OpenGL Shadow

Content

Shadowing using Stencil BufferShadowing using Projective Texture Shadow mapShadow volume

Shadows are important …

It is usually better to have an inaccurate shadow than none at all

Terminology

(area) light source

receiver shadow

occluder

umbra

penumbra

Hard Shadow Soft ShadowOccludee (receiver)

Soft Shadows (Ref)

–Point light generates hard shadows only

–Soft shadows are NOT generated by low pass filters

–Soft shadows are generally preferable, if possible.

Planar Projection Shadow (xz plane)

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Similarly,

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v: any pt on model

Planar Projection (general)l

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Planar Projection Shadow

Problems [& Solutions] Z-fighting [polygon offset, stencil] Shadow fall outside of the receiver [stencil] Semi-transparent shadows [blending +

stencil, ensuring each pixel is drawn at most once]

Planar receiver only Need to render each frame (But for static

objects, the shadows are view-independent) [render the shadow into texture]

Remark

Shadow polygons are blended with the floor, one-by-oneRepetitive blending …

To prevent this from happening, use stencil bufferWhen a polygon is drawn, increment the stencil count (therefore, the area is not drawn again)

glStencilFunc(GL_EQUAL, 1, 0xFF);

glStencilOp (GL_KEEP, GL_KEEP, GL_INCR);

glStencilFunc(GL_EQUAL, 1, 0xFF);

glStencilOp (GL_KEEP, GL_KEEP, GL_INCR);

Stencil Setting

Projective Texture Shadow

Overview

In the display callbackRender occluders to projective

shadow texture (with FBO)Set up texture unit 1 for projective

(shadow) texture; render receiverRender occluder to complete the

scene

Steps

Terrain: triangle stripFBO for render to shadow textureRender projective (shadow) texture

Terrain (trianglestrip)

0 1 2 3 44

0

1

2

3

x

z

[0,0] [0,1] [1,0] [1,1] [2,0] [2,1] …[4,0] [4,1]

[i-1,j] [i,j]

[i-1,j+1]

[i,j+1]

[i,j]

[i-1,j+1]

Set normal at

Render to Shadow Texture

Render occluder (unlit) as black objectsClear color set to (1,1,1)

Projective Shadow Texture

Set texture environment to modulate Shadowed part: dark (modulated by

GREY) Unshadowed part: unchanged

(modulated by WHITE)

Use the same projective parameters (fovy, …) as in render to texture

Tricks

This is normal!If you see something like this, the artifact comes from projective texture clamping: the shadow touches the texture border. It can be alleviated by increasing the fov or raising the light position

Shadow Map Primer

Image-based technique by Lance Williams (1978)Amenable to h/w implementation

Coordinate Transformation

World Space

Note the convention on X and X-1

Ee: transformed plane equations

Plight: projection matrix for light frustum

[s,t,r,q]T is actually [x,y,z,w]T (scale-biased clip coordinates)

Texgen (eye-linear)

We do not use texture matrix T for now

S: scale-bias matrix from [-1,1] to [0,1]

Depth Map Comparison

Render from eye’s point of view“Convert” the eye coordinates to light space via texgen facility

ObjectLight

EyeLight

Summary

EyeLight

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After converting to light space

Compare with depth texture:

Use comparison result (0 or 1) in fragment shading (can be bilinearly filtered): use the result directly to modulate the diffuse and specular intensity

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Zls > Zsm: in shadow

GL_ARB_shadow (ref)Require depth texture extensionEnable shadow comparison

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE_ARB, GL_COMPARE_R_TO_TEXTURE);

Not in shadow if r<=texture glTexParameteri(GL_TEXTURE_2D,

GL_TEXTURE_COMPARE_FUNC_ARB, GL_LEQUAL);

Shadow comparison should generate an INTENSITY result (or ALPHA result)

glTexParameteri(GL_TEXTURE_2D, GL_DEPTH_TEXTURE_MODE_ARB, GL_INTENSITY)

Set alpha test to discard false comparisons glAlphaFunc(GL_GEQUAL, 0.99f); glEnable(GL_ALPHA_TEST);

This comparison produces a boolean texture value

From GL_ARB_SHADOW

Implementation

Generate shadow map render the scene from light

position Lighting, texturing, color

buffer writes OFF Copy depth buffer content

to a depth texture

Note: shadow map is view independent (only need to be updated if scene or light moved)

Ambient pass Render the scene from

camera with only ambient lighting

Diffuse Pass Set up necessary manipulations

for coordinate transformation (eye coordinate to light coordinate)

Set up shadow map parameters (compare_mode, compare_func, deptht texture_mode)

Render the scene with lights on; the shadow map will generate a boolean texture result based on shadow test. The test will set the alpha values of the fragment. [in shadow = 0, otherwise, = 1]

The alpha test will let the lit (unshadowed) fragment pass, covering the original ambient fragments.

Final color is ambient pass + full color

[multitexture]Set shadowmap at the fourth texture unitUse the other three for original textures

FAQ

Fov for light need to be the same as te fov of camera?No. The shadow comparison is solely based on depth texture and the converted clip coordinates in light coordinate system. The projection matrices (where fov plays a part) are of no importance.

Aspect ratio of shadow map need to be the same as the aspect ratio of the window?No. (similar reason as in the left.) The projection matrices (where aspect ratio plays a part) are of no importance.

Trouble Shooting

The artifact on the base is because the shadow map coverage contains only part of the base. It can be remedied by increasing the fov of the light frustum (but sacrificing the depth texture resolution)

fov=45 fov=90

Using Shaders…

Texgen is not performedThe inverse of viewing matrix need to be computed directly (see projective texture)Only two passes are required First pass: generate shadow map Second pass: render the shadowed and lit part

(one pass less than fixed functionality)

Remove unwanted artifact (more precise control by rendering shadowed part only within light frustum)

Bias(1)

Bias(2)

Bias(3)

Alias(1)

Alias(2)

Issues

Bias factor Depth in shadow map often slightly

lower than that of the viewer

Aliasing due to point-sampling Percentage closer filtering: instead of

single point sample, take four closest. Bilinearly interpolate the results (in/out

shadow) to calculate the lighting Provide pseudo-soft shadows

Shadow Map

Advantage: Simple Shadow map is view independent

Disadvantage: Shadow distance from light position may

appear blocky Storage Light source in the view volume?

Shadow Volume

Geometry-based technique first proposed by Frank Crow in 1977

Need stencil buffer!

Silhouette: the border of front/back facing polygons w.r.t. light source

Point light [illustration here based on point light]

Z-pass Algorithm

Figure 6: When eye point is within the shadow volume, depth-pass stencil operation fails

Pitfalls of Z-Pass

1

Carmack’s reverse (Z-fail)

Shadow volume must be a closed volume (require front and back caps). Back cap can be at infinity !?

Z-fail Requires Capping

Only need to close the back gap for point light (directional light is closed by definition: all rays intersect to a single point at infinity)

Reusing front cap

Triangular-Fan

We can build the front cap by using the front facing triangles with respect to the light source. The geometries used in the front cap can then be extruded, with their ordering reversed, to create the back cap.

Algorithm (Shadow Volume)

Bottlenecks

Silhouette Determination

View Frustum Clipping1

Solution by Mark Kilgard

Solution by Eric Lengyel…Depth-clamping (Kilgard & Everitt): hardware solution

View Frustum Clipping2

Better, unless viewer enters shadow vol.

Depends on type of games: z-pass sufficient for Diablo ( 暗黑破壞神 )

Shadow Volume by Shaders

Unless CPU is totally occupied, may not be beneficial (silhouette computation; too many vertices processed by shaders)

Advantages

• Support omnidirectional lights

• Exact shadow boundaries

Disadvantages

• Fill-rate intensive

• Expensive to compute shadow volumes

• Hard shadow boundaries, not soft shadows

• Difficult to implement robustly

Area Light