

Continue to use a normal map for water

ripples

Artists author a flow map (a texture

containing 2D flow vectors) Use this flow map in a pixel shader to

distort the normal map in the direction of

flow


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Flow Texture Mapped onto Surface

Covers entire water surface


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Normal Map Mapped onto Surface

Tiled over the water surface


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Artists Author Flow Maps

Flow map provides a unique 2D vector for ever

the water surface Relatively low resolution: ~4 texels/meter

Impractical to paint directly

We use Houdini to create vector flow maps


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Houdini Importing Level Geom


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Houdini Combing Vector Fie


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Houdini Procedural Masks


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Houdini Applying Masks


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Houdini Water Normal Maps


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Left 4 Dead 2

Wanted to replace our scrolling normal maps w

flowing normal maps

Keep the rest of the water shader the same

This algorithm ultimately provides a new per-p

normal generated from the normal map and fl


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Related Work

Nelson Max and Barry Becker 1995. Flow visua

using moving textures. In Proceedings of theICASW/LaRC Symposium on Visualizing Time-V

Data, 7787.

Building on aspects of their algorithm and app

approach to flowing normal maps


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Flow Visualization

Inputs: flow field & noise texture

Distort a noise texture to visualize a flow field

The UV is offset by the 2D flow vector scaled b


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Flow Visualization Textures

Flow Texture

Noise Texture


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Flow Visualization Experiment


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Max & Beckers Observation

The beginning of the distortion looks convincin

Only distort a small amount

In general, distortion looks reasonable for thefirst 1/3 of uv space

hl l


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Smoothly Interpolating Layers

Blend the short animated segment in two laye

Each layer is offset half a phase so we can hiderestart for each layer

hl l


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Smoothly Repeating Flow

A G S


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A Great Start

We now have a method to flow a normal map

We want to apply this to a larger surface

But applying this to a large surface means tilingnormal map which will cause artifacts

P l 2 T M


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Portal 2 Test Map

P l 2 T M (P A


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Portal 2 Test Map (Programmer Ar

Fl V t W t S f


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Flow Vectors on Water Surface

Si l L N l Di t ti


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Single Layer Normal Distortion

D bl L N l Di t ti


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Double Layer Normal Distortion

T M j P bl


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Two Major Problems

Repetition The same normals will flow

through the same point on the mesh

Pulsing The surface appears to pulse in a

repeating pattern

R titi Vi li ti Si l L


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Repetition Visualization Single Lay

Double Layer


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Double Layer

Double Layer With Offset


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Double Layer With Offset

Repetition Solved by Offset


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Repetition Solved by Offset

Pulsing Solved by Noise


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Water Speed Affects Normals


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Water Speed Affects Normals

We scale down the strength of the normal in tangent spa

flow speed (Flow speed is the length of the 2D flow ve

Performance


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Performance

Compared to scrolling two normal maps:

Additional texture fetches: 2 - flow & noise Additional arithmetic pixel shader instructions

Water Flow in Portal 2


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Water Flow in Portal 2

Wanted to also flow debris in dirty water

Needed to modify our algorithm to support flo

color map

Debris Flow Example


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Debris Flow Example

Debris Flow Example


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Debris Flow Example

Debris Normal (Same as before


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Debris Normal (Same as before

Flowing Debris Using Same Algorit


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Flowing Debris Using Same Algorit

Flowing Normals


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Flowing Normals Flowing normals would repeat

an interval from zero to some

fraction with the peak (center)of the interval at half distortion

Flowing Debris


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Flowing Debris Flowing colors works better by

offsetting the interval from

fraction to +fraction so thepeak of the interval is at zero

(the at-rest position)

Flowing Debris Using Offset


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Flowing Debris Using Offset

Debris Flow


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Debris Flow

Portal 2 Test Map


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Portal 2 Test Map

Left 4 Dead 2


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Left 4 Dead 2

Future Work


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Future Work

Flow height maps and use tessellation hardwa

Multiple frequencies of normal maps

Render dynamic flow vectors per-frame so ani

objects cause flow changes

Use flow map with our physics simulation to h

objects flow on the water surface using the sam

Summary


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Summary

Use an artist-authored flow map

Flow the normals in two layers and combine

Use noise to reduce pulsing artifact

Offset each phase of animation to reduce repe

Flowing debris uses an offset distortion range tless distortion than the normal flow


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Thank You!

Water textures created by Alireza Razmpo

Alex Vlachos, [email protected]

siggraph2010_vlachos_waterflow

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