Shadow Mapping

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Why Shadows?

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Why Shadows?

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Why Shadows?

Shadows ...... make a scene look more three-dimensional

... tell us about the positions of objects relative to each other

... tell us where the light comes from

... should really be there

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Shadow Determination

Several techniques, e.g.Shadow Mapping

Shadow Volumes

Let’s take a closer look at shadow mapping2 pass algorithm

fast on today’s GPUs

relatively easy to implement

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Shadow Mapping Overview

1st pass:we assume the light source has a “view frustum” just like a camerarender scene from light source’s positionsave depth values onlywe end up with a shadow (depth-) map

2nd pass:render scene as usual, but transform vertices to light space, toofor each fragment, compare rasterized fragment depth to previously stored depth (read it from shadow map)

zfragment > zfrom_shadow_map => fragment lies in shadow

both fragments must be in light space!!!RTR Team 2009

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Scene – “Meta” View

EyeLight Source

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Scene – Light Source View

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Scene – Light Source View (Depth Only)

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Scene – Eye View

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Shadowed Fragment

Eye View

“Meta“ View

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Shadowed Fragment

“Meta“ View

Fragment DistanceDistance

from Shadow

Map

Eye View

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Lit Fragment

“Meta“ View

Eye View

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Lit Fragment

“Meta“ View

Fragment Distance Distancefrom

ShadowMap

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Eye View

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Coordinate Systems

Eye Light World

Model

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Coordinate Systems

Eye

Light

World

Model

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Transforming to World Space

Eye

Light

World

Model

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Transforming to Light Space

Eye

Light

World

Model

Light (Vlight)

rendering from the light source‘s point of view

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Transforming to Eye Space

Eye

Light

World

Model

rendering from the eye‘s point of view

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1st pass: Create Shadow Map

Create an FBO

// create the texture we'll use for the shadowmapglGenTextures(1, &shadow_tex_ID);glBindTexture(GL_TEXTURE_2D, shadow_tex_ID);glTexImage2D (GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT24, SM_width, SM_height, 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_BYTE, NULL);

glGenFramebuffers(1, &shadow_FBO); glBindFramebuffer(GL_FRAMEBUFFER, shadow_FBO);glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, shadow_tex_ID, 0);glDrawBuffer(GL_NONE); // essential for depth-only FBOs!!!glReadBuffer(GL_NONE); // essential for depth-only FBOs!!!

// then, just before renderingglBindFramebuffer(GL_FRAMEBUFFER, shadow_FBO);

// create the texture we'll use for the shadowmapglGenTextures(1, &shadow_tex_ID);glBindTexture(GL_TEXTURE_2D, shadow_tex_ID);glTexImage2D (GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT24, SM_width, SM_height, 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_BYTE, NULL);

glGenFramebuffers(1, &shadow_FBO); glBindFramebuffer(GL_FRAMEBUFFER, shadow_FBO);glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, shadow_tex_ID, 0);glDrawBuffer(GL_NONE); // essential for depth-only FBOs!!!glReadBuffer(GL_NONE); // essential for depth-only FBOs!!!

// then, just before renderingglBindFramebuffer(GL_FRAMEBUFFER, shadow_FBO);

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1st pass: Create Shadow Map

The “view” matrix must be set to Vlight

Note: No projection matrix used up to now!but light-”camera” involves another projection!

Turn off all effects when rendering to the shadow map

No textures, lighting, etc.

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2nd pass: Render from Eye’s POV

Transform vertices to eye space and project as usual

p‘ = Pcam * Vcam * M * p

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2nd pass: Render from Eye’s POV

Also transform vertices to light space and projectone possibility: Use Vcam

-1, the inverse view matrix of the camera

pproj_lightspace = (Plight * Vlight * Vcam-1) * Vcam * M * p

save as texture coordinates for accessing shadow map

Note: the light‘s projection matrix may be different from the eye‘s projection matrix

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2nd pass: Render from Eye’s POV

one last issue...

plightspace is in the interval [-1…..+1]

shadow map coordinates in range [0….+1]

scaling/translation necessary (*0.5, +0.5)

SMtexcoords = (Mtranslate*Mscale*Plight*Vlight*Vcam-1)* Vcam * M * p

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Shadow Mapping: Vertex Shader

texture_matrix = (Mtranslate*Mscale*Plight*Vlight*Vcam-1)

#version 140uniform mat4 M; // model matrixuniform mat4 V_cam; // view matrix for the camerauniform mat4 P_cam; // projection matrix for the camerauniform mat4 texture_matrix;

in vec4 vertex; // from the applicationout vec4 SM_tex_coord; // pass on to the FS

void main(void) {// standard transformation gl_Position = P_cam * V_cam * M * vertex;

// shadow texture coords in projected light space

SM_tex_coord = texture_matrix * V_cam * M * vertex;}

#version 140uniform mat4 M; // model matrixuniform mat4 V_cam; // view matrix for the camerauniform mat4 P_cam; // projection matrix for the camerauniform mat4 texture_matrix;

in vec4 vertex; // from the applicationout vec4 SM_tex_coord; // pass on to the FS

void main(void) {// standard transformation gl_Position = P_cam * V_cam * M * vertex;

// shadow texture coords in projected light space

SM_tex_coord = texture_matrix * V_cam * M * vertex;}

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Shadow Mapping: Fragment Shader

Fragment Shader:

#version 140uniform sampler2D shadow_map;

in vec4 SM_tex_coord; // passed on from VSout vec4 fragment_color; // final fragment color destination

void main(void) {// note the perspective division!vec3 tex_coords =

SM_tex_coord.xyz/SM_tex_coord.w;

// read depth value from shadow map float depth = texture(shadow_map,

tex_coords.xy).r;

float inShadow = (depth < tex_coords.z) ? 1.0 : 0.0;

// do something with that value ...}

#version 140uniform sampler2D shadow_map;

in vec4 SM_tex_coord; // passed on from VSout vec4 fragment_color; // final fragment color destination

void main(void) {// note the perspective division!vec3 tex_coords =

SM_tex_coord.xyz/SM_tex_coord.w;

// read depth value from shadow map float depth = texture(shadow_map,

tex_coords.xy).r;

float inShadow = (depth < tex_coords.z) ? 1.0 : 0.0;

// do something with that value ...}

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Add Offset to polygons when rendering shadow map

Artifacts

glPolygonOffset(1.1, 4.0); // these values work wellglPolygonOffset(1.1, 4.0); // these values work well

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Decrease ambient term

Filter shadow map ...

Artifacts

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GPU can do depth compare in hardware

and also PCF!

Filter shadow map

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE, GL_COMPARE_REF_TO_TEXTURE);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_FUNC, GL_LEQUAL);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE, GL_COMPARE_REF_TO_TEXTURE);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_FUNC, GL_LEQUAL);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

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Shadow Mapping: Fragment Shader

Fragment Shader for Hardware PCF:

#version 140uniform sampler2DShadow shadow_map;

in vec4 SM_tex_coord; // passed on from VSout vec4 fragment_color; // final fragment color destination

void main(void) {

float shadow = textureProj(shadow_map, SM_tex_coord); // do something with that value ...

}

#version 140uniform sampler2DShadow shadow_map;

in vec4 SM_tex_coord; // passed on from VSout vec4 fragment_color; // final fragment color destination

void main(void) {

float shadow = textureProj(shadow_map, SM_tex_coord); // do something with that value ...

}

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References

www.opengl.org/registryhttp://www.opengl.org/registry/doc/glspec31undep.20090528.pdf

http://www.opengl.org/registry/doc/GLSLangSpec.Full.1.40.07.pdf

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