Aaron Schultz

Real-time Soft Shadows in XNA

Idea: Objects close to a light shadow those

far away. Anything we can see from the light’s POV is lit. Everything hidden is dark.

Distance from light = Depth

Render a depth map!

How to render shadows?

Pass 1:From light’s POV, render scene into a texture.

Each pixel stores depth information.Pass 2:

From camera POV, render scene to screen. For each pixel, compute position in light space. Compare stored depth (from pass 1) to current depth.

if( distanceToLight > storedDistance ) //Hidden by occluder

In Shadow = true

What is shadow mapping?

Light Space Camera Space

It’s never that easy…

• Aliasing = Really bad jaggies• Shadow map texels Not 1:1 camera pixels

• Poor Precision = Banding, speckles, noise• How do we store depth? 16-bit? 32-bit? Floating-point?

Fixed-point?• Different depths mapped to same value• Rounding errors

What’s so difficult?

• Blur the shadow map or use hardware filtering• Still doing a binary compare = hard edges

• Constant depth bias• Helps prevent “shadow acne” caused by precision errors• Doesn’t work well with the stretched shadow map (different bias

needed at different points)• Causes Peter Panning

Naïve Solutions

From Common Techniques to Improve Shadow Depth Maps, MSDN

• Percentage Closer Filtering• Blur the *results* of multiple depth comparisons• Really expensive for any decent quality

• Different encoding• VSM• Encode (depth, depth2). Extract to use as variance, compute

probability of shadow• ESM• Encode depth. Extract

• Both permit blurring the shadow map and using hardware filtering• **If the hardware supports the correct surface formats**

Getting Smarter…

• Cascaded Shadow Maps• Put precision where we need it: close to the camera• Split camera frustum into pieces• Render separate shadow map for each sub-frustum

Really Smart…

Cascaded Shadow Maps, MSDN

• What I Want:• RG32 (2 channel, 16-bit per channel), HW filterable, for VSM• R32F (1 channel, 32-bit), HW filterable, for Basic and ESM

• What I Get:• Hardware filtering on floating-point formats disabled in XNA 4.0

(because actual support is only on Nvidia 8-series cards and up)• RG32 format (for VSM) unsupported on my machine• Access to…• RGBA1010102• Color (4 8-bit channels)• All floating-point formats• Requires encoding VSM and ESM into Color to permit HW filtering

(which is then mathematically wrong)

Implementation Notes

• Normalize all depth values to [0,1] in the shader. Makes everything easier.

• Remember the DirectX9 ½ pixel offset• So many places to mess up• Lots of converting between coordinate systems• Viewport settings, texture wrapping• Check hardware capabilities!• Get it to work first. Then optimize.• ESM not widely documented on the web• Most people do PCF wrong. Interpolation required!• XNA lets you focus on the algorithms (most of the

time)

Implementation Notes

(Demo)

Let’s see the goods