Shadows CS418 Computer Graphics John C. Hart Shadowing Shadows - - PowerPoint PPT Presentation

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May 23, 2023 343 likes •456 views

Shadows CS418 Computer Graphics John C. Hart Shadowing Shadows indicate light occlusion Fix object positions relative to each other Perspective Distortion screen y y view y clip -z z view 1 x view z

SLIDE 1

Shadows

CS418 Computer Graphics John C. Hart

SLIDE 2

Shadowing

Shadows indicate light occlusion
Fix object positions relative to each
ther

SLIDE 3

Perspective Distortion

screen

y zview yview 1 yclip

view clip view

y y z  

view view view view view view view view view view view view

1 1 1 1 1 x z x x y y y z z z z z                                                                         

SLIDE 4

Shadow Buffer

Williams, SIGGRAPH 78
Render scene twice from

viewpoint (x,y,z)screen = WPV (x,y,z)world – Once with light source on – Once only with ambient – Save a z-buffer

Render scene from light position

(x,y,z)light = WlPlVl (x,y,z)world – Save only z-buffer

For each viewpoint image pixel

compute (x,y,z)vis =WlPlVl V-1P-1W-1 (x,y,z)screen

If zvis < zlight at (x,y)vis = (x,y)light
Then pixel is in shadow

RGB Depth Viewpoint Light Position

SLIDE 5

Shadow Buffer

Williams, SIGGRAPH 78
Render scene twice from

viewpoint (x,y,z)screen = WPV (x,y,z)world – Once with light source on – Once only with ambient – Save a z-buffer

Render scene from light position

(x,y,z)light = WlPlVl (x,y,z)world – Save only z-buffer

For each viewpoint image pixel

compute (x,y,z)vis =WlPlVl V-1P-1W-1 (x,y,z)screen

If zvis < zlight at (x,y)vis = (x,y)light
Then pixel is in shadow

SLIDE 6

Shadow Buffer

Williams, SIGGRAPH 78
Render scene twice from

viewpoint (x,y,z)screen = WPV (x,y,z)world – Once with light source on – Once only with ambient – Save a z-buffer

Render scene from light position

(x,y,z)light = WlPlVl (x,y,z)world – Save only z-buffer

For each viewpoint image pixel

compute (x,y,z)vis =WlPlVl V-1P-1W-1 (x,y,z)screen

If zvis < zlight at (x,y)vis = (x,y)light
Then pixel is in shadow

SLIDE 7

Shadow Buffer

Williams, SIGGRAPH 78
Render scene twice from

viewpoint (x,y,z)screen = WPV (x,y,z)world – Once with light source on – Once only with ambient – Save a z-buffer

Render scene from light position

(x,y,z)light = WlPlVl (x,y,z)world – Save only z-buffer

For each viewpoint image pixel

compute (x,y,z)vis =WlPlVl V-1P-1W-1 (x,y,z)screen

If zvis < zlight at (x,y)vis = (x,y)light
Then pixel is in shadow

SLIDE 8

Shadow Buffer

Williams, SIGGRAPH 78
Render scene twice from

viewpoint (x,y,z)screen = WPV (x,y,z)world – Once with light source on – Once only with ambient – Save a z-buffer

Render scene from light position

(x,y,z)light = WlPlVl (x,y,z)world – Save only z-buffer

For each viewpoint image pixel

compute (x,y,z)vis =WlPlVl V-1P-1W-1 (x,y,z)screen

If zvis < zlight at (x,y)vis = (x,y)light
Then pixel is in shadow

SLIDE 9

Numerical Analysis

SLIDE 10

Percentage Closer Filtering

Reeves, Salesin, Cook,

SIGGRAPH 87

Shadow aliasing occurs when

numerical inaccuracy and discretization causes sample to compare to the wrong depth near an edge

Can be fixed by comparing

sample in a neighborhood (e.g. 3x3 pixels) and filtering the binary shadow answer

Can also be extended to

generate soft shadows