Graphics14-SurfaceDetectionMethods

8/18/2019 Graphics14-SurfaceDetectionMethods

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Course Website: http://www.comp.dit.ie/bmacnamee

Computer Graphics:Surface Detection Methods

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28Contents

Today we wi start to ta!e a oo! at "isibesurface detection techni#ues:

– Why surface detection$

–

%ac! face detection – Depth&buffer method

– '&buffer method

–

Scan&ine method


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28Why$

We must determine what is "isibe within ascene from a chosen "iewin) position

*or (D words this is !nown as visible

surface detection or hidden surfaceelimination


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28Two Main 'pproaches

,isibe surface detection a)orithms arebroady cassified as:

– Object Space Methods: Compares ob-ects

and parts of ob-ects to each other within thescene definition to determine which surfaces

are "isibe

– Image Space Methods: ,isibiity is decided

point&by&point at each pie position on thepro-ection pane

ma)e space methods are by far the more

common


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Object-space methods (Continuous):

Implemented in WCS considering the geometrical relationships

beteen the actual objects!It compares the objects " parts of

objects to each other ithin the scene definition to determine

hich surfaces as hole should e label as visible!

1Image Space methods (discrete):

Implemented in screen co-ordinate s#stem considering the

relationships b$ images of objects in their final configuration at

pi%el level!&isibilit# is decided point b# point at each pi%el positionon the projection plane!


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283%45CT S6'C5 '7G3T9MS

•3b-ect space a)orithms do their wor! on the ob-ectsthemse"es before they are con"erted to pies in the

frame buffer. The resoution of the dispay de"ice is

irree"ant here as this cacuation is done at the

mathematica e"e of the ob-ects

• for each object a in the scene determine which parts of object a are

visible (involves comparing the polyons in object a to other polygons

in a and to polygons in every other object in the scene)


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•Image space a)orithms do their wor! as the ob-ects arebein) con"erted to pies in the frame buffer. The

resoution of the dispay de"ice is important here as this

is done on a pie by pie basis.

•

for each pixel in the frame buffer determine which polygonis closest to the viewer at that pixel location colour the pixel

with the colour of that polygon at that location


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• There are aso times when we may notwant to cu out poy)ons that are behind

other poy)ons. f the frontmost poy)on is

transparent then we want to be abe toAsee throu)hA it to the poy)ons that are

behind it as shown beow:


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Which ob-ects are transparent in the

abo"e scene$


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more detaiE chec!s woud then be made on the ob-ect in

the bo.

• There are many ways to define the boundin) bo. The

simpest way is to ta!e the minimum and maimum E FE

and ? "aues to create a bo. Fou can aso ha"e

boundin) boes that rotate with the ob-ectE boundin)

spheresE boundin) cyindersE etc.


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28%ac!&*ace Detection

The simpest thin) we can do is find thefaces on the bac!s of poyhedra and discard

them


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28%ac!&*ace Detection =cont>

We !now from before that a point = x, y, z > isbehind a poy)on surface if:

where A, B, C H D are the pane parametersfor the surface

This can actuay be made e"en easier if we

or)anise thin)s to suit ourse"es

0


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5nsure we ha"e a ri)ht handed system withthe "iewin) direction aon) the ne)ati"e z &ais


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n )enera bac!&face detection can beepected to eiminate about haf of the

poy)on surfaces in a scene from further

"isibiity testsMore compicated surfaces

thou)h scupper us

We need better techni#uesto hande these !ind of

situations


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28Depth&%uffer Method

Compares surface depth "aues throu)houta scene for each pie position on the

pro-ection pane

Nsuay appied to scenes ony containin)poy)ons

's depth "aues can be computed easiyE

this tends to be "ery fast 'so often caed the J&buffer method


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28Depth&%uffer Method =cont>

/ m a ) e s t a ! e n f r o m 9 e a r n H % a ! e r E K C o m p u t e r G r a p h i c

s w i t h 3 p e n G 7 L = 2 + >


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28Depth&%uffer ')orithm

B. nitiaise the depth buffer and frame bufferso that for a buffer positions = x, y>

depth%uff=E y> O B.

frame%uff=E y> O b)Coour


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28Depth&%uffer ')orithm =cont>

2. 6rocess each poy)on in a sceneE one ata time

– *or each pro-ected = x, y> pie position of a

poy)onE cacuate the depth z =if not aready!nown>

– f J P depth%uff=E y>E compute the surface

coour at that position and set

depth%uff=E y> O J

frame%uff=E y> O surfCoour=E y>

'fter a surfaces are processed depth%uffand frame%uff wi store correct "aues


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28C'7CN7'T3< 3* ?


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28Cacuatin) Depth

't any surface position the depth iscacuated from the pane e#uation as:

*or any scan ine ad-acent x positions differ

by QBE as do ad-acent y positions

C

D By Ax z

−−−=

C

D By x A z

−−+−=

)1('

C

A z z −='


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28terati"e Cacuations

The depth&buffer a)orithm proceeds bystartin) at the top "erte of the poy)on

Then we recursi"ey cacuate the x&

coordinate "aues down a eft ed)e of thepoy)on

The x "aue for the be)innin) position on

each scan ine can be cacuated from thepre"ious one

m x x 1

' −= where m is the sope


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28terati"e Cacuations =cont>

Depth "aues aon) the ed)e bein)considered are cacuated usin)

C

Bm

A

z z

+

−='


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28terati"e Cacuations =cont>

top scan ine

bottom scan ine

y scan ine

y & B scan ine

x x’


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28 '&%uffer Method

The '&buffer method is an etension of thedepth&buffer method

The '&buffer method is "isibiity detection

method de"eoped at 7ucasfim Studios forthe renderin) system 5F5S =0enders

."erythin) 1ou ."er Saw>


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28 '&%uffer Method =cont>

The '&buffer epands on the depth buffermethod to aow transparencies

The !ey data structure in the '&buffer is the

accumulation buffer

(+


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f depth is RO E then the surface data fied

stores the depth of that pie position as

before

f depth P then the data fied stores a

pointer to a in!ed ist of surface data

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Surface information in the '&buffer incudes: – G% intensity components – 3pacity parameter – Depth

– 6ercent of area co"era)e

– Surface identifier

– 3ther surface renderin) parameters

The a)orithm proceeds -ust i!e the depthbuffer a)orithm

The depth and opacity "aues are used to

determine the fina coour of a pie

(


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28Scan&7ine Method

'n ima)e space method for identifyin)"isibe surfaces

Computes and compares depth "aues

aon) the "arious scan&ines for a scene

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28Scan&7ine Method =cont>

To faciitate the search for surfaces crossin)a )i"en scan&ine an acti"e ist of ed)es is

formed for each scan&ine as it is processed

The acti"e ist stores ony those ed)es thatcross the scan&ine in order of increasin)

'so a fa) is set for each surface to indicate

whether a position aon) a scan&ine is eitherinside or outside the surface

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28Scan&7ine Method

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Two important tabes are maintained: – The ed)e tabe

– The surface facet tabe

The ed)e tabe contains: – Coordinate end points of each ine in the

scene

– The in"erse sope of each ine

– 6ointers into the surface facet tabe to

connect ed)es to surfaces

00

S = >


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The surface facet tabes contains: – The pane coefficients

– Surface materia properties

–

3ther surface data – Maybe pointers into the ed)e tabe

0

S 7i M h d = >


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6ie positions across each scan&ine areprocessed from eft to ri)ht

't the eft intersection with a surface the

surface fa) is turned on 't the ri)ht intersection point the fa) is

turned off

We ony need to perform depth cacuationswhen more than one surface has its fa)

turned on at a certain scan&ine position

0

S 7i M th d 5


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28Scan 7ine Method 5ampe

/ m

a ) e s t a ! e n f r o m 9 e a r n H

% a ! e r E K C o m p u t e r G r a p h i c s w i t h 3 p e n G 7 L = 2 + >

08

S 7i M th d 7i it ti


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28Scan&7ine Method 7imitations

The scan&ine method runs into troube whensurfaces cut throu)h each other or

otherwise cycicay o"erap

Such surfaces need to be di"ided

/ m

a ) e s t a ! e n f r o m 9 e a r n H

% a ! e r E K C o m p u t e r G r a p h i c s w i t h 3 p e n G 7 L = 2 + >

0@

W'


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28Summary

We need to ma!e sure that we ony draw"isibe surfaces when renderin) scenes

There are a number of techni#ues for doin)this such as

– %ac! face detection

– Depth&buffer method

– '&buffer method

–

Scan&ine method


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Graphics14-SurfaceDetectionMethods

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