Computer Graphics

Lecture 07 – Shadows

Edirlei Soares de Lima

<edirlei.lima@universidadeeuropeia.pt>

Shadows

•

Shadows appear when light rays are blocked: when an object

is between a light source and another object. It prevents the

light rays from reaching the other object.

–

The fist object casts a shadow on the second one.

Shadows in Computer Graphics

•

In ray tracing rendering, shadows can be added very easily:

–

Light comes from some direction l;

–

When computing the color of a point p on a surface, the point is in

shadow if we cast a ray in direction l and it hits an object. Otherwise,

the object is not in a shadow.

How to implement shadows in

rasterized renderings?

•

Shadow Mapping!

Shadow Mapping

•

Basic idea: if we looked our scene from the view point of the

light source, all of the objects that we can see would appear

in light. Anything behind those objects, however, would be in

shadow.

Shadow Mapping

•

Algorithm:

–

Step 1: render the scene from the light's point of view (without

calculating light, color or any other shading process).

–

Step 2: extract and save (usually in a texture) the depth buffer from

the rendering (z-buffer generated by the projection process).

•

This texture is called shadow map. If there are multiple lights, a separate

depth map must be used for each light.

–

Step 3: perform a normal rendering pass, and when evaluating

whether a fragment is visible to the light source, project its location in

the shadow map and compare the looked-up value d_m_a_p with the

actual distance d to the light source.

•

If the distances are the same, the fragment’s point is illuminated; if the d > d_m_a_p,

that implies there is a different surface closer to the source, so it is shadowed.

Shadow Mapping Shader

•

In order to implement shadow mapping in a Unity shader, we

need to implement two processes:

–

Cast Shadows: implement a special pass (ShadowCaster) in the shader

to produce the depth data when rendering the scene from the light's

point of view.

•

When Unity detects this pass in a Shader, it automatically renders the object in the

shadow map.

–

Receive Shadows: modify the vertex and fragment programs of the

shader in order to access the shadow map, verify which fragments are

shadowed, and compute their colors appropriately.

Shadow Mapping Shader

•

Cast Shadows:

SubShader{

Pass{

Tags{"LightMode" = "ForwardBase"}

...

}

Pass{

Tags{"LightMode" = "ForwardAdd"}

..

.

}

Pass{

Tags{"LightMode" = "ShadowCaster"}

CGPROGRAM

#

pragma target 3.0

pragma vertex MyShadowVertexProgram

pragma fragment MyShadowFragmentProgram

include "ShadowsShader.cginc"

ENDCG

}

Shadow Mapping Shader

•

Cast Shadows:

#

if !defined(SHADOWSSHADER_INCLUDED)

define SHADOWSSHADER_INCLUDED

#include "UnityCG.cginc"

struct VertexData{

float4 position : POSITION;

Using the standard shader on the

ground to visualize the shadows.

};

float4 MyShadowVertexProgram(VertexData vert) : SV_POSITION{

return UnityObjectToClipPos(vert.position);

}

float4 MyShadowFragmentProgram() : SV_TARGET {

return 0;

}

#endif

Shadow Mapping Shader

•

Cast Shadows (bias – light setting to control shadow distance):

#

if !defined(SHADOWSSHADER_INCLUDED)

define SHADOWSSHADER_INCLUDED

#include "UnityCG.cginc"

struct VertexData {

float4 position : POSITION;

float3 normal : NORMAL;

};

float4 MyShadowVertexProgram(VertexData vert) : SV_POSITION{

float4 position = UnityClipSpaceShadowCasterPos(vert.position.xyz,

vert.normal);

return UnityApplyLinearShadowBias(position);

}

float4 MyShadowFragmentProgram() : SV_TARGET {

return 0;

}

#endif

Shadow Mapping Shader

•

Receive Shadows:

Pass{

Tags{"LightMode" = "ForwardBase"}

CGPROGRAM

#

.

pragma target 3.0

pragma multi_compile _ SHADOWS_SCREEN

..

}

struct VertexToFragment {

...

#

if defined(SHADOWS_SCREEN)

float4 shadowcoords : TEXCOORD2;

endif

#

Used to store the screen-space

coordinates of shadow map.

};

Shadow Mapping Shader

•

Receive Shadows:

VertexToFragment MyVertexProgram(VertexData vert){

...

#if defined(SHADOWS_SCREEN)

v2f.shadowcoords = ComputeScreenPos(v2f.position);

endif

#

.

Computes the screen-space

..

coordinates of the shadow map

based on the vertex position.

}

UnityLight CreateLight(VertexToFragment v2f) {

Access the shadow

map values

...

#if defined(SHADOWS_SCREEN)

(

_ShadowMapTexture).

float attenuation = tex2D(_ShadowMapTexture,

v2f.shadowcoords.xy / v2f.shadowcoords.w);

#else

UNITY_LIGHT_ATTENUATION(attenuation, 0, v2f.worldpos);

#endif

light.color = _LightColor0.rgb * attenuation; Multiplies the light

...

color by the attenuation

factor (shadow map).

}

Shadow Mapping Shader

Screen-Space Shadow Map

Shadow Map

Rendered Scene

Shadow Mapping Shader

•

Add support to multiple shadows (directional lights):

Pass{

Tags{"LightMode" = "ForwardBase"}

CGPROGRAM

#

.

pragma target 3.0

pragma multi_compile_fwdadd_fullshadows

..

}

Pass{

Tags{"LightMode" = "ForwardAdd"}

Blend One One

ZWrite Off

CGPROGRAM

#

.

pragma target 3.0

pragma multi_compile_fwdadd_fullshadows

..

}

Shadow Mapping – Frame Debbuger

Shadow Map – Light 1

Shadow Map – Light 2

Unity Shadows Settings

•

Shadow quality settings: Edit -> Project Settings -> Quality

Shadow settings per light:

Unity Shadow Maps

•

By default, Unity renders the scene four times per light (cascade

technique). The shadow maps are split into four quadrants,

each being rendered from a different point of view.

Shadow Quality

•

Hard vs. Soft Shadows:

Hard Shadows

Soft Shadows

Shadow Quality

Shadow problems:

•

–

Numerical precision limitations (shadow acne):

No bias

With bias

–

Shadow map resolution:

No blur

With Blur

Shadow Quality

•

Cascades Bands: