Viewing 3 - Home | Computer Science at UBC
Transcript of Viewing 3 - Home | Computer Science at UBC
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Tamara Munzner
http://www.ugrad.cs.ubc.ca/~cs314/Vjan2016
Viewing 3
University of British Columbia CPSC 314 Computer Graphics
Jan-Apr 2016
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View Volumes • specifies field-of-view, used for clipping • restricts domain of z stored for visibility test
z
perspective view volume orthographic view volume
x=left
x=right
y=top
y=bottom z=-near z=-far x VCS x
z
VCS
y y
x=left y=top
x=right
z=-far z=-near y=bottom
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Canonical View Volumes • standardized viewing volume representation
perspective orthographic orthogonal parallel
x or y
-z
1
-1
-1 front plane
back plane
x or y
-z front plane
back plane
x or y = +/- z
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Why Canonical View Volumes?
• permits standardization • clipping
• easier to determine if an arbitrary point is enclosed in volume with canonical view volume vs. clipping to six arbitrary planes
• rendering • projection and rasterization algorithms can be
reused
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Normalized Device Coordinates
• convention • viewing frustum mapped to specific
parallelepiped • Normalized Device Coordinates (NDC) • same as clipping coords
• only objects inside the parallelepiped get rendered
• which parallelepiped? • depends on rendering system
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Normalized Device Coordinates
left/right x =+/- 1, top/bottom y =+/- 1, near/far z =+/- 1
-z
x
Frustum
z=-n z=-f
right
left z
x
x= -1 z=1
x=1
Camera coordinates NDC
z= -1
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Understanding Z
• z axis flip changes coord system handedness • RHS before projection (eye/view coords) • LHS after projection (clip, norm device coords)
x
z
VCS
y x=left
y=top
x=right
z=-far z=-near y=bottom
x
z
NDCS
y
(-1,-1,-1)
(1,1,1)
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Understanding Z near, far always positive in GL calls THREE.OrthographicCamera(left,right,bot,top,near,far); mat4.frustum(left,right,bot,top,near,far, projectionMatrix);
orthographic view volume
x
z
VCS
y x=left
y=top
x=right
z=-far z=-near y=bottom
perspective view volume
x=left
x=right
y=top
y=bottom z=-near z=-far x VCS
y
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Understanding Z
• why near and far plane? • near plane:
• avoid singularity (division by zero, or very small numbers)
• far plane: • store depth in fixed-point representation
(integer), thus have to have fixed range of values (0…1)
• avoid/reduce numerical precision artifacts for distant objects
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Orthographic Derivation
• scale, translate, reflect for new coord sys
x
z
VCS
y x=left
y=top
x=right
z=-far z=-near y=bottom
x
z
NDCS
y
(-1,-1,-1)
(1,1,1)
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Orthographic Derivation
• scale, translate, reflect for new coord sys
x
z
VCS
y x=left
y=top
x=right
z=-far z=-near y=bottom
x
z
NDCS
y
(-1,-1,-1)
(1,1,1)
byay +⋅='1'
1'
−=→=
=→=
yboty
ytopy
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Orthographic Derivation
• scale, translate, reflect for new coord sys
byay +⋅='1'
1'
−=→=
=→=
yboty
ytopy
bottop
bottopb
bottop
topbottopb
bottop
topb
btopbottop
−
−−=
−
⋅−−=
−
⋅−=
+−
=
2)(
21
21
bbota
btopa
+⋅=−
+⋅=
1
1
bottopa
topbota
topabota
botatopa
botabtopab
−=
+−=
⋅−−⋅−=−−
⋅−−=⋅−
⋅−−=⋅−=
2
)(2
)()1(1
11
1,1
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Orthographic Derivation
• scale, translate, reflect for new coord sys
x
z
VCS
y x=left
y=top
x=right
z=-far z=-near y=bottom
byay +⋅='1'
1'
−=→=
=→=
yboty
ytopy
bottop
bottopb
bottopa
−
+−=
−=
2
same idea for right/left, far/near
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Orthographic Derivation
• scale, translate, reflect for new coord sys
P
nearfar
nearfar
nearfar
bottop
bottop
bottop
leftright
leftright
leftright
P
−
+−
−
−
−
+−
−
−
+−
−
=
1000
200
02
0
002
'
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Orthographic Derivation
• scale, translate, reflect for new coord sys
P
nearfar
nearfar
nearfar
bottop
bottop
bottop
leftright
leftright
leftright
P
−
+−
−
−
−
+−
−
−
+−
−
=
1000
200
02
0
002
'
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Orthographic Derivation
• scale, translate, reflect for new coord sys
P
nearfar
nearfar
nearfar
bottop
bottop
bottop
leftright
leftright
leftright
P
−
+−
−
−
−
+−
−
−
+−
−
=
1000
200
02
0
002
'
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Orthographic Derivation
• scale, translate, reflect for new coord sys
P
nearfar
nearfar
nearfar
bottop
bottop
bottop
leftright
leftright
leftright
P
−
+−
−
−
−
+−
−
−
+−
−
=
1000
200
02
0
002
'
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Projective Rendering Pipeline
OCS - object/model coordinate system
WCS - world coordinate system
VCS - viewing/camera/eye coordinate system
CCS - clipping coordinate system
NDCS - normalized device coordinate system
DCS - device/display/screen coordinate system
OCS O2W VCS
CCS
NDCS
DCS
modeling transformation
viewing transformation
projection transformation
viewport transformation
perspective divide
object world viewing
device
normalized device
clipping
W2V V2C
N2D
C2N
WCS
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Projection Warp
• warp perspective view volume to orthogonal view volume
• render all scenes with orthographic projection! • aka perspective warp
Z
x
z=α z=d
Z
x
z=0 z=d
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Perspective Warp • perspective viewing frustum transformed to
cube • orthographic rendering of cube produces same
image as perspective rendering of original frustum
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Predistortion
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Projective Rendering Pipeline
OCS - object/model coordinate system
WCS - world coordinate system
VCS - viewing/camera/eye coordinate system
CCS - clipping coordinate system
NDCS - normalized device coordinate system
DCS - device/display/screen coordinate system
OCS O2W VCS
CCS
NDCS
DCS
modeling transformation
viewing transformation
projection transformation
viewport transformation
perspective divide
object world viewing
device
normalized device
clipping
W2V V2C
N2D
C2N
WCS
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Separate Warp From Homogenization
• warp requires only standard matrix multiply • distort such that orthographic projection of distorted
objects is desired persp projection • w is changed
• clip after warp, before divide • division by w: homogenization
CCS NDCS
alter w / w
VCS projection
transformation
viewing normalized device
clipping
perspective division
V2C C2N
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Perspective Divide Example
• specific example • assume image plane at z = -1
• a point [x,y,z,1]T projects to [-x/z,-y/z,-z/z,1]T ≡ [x,y,z,-z]T
-z
1
z
y
x
− z
z
y
x
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Perspective Divide Example
−
−
−
≡
−
=
⋅
−
=
1
1
/
/
10100
0100
0010
0001
1
zy
zx
z
z
y
x
z
y
x
z
y
x
T
alter w / w
projection transformation
perspective division
• after homogenizing, once again w=1
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Perspective Normalization • matrix formulation
• warp and homogenization both preserve relative depth (z coordinate)
−
⋅−=
−
⋅−
−
d
z
d
dz
y
x
z
y
x
d
d
d
d
d
α
α
α
α
α
)(
101
00
00
0010
0001
−
−
=
zd
ddz
ydz
x
z
y
x
p
p
p
α
α1
/
/
2
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Perspective To NDCS Derivation
x
z
NDCS
y
(-1,-1,-1)
(1,1,1) x=left
x=right
y=top
y=bottom z=-near z=-far x
VCS
y
z
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Perspective Derivation
simple example earlier:
complete: shear, scale, projection-normalization €
x '
y '
z'
w '
=
1 0 0 0
0 1 0 0
0 0 1 0
0 0 1/d 0
x
y
z
1
€
′ x
′ y
′ z
′ w
=
E 0 A 0
0 F B 0
0 0 C D
0 0 −1 0
x
y
z
1
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Perspective Derivation
earlier:
complete: shear, scale, projection-normalization €
x '
y '
z'
w '
=
1 0 0 0
0 1 0 0
0 0 1 0
0 0 1/d 0
x
y
z
1
€
′ x
′ y
′ z
′ w
=
E 0 A 0
0 F B 0
0 0 C D
0 0 −1 0
x
y
z
1
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Perspective Derivation
earlier:
complete: shear, scale, projection-normalization €
x '
y '
z'
w '
=
1 0 0 0
0 1 0 0
0 0 1 0
0 0 1/d 0
x
y
z
1
€
′ x
′ y
′ z
′ w
=
E 0 A 0
0 F B 0
0 0 C D
0 0 −1 0
x
y
z
1
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Perspective Derivation
€
x '
y '
z'
w '
=
E 0 A 0
0 F B 0
0 0 C D
0 0 −1 0
x
y
z
1
€
y '= Fy + Bz, y '
w '=Fy + Bz
w', 1=
Fy + Bz
w ', 1=
Fy + Bz
−z,
1 = Fy
−z+ B
z
−z, 1 = F
y
−z− B, 1= F
top
−(−near)− B,
€
x '= Ex + Az
y '= Fy + Bz
z'= Cz + D
w '= −z
x = left → ′x / ′w = −1
x = right → ′x / ′w =1
y = top → ′y / ′w =1
y = bottom → ′y / ′w = −1
z = −near → ′z / ′w = −1
z = − far → ′z / ′w =1
€
1 = Ftop
near− B
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Perspective Derivation
• similarly for other 5 planes • 6 planes, 6 unknowns
€
2n
r − l0
r + l
r − l0
02n
t −b
t + b
t − b0
0 0−( f + n)
f − n
−2 fn
f − n0 0 −1 0
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Projective Rendering Pipeline
OCS - object/model coordinate system
WCS - world coordinate system
VCS - viewing/camera/eye coordinate system
CCS - clipping coordinate system
NDCS - normalized device coordinate system
DCS - device/display/screen coordinate system
OCS O2W VCS
CCS
NDCS
DCS
modeling transformation
viewing transformation
projection transformation
viewport transformation
perspective divide
object world viewing
device
normalized device
clipping
W2V V2C
N2D
C2N
WCS
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NDC to Device Transformation • map from NDC to pixel coordinates on display
• NDC range is x = -1...1, y = -1...1, z = -1...1 • typical display range: x = 0...500, y = 0...300
• maximum is size of actual screen • z range max and default is (0, 1), use later for visibility
x y
viewport NDC
0 500
300
0
-1 1
1
-1
x
y
gl.viewport(0,0,w,h); gl.depthRange(0,1); // depth = 1 by default
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Origin Location • yet more (possibly confusing) conventions
• GL origin: lower left • most window systems origin: upper left
• then must reflect in y • when interpreting mouse position, have to flip your y
coordinates x
y
viewport NDC
0 500
300
0
-1 1
1
-1
x
y
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N2D Transformation • general formulation
• reflect in y for upper vs. lower left origin • scale by width, height, depth • translate by width/2, height/2, depth/2
• FCG includes additional translation for pixel centers at (.5, .5) instead of (0,0)
x y
viewport NDC
0 500
300
0
-1 -1
1
1 height
width
x
y
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N2D Transformation
x y
viewport NDC
0 500
300
0
-1 -1
1
1 height
width
x
y
€
xD
yD
zD
1
=
1 0 0width
2−1
2
0 1 0height
2−1
2
0 0 1depth
20 0 0 1
width
20 0 0
0height
20 0
0 0depth
20
0 0 0 1
1 0 0 0
0 −1 0 0
0 0 1 0
0 0 0 1
xN
yN
zN
1
=
width (xN
+ 1) −1
2height(− y
N+1) −1
2depth(z
N+1)
21
reminder: NDC z range is -1 to 1
Display z range is 0 to 1. gl.depthRange(n,f) can constrain further, but depth = 1 is both max and default
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Device vs. Screen Coordinates • viewport/window location wrt actual display not available
within GL • usually don’t care
• use relative information when handling mouse events, not absolute coordinates
• could get actual display height/width, window offsets from OS • loose use of terms: device, display, window, screen...
x
display viewport
0 1024
768
0
300
500
display height
display width
x offset
y offset y
viewport
x 0 0 y
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Projective Rendering Pipeline
OCS - object coordinate system
WCS - world coordinate system
VCS - viewing coordinate system
CCS - clipping coordinate system
NDCS - normalized device coordinate system
DCS - device coordinate system
OCS WCS VCS
CCS
NDCS
DCS
modeling transformation
viewing transformation
projection transformation
viewport transformation
alter w
/ w
object world viewing
device
normalized device
clipping
perspective division
glVertex3f(x,y,z)
glTranslatef(x,y,z) glRotatef(a,x,y,z) ....
gluLookAt(...)
glFrustum(...)
glutInitWindowSize(w,h) glViewport(x,y,a,b)
O2W W2V V2C
N2D
C2N
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Coordinate Systems viewing (4-space, W=1)
clipping (4-space parallelepiped, with COP moved backwards to infinity
normalized device (3-space parallelepiped)
device (3-space parallelipiped)
projection matrix
divide by w
scale & translate
framebuffer
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Perspective Example
tracks in VCS: left x=-1, y=-1 right x=1, y=-1
view volume left = -1, right = 1 bot = -1, top = 1 near = 1, far = 4
z=-1
z=-4
x
zVCS
top view
-1 -1 1
1
-1 NDCS
(z not shown)
real midpoint
0 xmax-1 0 DCS
(z not shown)
ymax-1
x=-1 x=1
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Perspective Example
view volume • left = -1, right = 1 • bot = -1, top = 1 • near = 1, far = 4
€
2n
r − l0
r + l
r − l0
02n
t −b
t + b
t − b0
0 0−( f + n)
f − n
−2 fn
f − n0 0 −1 0
€
1 0 0 0
0 1 0 0
0 0 −5 /3 −8 /3
0 0 −1 0
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Perspective Example
/ w
€
xNDCS = −1/ zVCS
yNDCS =1/zVCS
zNDCS =5
3+
8
3zVCS
€
1
−1
−5zVCS/3− 8 / 3
−zVCS
=
1
1
−5 / 3 −8 /3
−1
1
−1
zVCS
1