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Massive Point Light Soft Shadows
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
Basics of Soft Shadows
• 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
Basics of Soft Shadows
• Blocker search // is it a blocker? if (shadMapDepth < receiver) { blockerSum += shadMapDepth; blockerCount++; foundBlocker = 1; }… // return average depth of the blockers result = blockerSum / blockerCount;
Basics of Soft Shadows
• 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
Basics of Soft Shadows
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
Screen-Space Anisotropic Filter Kernel
• 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
Screen-Space Anisotropic Filter Kernel
• 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 Anisotropic Filter Kernel
• 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
Massive Soft Point Light Shadows
32 Point Light Soft Shadows filtered in screen-space
Massive Soft Point Light Shadows
64 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
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/