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Geometric Geometric Transformations Transformations Ceng 477 Introduction to Computer Graphics Fall 2007 Computer Engineering METU
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Geometric Geometric TransformationsTransformations

Ceng 477 Introduction to Computer GraphicsFall 2007

Computer EngineeringMETU

2D Geometric 2D Geometric TransformationsTransformations

Basic Geometric Transformations

● Geometric transformations are used to transform the objects and the camera in a scene (for animation or modelling) and are also used to transform World Coordinates to View Coordinates

● Given the shape, transform all the points of the shape? Transform the points and/or vectors describing it.

● For example: Polygon: corner points Circle, Ellipse: center point(s), point at angle 0

● Some transformations preserves some of the attributes like sizes, angles, ratios of the shape.

Translation

● Simply move the object to a relative position.

TPP

PTP

+=

y'x'

=tt

=yx

=

t+y=y't+x=x'

y

x

yx

'

' ⎥⎦

⎤⎢⎣

⎡⎥⎦

⎤⎢⎣

⎡⎥⎦

⎤⎢⎣

T

P

P'

Rotation

● A rotation is defined by a rotation axis and a rotation angle.

● For 2D rotation, the parameters are rotation angle (θ) and the rotation point (xr ,yr ).

● We reposition the object in a circular path arround the rotation point (pivot point)

θ

xr

yr

Rotation

● When (xr ,yr )=(0,0) we have

r P

P'

θφθφθφθφθφθφ

cossinsincos)sin(sinsincoscos)cos(

rrryrrrx

+=+=′−=+=′

φφ

sincos

ryrx

==The original coordinates are:

θθθθ

cossinsincos

yxyyxx

+=′−=′Substituting them in the

first equation we get:

In the matrix form we have:

where

PRP ⋅=′

⎥⎦

⎤⎢⎣

⎡ −=

θθθθ

cossinsincos

R

Rotation

● Rotation around an arbitrary point (xr ,yr )

● This equations can be written as matrix operations (we will see when we discuss homogeneous coordinates).

r

P

P'

θθθθ

cos)(sin)(sin)(cos)(

rrr

rrr

yyxxyyyyxxxx

−+−+=′−−−+=′

(xr ,yr )

Scaling

● Change the size of an object. Input: scaling factors (sx ,sy )

PSP

S

' ⋅

⎥⎦

⎤⎢⎣

=

ss

=

ys=y'xs=x'

y

x

yx

00

PP'

non-uniform vs.uniform scaling

Homogenous Coordinates● All transformations can be represented by matrix

operations.

● Translation is additive, rotation and scaling is multiplicative (+ additive if you rotate around an arbitrary point or scale around a fixed point); making the operations complicated.

● Adding another dimension to transformations make translation also representable by multiplication. Cartesian coordinates vs homogenous coordinates.

⎥⎥⎥

⎢⎢⎢

⎡⋅⋅

⎥⎥⎥

⎢⎢⎢

hyhxh

=hyx

=Phy=y

hx=x h

hhh

● Many points in homogenous coordinates can represent the same point in Cartesian coordinates.

● In homogenous coordinates, all transformations can be written as matrix multiplications.

Transformations in Homogenous C.

● Translation

● Rotation

● Scaling

( )

( ) Pt,tT=P

tt

=t,tT

yx'

y

x

yx

⎥⎥⎥

⎢⎢⎢

1001001

( )

( ) PθR=P

θθθθ

=θR

' ⋅

⎥⎥⎥

⎢⎢⎢

⎡ −

1000cossin0sincos

( )

( ) Ps,sS=P

ss

=s,sS

yx

y

x

yx

⋅′

⎥⎥⎥

⎢⎢⎢

1000000

Composite Transformations

● Application of a sequence of transformations to a point:

PMPMMP 12

⋅=⋅⋅=′

Composite Transformations

● First: composition of similar type transformations

● If we apply to successive translations to a point:

( ) ( ) ( )yyxxyy

xx

y

x

y

x

yxyx t+t,t+tT=tttt

=tt

tt

=t,tTt,tT 212121

21

1

1

2

2

1122

1001001

1001001

1001001

⎥⎥⎥

⎢⎢⎢

⎡++

⎥⎥⎥

⎢⎢⎢

⎡⋅⎥⎥⎥

⎢⎢⎢

⎡⋅

PTT

PTTP

⋅⋅=

⋅⋅=′

)},(),({

}),({),(

1122

1122

yxyx

yxyx

tttt

tttt

( ) ( ) ( )yyxxyy

xx

y

x

y

x

yxyx ss,ss=ssss

=ss

ss

=s,ss,s 212121

21

1

1

2

2

1122

1000000

1000000

1000000

⋅⋅⎥⎥⎥

⎢⎢⎢

⎡⋅

⎥⎥⎥

⎢⎢⎢

⎡⋅⎥⎥⎥

⎢⎢⎢

⎡⋅ SSS

Composite Transformations

( ) ( )

( )φ+θ=φ+θφ+θφ+θφ+θ

=φθ+φθφθ+φθφθφθφθφθ

=θθθθ

=φθ

R

RR

⎥⎥⎥

⎢⎢⎢

⎡ −

⎥⎥⎥

⎢⎢⎢

⎡−

−−−

⎥⎥⎥

⎢⎢⎢

⎡ −⋅⎥⎥⎥

⎢⎢⎢

⎡ −⋅

1000)cos()sin(0)sin()cos(

1000coscossinsinsincoscossin0cossinsincossinsincoscos

1000cossin0sincos

1000cossin0sincos

ϕϕϕϕ

Rotation around a pivot point– Translate the object so that the

pivot point moves to the origin

– Rotate around origin

– Translate the object so that the pivot point is back to its original position

( ) ( ) ( )

( )( )

⎥⎥⎥

⎢⎢⎢

⎡−−

−−

⎥⎥⎥

⎢⎢⎢

⎡−−

⋅⎥⎥⎥

⎢⎢⎢

⎡ −⋅

⎥⎥⎥

⎢⎢⎢

−−⋅⋅

100sincos1cossinsincos1sincos

1001001

1000cossin0sincos

1001001

θxθyθθθy+θxθθ

=yx

θθθθ

yx

=y,xθy,x

rr

rr

r

r

r

r

rrrr TRT

( )rr y,x −−T

( )θR

( )rr y,xT

Scaling with respect to a fixed point

– Translate to origin

– Scale

– Translate back

( ) ( ) ( )

( )( )

⎥⎥⎥

⎢⎢⎢

⎡−−

⎥⎥⎥

⎢⎢⎢

⎡−−

⋅⎥⎥⎥

⎢⎢⎢

⎡⋅⎥⎥⎥

⎢⎢⎢

−−⋅⋅

1001010

1001001

1000000

1001001

yfy

xfx

f

f

y

x

f

f

ffyxff

syssxs

=yx

ss

yx

=y,xs,sy,x TST

( )ff y,x −−T

( )yx s,sS

( )ff y,xT

Order of matrix compositions

● Matrix composition is not commutative. So, be careful when applying a sequence of transformations.

pivot same pivot

Other Transformations

● Reflection

⎥⎥⎥

⎢⎢⎢

100

010

001

⎥⎥⎥

⎢⎢⎢

⎡−

100

010

001

⎥⎥⎥

⎢⎢⎢

100

010

001

● Shear: Deform the shape like shifted slices.

(1,1)(3,1)

(2,1)

x ' x shx y y ' y

1 shx 00 1 00 0 1

(0,1)

Transformations Between the Coordinate Systems

● Between different systems: Polar coordinates to cartesian coordinates

● Between two cartesian coordinate systems. For example, relative coordinates or window to viewport transformation.

● How to transform from x,y to x',y' ?

● Superimpose x',y' to x,y

● Transformation:

– Translate so that (x0 ,y0 ) moves to (0,0) of x,y

– Rotate x' axis onto x axis

xx

y

y'

x'

x0

y0

( ) ( )00, yxTθR −−⋅−

xx

y

y'x'

● Alternate method for rotation: Specify a vector V for positive y' axis:

xx

y

y'

x'

V( )yx v,v==

y'

VVv

:direction in ther unit vecto

( ) ( )yxxy u,u=v,v=x'

−uv o90 clockwise rotate direction, in ther unit vecto

● Elements of any rotation matrix can be expressed as elements of a set of orthogonal unit vectors:

⎥⎥⎥

⎢⎢⎢

⎡ −

⎥⎥⎥

⎢⎢⎢

10000

10000

yx

xy

yx

yx

vvvv

=vvuu

=R

xx

yy'

x'

P

0P

| |0

0

PPPPv

−−=

● Example:

xx'

yy'

P

0P

( ) ( )

| |( ) ( )

( )

( ) ( )

⎥⎥⎥

⎢⎢⎢

⎡⋅

⎥⎥⎥

⎢⎢⎢

⎥⎥⎥

⎢⎢⎢

−−

⎥⎥⎥

⎢⎢⎢

−⋅

⎥⎥⎥

⎢⎢⎢

⎡ −

−−⋅

⎟⎠⎞

⎜⎝⎛

−−

111

1.220.6

1.610.8111

121

443 100

20.80.6

10.60.8

100

110

201

100

00.80.6

00.60.812,

0.60.8,

0.6,0.82.52

2.51.5

21.51.5,2

3.5,32,1

220

0

=T

==,=

=,=+

==

==

M

TvuRM

=uPPPPv

PP

0

TM T

Let triangle T be defined asthree column vectors:

Affine Transformations

● Coordinate transformations of the form:

● Translation, rotation, scaling, reflection, shear. Any affine transformation can be expressed as the combination of these.

● Rotation, translation, reflection: preserve angles, lengths, parallel lines

yyyyx

xxyxx

b+ya+xa=y'

b+ya+xa=x'

3 DIMENSIONAL3 DIMENSIONAL TRANSFORMATIONSTRANSFORMATIONS

3D Transformations

● x,y,z coordinates. Usual notation: Right handed coordinate system

● Analogous to 2D we have 4 dimensions in homogenous coordinates.

● Basic transformations:

– Translation

– Rotation

– Scaling

x

y

z

z

x

y

y

z

x

Translation

● move the object to a relative position.

z

y

x

z

y

x

⎥⎥⎥⎥

⎢⎢⎢⎢

⎥⎥⎥⎥

⎢⎢⎢⎢

=

⎥⎥⎥⎥

⎢⎢⎢⎢

′′′

11000100010001

1zyx

ttt

zyx

z

y

x

PTP ⋅=′

P

P′

Rotation

● Rotation arround the coordinate axes

x axis y axis z axis

z

x

y

z

x

y

z

x

y

z

x

y

z

x

y

z

x

y

Counterclockwise when looking along the positive half towards origin

Rotation around coordinate axes

● Arround x

● Arround y

● Arround z

⎥⎥⎥⎥

⎢⎢⎢⎢

⎡−

=

10000cossin00sincos00001

)(θθθθ

θxR PRP ⋅=′ )(θx

⎥⎥⎥⎥

⎢⎢⎢⎢

⎡ −

=

1000010000cossin00sincos

)(θθθθ

θzR PRP ⋅=′ )(θz

⎥⎥⎥⎥

⎢⎢⎢⎢

−=

10000cos0sin00100sin0cos

)(θθ

θθ

θyR PRP ⋅=′ )(θy

Rotation Arround a Parallel Axis

● Rotating the object around a line parallel to one of the axes: Translate to axis, rotate, translate back.

z

y

x

z

y

x

z

y

xz

y

x

Translate Rotate Translate back

PTRTP ⋅−−⋅⋅=′ ),,0()(),,0( ppxpp zyzy θ

Figure from the textbook

Rotation Around an Arbitrary Axis

● Translate the object so that the rotation axis passes though the origin

● Rotate the object so that the rotation axis is aligned with one of the coordinate axes

● Make the specified rotation

● Reverse the axis rotation

● Translate back

z

y

x

Rotation Around an Arbitrary Axis

Rotation Around an Arbitrary Axis

),,( 12121212 zzyyxx −−−=−= PPV),,( cba==

VVuu is the unit vector along V:

First step: Translate P1 to origin:

⎥⎥⎥⎥

⎢⎢⎢⎢

−−−

=

1000100010001

1

1

1

zyx

T

Next step: Align u with the z axiswe need two rotations: rotate around x axis to get uonto the xz plane, rotate around y axis to get u aligned with z axis.

Rotation Around an Arbitrary Axis

z

x

u

Align u with the z axis 1) rotate around x axis to get u into the xz plane, 2) rotate around y axis to get u aligned with the z axis

y

α

z

xu

y

βz

x

uu'

αuz

y

Dot product and Cross Product

● v dot u = vx * ux + vy * uy + vz * uz. That equals also to |v|*|u|*cos(a) if a is the angle between v and u vectors. Dot product is zero if vectors are perpendicular.

v x u is a vector that is perpendicular to both vectors you multiply. Its length is |v|*|u|*sin(a), that is an area of parallelogram built on them. If v and u are parallel then the product is the null vector.

Rotation Around an Arbitrary Axis

z

x

uu'

α

Align u with the z axis 1) rotate around x axis to get u into the xz plane, 2) rotate around y axis to get u aligned with the z axis

uz

We need cosine and sine of α for rotation

),,0( cb=′u

22 cos cbddc

z

z +==′⋅′

=uuuuα

bxzxz uuuuuu =′=×′ αsin

αsindb =

db

dc

== αα sin cos

⎥⎥⎥⎥⎥⎥

⎢⎢⎢⎢⎢⎢

⎡−

=

1000

00

000001

)(

dc

db

db

dc

x αRProjection of u onyz plane

Rotation Around an Arbitrary Axis

z

x

u

u''= (a,0,d)

Align u with the z axis 1) rotate around x axis to get u into the xz plane,2) rotate around y axis to get u aligned with the z axis

duuuu

z

z =⋅′′⋅′′

=βcos

)(sin ayzyz −⋅=′′=×′′ uuuuuu β

⎥⎥⎥⎥

⎢⎢⎢⎢

⎡ −

=

−==

100000001000

)(

sin cos

da

y β

ββ

R

),,()()()()()(),,()( 111111 zyxzyx xyzyx −−−⋅⋅⋅⋅−⋅−⋅= TRRRRRTR αβθβαθ

β

Rotation, ... Alternative MethodAny rotation around origin can be represented by 3 orthogonal unit vectors:

⎥⎥⎥⎥

⎢⎢⎢⎢

=

1000000

333231

232221

131211

rrrrrrrrr

R

Define a new coordinate system with the given rotation axis u using:

),,( zyx uuu ′′′

This matrix can be thought of asrotating the unit r1* , r2* , and r3* vectorsonto x, y, and z axes.

So, to align a given rotation axis u onto the z axis,we can define an (orthogonal) coordinate system and form this R matrix

zyx

x

xy

z

uuuuuuuu

uu

′×′=′

××

=′

=′

Rotation, ... Alternative Method

⎥⎥⎥⎥⎥⎥

⎢⎢⎢⎢⎢⎢

⋅−⋅−

=

⋅−⋅−=×−=′×′=′

−=+

−=

××

=××

=′

==′

10000

00

0

)/,/,(),,()/,/,0(

)/,/,0(),,0()0,0,1(),,(),,(

22

dc

dca

dbdccbbccba

cba

zyx

xx

xy

z

R

uuuuuuu

uuu

uu

Check if this is equal to

)()( αβ xy RR ⋅

Scaling

● Change the coordinates of the object by scaling factors.

x 'y 'z '1

sx 0 0 00 s y 0 00 0 sz 00 0 0 1

xyz1

z

y

x

z

y

xPSP ⋅=′

P

P′

Scaling with respect to a Fixed Point

● Translate to origin, scale, translate back

z

y

x

z

y x

z

y

xz

y

x

Translate Scale Translate back

PTSTP ⋅−−−⋅⋅=′ ),,(),,( ffffff zyxzyx

Scaling with respect to a Fixed Point

⎥⎥⎥⎥

⎢⎢⎢⎢

−−−

=−−−⋅⋅=

⎥⎥⎥⎥

⎢⎢⎢⎢

−−−

⎥⎥⎥⎥

⎢⎢⎢⎢

=

⎥⎥⎥⎥

⎢⎢⎢⎢

⎥⎥⎥⎥

⎢⎢⎢⎢

⎥⎥⎥⎥

⎢⎢⎢⎢

=−−−⋅⋅

1000)1(00)1(00)1(00

),,(),,(

100000

0000

1000100010001

1000100010001

1000000000000

1000100010001

),,(),,(

zfz

yfy

xfx

ffffff

fzz

fyy

fxx

f

f

f

f

f

f

z

y

x

f

f

f

ffffff

szssyssxs

zyxzyx

zssyssxss

zyx

zyx

ss

s

zyx

zyxzyx

TST

TST

Reflection

● Reflection over planes, lines or points

1 0 0 00 1 0 00 0 1 00 0 0 1

z

y

x

z

y

x

z

y

x

1 0 0 00 1 0 00 0 1 00 0 0 1

1 0 0 00 1 0 00 0 1 00 0 0 1

z

y

x

1 0 0 00 1 0 00 0 1 00 0 0 1

Shear

● Deform the shape depending on another dimension

SH z

1 0 a 00 1 b 00 0 1 00 0 0 1

x and y value depends on z value of the shape

SH x

1 0 0 0a 1 0 0b 0 1 00 0 0 1

y and z value depends on x value of the shape

OpenGL Geometric-Transformation Functions

● In the core OpenGL library,

– a separate function is available for each basic transformation (translate, rotate, scale)

– all transformations are specified in 3D

● Parameters

– Translation: translation amount in x, y, z axes

– Rotation: angle, orientation of the rotation axis that passes through the origin

– Scaling: scaling factors for three coordinates

Basic OpenGL Transformations

● glTranslate* (tx, ty, tz);

– For 2D applications set tz = 0

● glRotate* (theta, vx, vy, vz);

– theta in degrees

– The rotation axis is defined by the vector (vx,vy,vz), i.e., P0 = (0,0,0) P1 = (vx,vy,vz)

● glScale* (sx, sy, sz);

– Use negative values to get reflection transformation

OpenGL Matrix Operations

● glMatrixMode (GL_MODELVIEW);

– modelview mode to tell OpenGL that we will be specifying geometric transformations. The command simply says that the current matrix operations will be applied on the 4 by 4 modelview matrix.

– the other mode is the projection mode, which specifies the matrix that is used of projection transformations (i.e., how a scene is projected onto the screen)

– There are also color and texture modes that we will discuss later

OpenGL Matrix Operations

● Once you are in the modelview mode, a call to a transformation routine generates a matrix that is multiplied by the current matrix for that mode

● Whatever object defined is multiplied with the current matrix

● The contents of the current matrix can also be manipulated explicitly

where elements16 is a single subscripted array that specifies a matrix in column-major order

OpenGL Matrix Operations

● Example:

for (int k=0; k<16;k++)

elements16[k]=(float)k;

will produce the matrix

⎥⎥⎥⎥

⎢⎢⎢⎢

=

0.150.110.70.30.140.100.60.20.130.90.50.10.120.80.40.0

M

OpenGL Matrix composition

● glMultMatrix* (otherElements16)

– The current matrix is postmultiplied with the matrix specified in otherElements16

what does this imply?

MMM ′⋅= currcurr

In a sequence of transformation commands, the lastone specified in the code will be the first transformation to be applied.

OpenGL Matrix Stacks

● OpenGL maintains a matrix stack for all the four matrix modes

● When we apply geometric transformations using OpenGL functions, the 4 by 4 matrix at the top of the matrix stack is modified

● The top is also called the current matrix

● If we want to create multiple transformation sequences and save the composition results we can make use of the OpenGL matrix stack

OpenGL Matrix Stacks

● Initially, there is only the identity matrix in the stack

● To find out how many matrices are currently in the stack:– glGetIntegerv(GL_MODELVIEW_STACK_DEPTH,numMats)

● glPushMatrix ();

– The current matrix is copied and stored in the second stack position

● glPopMatrix ();

– Destroys the matrix at the top and the second matrix in the stack becomes the current matrix

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