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XNA 4.0 - shimmering shadow maps

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I've been experiencing shadow map shimmering in my game right now and I've been looking at ways to solve it. I've been reading through this article and it has some really great things in it. but I can't quite wrap my head around what it's talking about on shimmering shadows. It says to move the light in texel-sized increments. I know what a texel is, but I have no idea how to convert where my shadow map is to a texel increment.

Can anyone translate that into non-GPU-wizard talk?

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Yeah it's a pretty simple concept, but it's kinda hard to explain. How are you setting up your shadow maps now? I can try and talk you through how to modify it to get rid of the shimmering.

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I'm taking my scene perspective camera and creating a region that encapsulates the whole view frustum of said camera. Most of the code was lifted from a forum post on this site =]

Here's the code:

public static OrthogonalCamera DLShadowCamera(PerspectiveCamera pCamera, float high, Vector3 direction)
// Calculate the world space location centroid
Vector3[] cornersWS = pCamera.ViewFrustum.GetCorners();

Vector3 centroid = new Vector3(0,0,0);

float low = high;

for (int i = 0; i < cornersWS.Length; i++)
centroid += cornersWS;

if (cornersWS.Y < 0)
cornersWS.Y = 0.0f;

if (low > cornersWS.Y)
low = cornersWS.Y;

centroid /= cornersWS.Length;

centroid.Y = (high - low) / 2;
// Calculate the position of the direction light.

float distance = centroid.Y / direction.Y;

Matrix lightViewMatrix = Matrix.CreateLookAt(centroid + (direction * distance), centroid, Vector3.Up);

// Calculate the light space locations of the 8 corners of the
// (world space) box. The lightViewMatrix is used to
// transform each world space frustum corner into light space.
// fake point[0].Y and point[1].Y to the high and low of the world space
cornersWS[0].Y = high;
cornersWS[1].Y = low;

Vector3[] cornersLS = new Vector3[cornersWS.Length];
Vector3.Transform(cornersWS, ref lightViewMatrix, cornersLS);

// Calculate the bounding box for the light space frustum
// corners. The bounding box is used to construct the proper
// orthographic projection matrix for the directional light.

BoundingBox box = BoundingBox.CreateFromPoints(cornersLS);

OrthogonalCamera camera = new OrthogonalCamera(
centroid + (direction * distance), centroid,
box.Min.X, box.Max.X, box.Min.Y, box.Max.Y, -box.Max.Z, -box.Min.Z);

return camera;

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Alright, I get it. Thanks Matt.

I don't think I'll be able to use a Bounding Sphere as I have some VERY tall geometry (100 times taller than a player) that will always need to be covered by the bounding region. But I get your point on keeping the size of the projection a constant. I should be able to do that with a pseudo fit-to-scene projection.

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Got it working on the first try. Truly a fantastic explanation matt.

In case anyone else is following a long and is lazier than I am, here's the result of the discussion:

[source = csharp]
public static OrthogonalCamera DLShadowCamera(PerspectiveCamera pCamera, float high, float low, Vector2 boxSize, int texSize, Vector3 direction)
Vector3 min = pCamera.Position - new Vector3(boxSize.X / 2, 0, boxSize.Y / 2);
min.Y = low;

Vector3 max = pCamera.Position + new Vector3(boxSize.X / 2, 0, boxSize.Y / 2);
max.Y = high;

BoundingBox boxWS = new BoundingBox(min, max);

Vector3 shadowCamPos = pCamera.Position;
shadowCamPos.Y = high - low;

Matrix shadowViewMatrix = Matrix.CreateLookAt(shadowCamPos, (shadowCamPos + (direction * 10)), Vector3.Up);

Vector3[] cornersWS = boxWS.GetCorners();

Vector3[] cornersLS = new Vector3[cornersWS.Length];

Vector3.Transform(cornersWS, ref shadowViewMatrix, cornersLS);

BoundingBox box = BoundingBox.CreateFromPoints(cornersLS);

Matrix shadowProjMatrix = Matrix.CreateOrthographicOffCenter(box.Min.X, box.Max.X, box.Min.Y, box.Max.Y, -box.Max.Z, -box.Min.Z);

Matrix shadowViewProjMatrix = shadowViewMatrix * shadowProjMatrix;

Vector3 shadowOrigin = Vector3.Transform(Vector3.Zero, shadowViewProjMatrix);
shadowOrigin *= (texSize / 2.0f);

Vector2 roundedOrigin = new Vector2((float)Math.Round(shadowOrigin.X), (float)Math.Round(shadowOrigin.Y));

Vector2 rounding = roundedOrigin - new Vector2(shadowOrigin.X, shadowOrigin.Y);
rounding /= (texSize / 2.0f);

Matrix roundMatrix = Matrix.CreateTranslation(new Vector3(rounding.X, rounding.Y, 0.0f));
shadowViewProjMatrix *= roundMatrix;

return new OrthogonalCamera(shadowCamPos, shadowViewProjMatrix);

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Hate to revive an old post but I'm implementing CSM using bounding spheres. However, I have a question.

Vector3 shadowCamPos = sphere.Center + direction * backupDist;

Is the direction the light direction? Also when the camera faces a particular direction, the shadows disappear.

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Yes, that direction would be direction towards the directional light, basically what you would use for N dot L.

I'm not sure why your shadows would disappear...I would try to break into the debugger when that happens and step through your code for creating the shadow matrices. Capturing in PIX/Nsight/PerfStudio might be helpful too. Edited by MJP

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Here an XNA version of the code. I prototype in XNA before I port it to SlimDx. I tired pix and stepping through debugging. I didn't see anything unusual.

Here a video of what's happening.

const int SHADOW_RESOLUTION = 4096;
static SpriteBatch spriteBatch = Engine.SpriteBatch;
static int width = Engine.BackBufferWidth;
static int height = Engine.BackBufferHeight;
static RenderTarget2D depthMap = new RenderTarget2D(Engine.Graphics.GraphicsDevice, width, height, false, SurfaceFormat.Rg32, DepthFormat.Depth24);
static RenderTarget2D shadowMap = new RenderTarget2D(Engine.Graphics.GraphicsDevice, SHADOW_RESOLUTION, SHADOW_RESOLUTION, false, SurfaceFormat.Rg32, DepthFormat.Depth24);
public static RenderTarget2D shadowOcclusion = new RenderTarget2D(Engine.Graphics.GraphicsDevice, width, height, false, SurfaceFormat.Color, DepthFormat.Depth24);
static Effect depthMapEffect = Engine.Game.Content.Load<Effect>("Effects\\Shadows\\DepthMap");
static Effect shadowMapEffect = Engine.Game.Content.Load<Effect>("Effects\\Shadows\\ShadowMap");
static Effect shadowOcclusionEffect = Engine.Game.Content.Load<Effect>("Effects\\Shadows\\ShadowOcclusion2");
static Matrix shadowMatrix = Matrix.Identity;
static Vector3[] farFrustumCornersVS = new Vector3[4];
static Vector3[] frustumCornersVS = new Vector3[8];
public static void Draw()
public static void Initialize()
shadowOcclusionEffect.Parameters["OcclusionTextureSize"].SetValue(new Vector2(width, height));
shadowOcclusionEffect.Parameters["ShadowMapSize"].SetValue(new Vector2(shadowMap.Width, shadowMap.Height));
private static void RenderDepthMap()
Engine.Graphics.GraphicsDevice.Clear(ClearOptions.Target, Color.White, 1.0f, 0);
private static void ShadowMatrix()
BoundingSphere sphere = BoundingSphere.CreateFromFrustum(Camera.BoundingFrustum);
float NearClipOffset = 500.0f;
float NearClip = 1.0f;
float backupDist = NearClipOffset + NearClip + sphere.Radius;
Vector3 position = sphere.Center + (new Vector3(1, 1, 1) * backupDist);
Matrix view = Matrix.CreateLookAt(position, sphere.Center, Vector3.Up);
float bounds = sphere.Radius * 2.0f;
float farClip = 2000 + sphere.Radius;
Matrix projection = Matrix.CreateOrthographic(bounds, bounds, NearClip, farClip);
shadowMatrix = view * projection;
Vector3 shadowOrigin = Vector3.Transform(Vector3.Zero, shadowMatrix);
shadowOrigin *= (SHADOW_RESOLUTION / 2.0f);
Vector3 roundedOrigin = new Vector3((float)Math.Round(shadowOrigin.X), (float)Math.Round(shadowOrigin.Y), 0);
Vector3 rounding = roundedOrigin - shadowOrigin;
rounding /= (SHADOW_RESOLUTION / 2.0f);
Matrix roundMatrix = Matrix.CreateTranslation(rounding.X, rounding.Y, 0.0f);
shadowMatrix *= roundMatrix;
private static void RenderShadowMap()
Engine.Graphics.GraphicsDevice.Clear(ClearOptions.Target, Color.White, 1.0f, 0);
private static void RenderShadowOcclusion()
Engine.Graphics.GraphicsDevice.Clear(ClearOptions.Target, Color.White, 1.0f, 0);
Matrix viewMatrix = Camera.View;
Vector3.Transform(Camera.BoundingFrustum.GetCorners(), ref viewMatrix, frustumCornersVS);
for (int i = 0; i < 4; i++)
farFrustumCornersVS = frustumCornersVS[i + 4];
Matrix inverseView = Camera.World;
public static void DrawRenderTargets()
spriteBatch.Begin(0, BlendState.Opaque, SamplerState.PointClamp, null, null);
spriteBatch.Draw(shadowMap, new Rectangle(0, 0, 256, 256), Color.White);
spriteBatch.Draw(depthMap, new Rectangle(0, 256, 256, 256), Color.White);
spriteBatch.Draw(shadowOcclusion, new Rectangle(0, 512, 256, 256), Color.White);

float4x4 World;
float4x4 ViewProjection;
float4x4 InverseView;
float4x4 LightViewProjection;
float2 ShadowMapSize;
float2 OcclusionTextureSize;
float3 FrustumCornersVS [4];
static const float ShadowBias = 0.01f;
static const float DarkenFactor = 100.0f;
texture DepthTexture;
sampler2D DepthTextureSampler = sampler_state
Texture = <DepthTexture>;
MinFilter = Point;
MagFilter = Point;
MipFilter = None;
texture ShadowMap;
sampler2D ShadowMapSampler = sampler_state
Texture = <ShadowMap>;
MinFilter = Anisotropic;
MagFilter = Anisotropic;
MipFilter = None;
MaxAnisotropy = 16;
float2 sampleShadowMap(float2 UV)
if (UV.x < 0 || UV.x > 1 || UV.y < 0 || UV.y > 1)
return float2(1, 1);
return tex2D(ShadowMapSampler, UV).rg;
struct Shadow_PSIn
float4 Position : POSITION;
float3 TexCoordAndCornerIndex : TEXCOORD0;
struct Shadow_VSOut
float4 Position : POSITION;
float2 TexCoord : TEXCOORD0;
float3 FrustumCornerVS : TEXCOORD1;
// Vertex shader for rendering the full-screen quad used for calculating
// the shadow occlusion factor.
Shadow_VSOut ShadowTermVS (Shadow_PSIn input)
Shadow_VSOut output;
// Offset the position by half a pixel to correctly align texels to pixels
output.Position.x = input.Position.x - (1.0f / OcclusionTextureSize.x);
output.Position.y = input.Position.y + (1.0f / OcclusionTextureSize.y);
output.Position.z = input.Position.z;
output.Position.w = 1.0f;

// Pass along the texture coordiante and the position of the frustum corner
output.TexCoord = input.TexCoordAndCornerIndex.xy;
output.FrustumCornerVS = FrustumCornersVS[input.TexCoordAndCornerIndex.z];
return output;
// Pixel shader for computing the shadow occlusion factor
float4 ShadowTermPS(Shadow_VSOut input) : COLOR0
// Reconstruct view-space position from the depth buffer
float pixelDepth = tex2D(DepthTextureSampler, input.TexCoord).r;
float4 position = float4(pixelDepth * input.FrustumCornerVS, 1.0f);
// Determine the depth of the pixel with respect to the light
float4x4 inverseLVP = mul(InverseView, LightViewProjection);
float4 positionLight = mul(position, inverseLVP);

float lightDepth = (positionLight.z / positionLight.w) - ShadowBias;

// Transform from light space to shadow map texture space.
float2 shadowTexCoord = 0.5 * positionLight.xy / positionLight.w + float2(0.5f, 0.5f);
shadowTexCoord.x = shadowTexCoord.x;
shadowTexCoord.y = 1.0f - shadowTexCoord.y;

// Offset the coordinate by half a texel so we sample it correctly
shadowTexCoord += (0.5f / ShadowMapSize);
float shadow = 1;
float2 shadowMap = sampleShadowMap(shadowTexCoord);
lightDepth += 0.01f;
if (lightDepth < 1)
// Check if we're in shadow
//float lit_factor = (lightDepth < shadowMap.x);
//// Variance shadow mapping
//float E_x2 = shadowMap.y;
//float Ex_2 = shadowMap.x * shadowMap.x;
//float variance = min(max(E_x2 - Ex_2, 0.0) + 1.0f / 5000, 1);
//float m_d = (shadowMap.x - lightDepth);
//float p = variance / (variance + m_d * m_d);
//shadow = clamp(max(lit_factor, p), 0.35, 1.0f);
//Calculate the Shadow Factor
float shadowFactor = exp((DarkenFactor * 0.5f) * (shadowMap - lightDepth));
shadowFactor = clamp(shadowFactor, 0.45, 1.0);
shadowFactor = saturate(shadowFactor);
shadow = shadowFactor;
return float4(shadow, shadow, shadow, 0);
technique ShadowOcclusion
pass p0
VertexShader = compile vs_3_0 ShadowTermVS();
PixelShader = compile ps_3_0 ShadowTermPS();

Edited by mgoss

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