Massive Point Light Soft Shadows

transcript

Wolfgang EngelConfetti Special Effects Inc.

September 14th, 2010

Confetti Special Effects

• Think-Tank for advanced real-time graphics research

• Targeting video game and movie industry• Website: (click on “Like” on this page )http://www.facebook.com/pages/Confetti-Special-Effects-Inc/159613387880?v=wall

Agenda

• Motivation• Basics of Soft Shadows• Point Light Soft Shadow Rendering

Architecture• Min-Z Map• Collect Shadow Data in Screen-Space• Screen-Space Anisotropic Filtering

Motivation

• We can render thousands of point lights• We need at least dozens of point light soft

shadows and many more regular point light shadows

• Fundamental for future development of more complex light and shadow effects like Global Illumination with a Global Occlusion / Shadow term

Image courtesy of Randy Fernando [Fernando]

Basics of Soft Shadows

• Terminology

• PCSS searches red region for blockers– Blocker is defined by being closer to light than receiving

point– Averages depth values of blockers– Averaged depth value used for penumbra size estimation

• Blocker search // is it a blocker? if (shadMapDepth < receiver) { blockerSum += shadMapDepth; blockerCount++; foundBlocker = 1; }… // return average depth of the blockers result = blockerSum / blockerCount;

• How to calculate the scale factor for the penumbra

• dBlocker – the result of the blocker search

• dReceiver – the depth of the pixel that is currently rendered

• wlight – the light size

Rendering Architecture

For Point Light Screen-Space soft shadows we are going to change mainly two things:

1. Replace blocker search with a minimum Z-map or dilated shadow map [Gumbau] -> Blocker search is expensive

2. Anisotropic Screen-Space Filter Kernel instead of Light-Space filter kernel-> Screen-Space is less expensive

The algorithm in steps:1. Calculate the cube shadow map2. Generate min Z map [Gumbau]3. Blend the “unfiltered” exponential shadow map data of all

cube maps into a screen-space shadow map4. Based on each min Z map, calculate

a. The x and y offset values for the filter kernel based on• Adjustment based on distance from camera• Penumbra size [Fernando]• Anisotropic kernel adjustment [Geusebroek]

b. Early out value (optimization) [Gumbau]-> store the end result in a screen-space texture

5. Apply a screen-space anisotropic Gaussian filter kernel based on 4.

Rendering Architecture

Min-Z Map

• Blocker search • is used to determine the distance of the shadow

blockers -> dBlocker

• this is used to determine the penumbra width• Minimum Z map represents the minimum Z values ==

closest to the lights of the whole scene~ kind of like blocker data

Min-Z Map

• Generated from the shadow map• Into a lower res render target == coarse shadow map

== one pixel represents an area of the orig. shadow map-> runs only CoarseMapsizeX * CoarseMapsizeY times -> fast

• 2 pass filter kernel that returns the minimum Z values of its area in light space

Min-Z Map

• Issue: maximum size of penumbra restricted by size of filter kernel -> no way to figure out max size of the penumbra-> sensible user defined constant value that scales up filter kernel• too high -> artefacts• too low -> loose of softeness

• In other words: filter kernel is determined by a• value representing the light size• + value that is the magic user defined constant

Min-Z Map

• Advantage of Min-Z map approach• much faster• … therefore allows soft cube shadow maps

• Disadvantage: min-Z value not only from blockers but for the whole lit scene -> min-Z aliasing

Collect Shadow Data in Screen-Space

Cube map == exponential shadow map [Salvi]float depth = tex2D(ShadowSampler, pos.xy).x;shadow = saturate(2.0 - exp((pos.z - depth) * k));

• Approximate step function (z-d> 0) byexp(k*(z-d)) = exp(k*z) * exp(-k*d)

Good overview on the latest development in [Bavoil]

• All the shadow data is just blended via BLEND_ADD into a screen-space texture

• This texture can be called shadow collector or shadow mask

Collect Shadow Data in Screen-Space

Screen-Space Anisotropic Filter Kernel

• Why a screen-space filter kernel?• In light space we filter per shadow map • In screen-space we filter only once for all

shadow maps-> many light sources -> advantage

• What we need to do:• Determine the filter kernel offset values

that scale the filter kernel• Write “offset” values for all shadow maps

into a screen-space render target• … the Gauss filter will read those values

later from there

• What are the values required for the Gauss filter?1. The x and y offset values for the filter kernel are

based ona. Adjustment based on distance from camerab. Penumbra size & Light size[Fernando]c. Anisotropic kernel adjustment [Geusebroek]

2. Early out value (optimization) [Gumbau]• … store those values in a 16:16 fp render target or

calculate them on the fly while filtering

Distance to the Camera

• Screen-space filter kernel is getting bigger with increasing distance because of the projection-> decrease kernel size with with increasing distance

• Simple way to do this is 1.0 / (distance2 * bias)~ light attenuation• This requires a linear depth value in camera space [Gillham]:float depthLin= (-NearClip * Q) / (Depth - Q);

Q = FarClip / (FarClip – NearClip)Depth = value from depth buffer

• Source code:// scale based on distance to the viewer sampleStep.xy = TexelSize.zw * sqrt(1.0f / ((depthLin.xx * depthLin.xx) * bias));

Penumbra Size

• To calculate the Penumbra, [Fernando] suggested the following equation

• We use as is, just modified by the “distance to camera” scaling value

Anisotropic Filter Kernel Adjustment• Anisotropic filter kernel: round filter kernel

projected into ellipse following the orientation of the geometry [Geusebroek]-> need to determine the shape and orientation of this ellipse

float Aniso = saturate(sqrt(dot( viewVec, normal )));

Screen-Space Anisotropic Filter Kernel• Screen-space challenges

• Filter kernel can smear values into the penumbra around corners of geometry

• Compare Z value of pixel with Z value of shadow map tap bool isValidSample = bool( abs(sampleDepth - d) < errDepth );if (isValidSample && isShadow){

// the sample is considered validsumWeightsOK += weights[i+1]; // accumulate valid weightsShadow += sampleL0.x * weights[i+1]; // accumulate weighted shadow value

• Screen-space challenges• “Light in a box” or occlusion of shadow data in

general -> should not affect Gauss filter -> need to deal with occlusion or ignore it (game specific)

• Overlapping shadows in screen-space-> starts with the philosophical question: what kind of entity are shadows?

Tips & Tricks• Seriously! Who needs 64 Point light shadows

perceptually correct on screen • switch off when the lights are fast• Far away• .. . or in all other cases you can think off

• How to render shadow data into a cube map-> fill up a texture array; then type cast to cube maps

• Try Dual-Paraboloid Shadow Maps … might be faster with DX10 / 11 … I didn’t try so far

Massive Soft Point Light Shadows

16 Point Light Soft Shadows filtered in screen-space

Acknowledgements

• Jesus Gumbau • Peter Santoki• Yue Yu• Timothy Martin• Carlos Dominguez

Confetti Special Effects

• Is looking for– Investors– Contract work– Offers support to young researchers at Universities

that are interested in game related real-time research

Thank you

wolf@conffx.com

References• [Bavoil] Louis Bavoil, “Advanced Soft Shadow Mapping Techniques”

http://developer.download.nvidia.com/presentations/2008/GDC/GDC08_SoftShadowMapping.pdf

• [Fernando] Randy Fernando, “Percentage-Closer Soft Shadows”, SIGGRAPH 2005• [Forsyth] Tom Forsyth, “Making Shadow Buffers Robust Using Multiple Dynamic Frustums”,

ShaderX4, pp. 331 – 345• [Geusebroek] Jan-Mark Geusebroek, Arnold W. M. Smeulders, J. van de Weijer, “Fast

anisotropic Gauss filtering”, IEEE Transactions on Image Processing, Volume 12 (8), page 938-943, 2003

• [Gilham] David Gilham, "Real-Time Depth-of-Field Implemented with a Post-Processing only Technique", ShaderX5: Advanced Rendering, Charles River Media / Thomson, pp 163 - 175, ISBN 1-58450-499-4

• [Gumbau] Jesus Gumbau, Miguel Chover, and Mateu Sbert, “Screen-Space Soft Shadows”, GPU Pro, pp. 477 - 490

• [Waliszewski] Arkadiusz Waliszewski, “Floating-point Cube Maps”, ShaderX2 – Shader Programming Tips and Tricks with DirectX9, Wordware Inc., pp. 319 – 323. http://www.realtimerendering.com/blog/shaderx2-books-available-for-free-download/

Massive Point Light Soft Shadows

Engineering