9-culling-visibility.ppt

(1)

If you cannot see it, ignore it

• CULLING

– Do not render the primitives that are not visible

– Throw away the primitives that do not change

the scene

• Or with negligible impact

– Do it as soon as possible! (in the pipeline)

(2)

Notation

• _{We call it}

_culling

_{if we discard primitives}

– Or groups of primitives

• If a primitive is divided into a visible part and a not

visible one, we call it

clipping

• _{If we discard a fragment, this is}

_fragment

testing

(3)

Notation

• _{We call it}

_culling

_{if we discard primitives}

– Or groups of primitives

• If a primitive is divided into a visible part and a not

visible one, we call it

clipping

• _{If we discard a fragment, this is}

_{fragment testing}

(in most cases)

an

optimization

!

(in most cases)

(4)

Forms of Culling

• Occlusion

culling

– Not visible ... because it is covered

• View-frustum

culling

– Not visible... because it is outside the view frustum

• Backface

culling

– Not visible... because it is in the inside of a closed figure

• Importance

culling

(5)

Forms of Culling

• Executed in hardware

– automated

– Very efficient (very small overhead)

– But discard late and only one primitive at the time

• Executed by the application (SW)

– Require algorithms and data structures

– Less efficient (quite large overhead)

(6)

Forms of Culling

• Conservative

– Only discard when sure

– Sometimes they draw objects not visible

• Non Conservative

– Discrad according to an heuristics

– Sometimes they DO NOT draw a (partly) visible

object!

Less efficient but sound.

(7)

• We call artifact a rendering error,

– A discrepancy between the model and the image

– Or between reality and the model

– In most cases, it draws on the screen a

non-existant detail

parentesi notazione parentesi notazione parentesi notazione

(8)

Culling and Visibility

Computer Graphics Pagina 8

• We call artifact a rendering error,

– A discrepancy between the model and the image

– Or between reality and the model

– In most cases, it draws on the screen a non-existant

detail

parentesi notazione parentesi notazione parentesi notazione

(9)

Forms of Culling

• Occlusion

culling

– Not visible ... because it is covered

• View-frustum

culling

– Not visible... because it is outside the view frustum

• Backface

culling

– Not visible... because it is in the inside of a closed figure

• Importance

culling

(10)

Backface Culling

• Idea:

– We are modeling the surface of a

closed

object

...

– I cannot see the inside

(11)

(HW) Backface Culling: Where?

Final pixels

(12)

(HW) Backface Culling: How much?

Throw away approx 50% of the faces...

The speed-up?

x2

?

...only after this step!

Final pixels

(13)

(HW) Backface culling: when?

• I cannto use it always!

– Need to know

a priori

that the object is closed

primitives

here

OpenGL state

(14)

(HW) Backface culling: How?

• Turn it on and off:

glEnable( GL_CULL_FACE );

glDisable( GL_CULL_FACE );

• Deciding whether to discard the front

orback-facing faces:

glCullFace(GL_FRONT );

glCullFace(GL_BACK );

These commands only change

the state. “not the screen"

(15)

Metodologie di Culling

• _Occlusion

_culling

– Not visible ... because it is covered

• View-frustum

culling

– Not visible... because it is outside the view frustum

• Backface

culling

– Not visible... because it is in the inside of a closed figure

• Importance

culling

(16)

(HW) View-frustum culling

• _{Given a primitive}

– triangle, segment, point

• _{Is it inside the view frustum?}

– (or partly inside- conservative!)

y

x

• Better working in normalized

coordinates

(17)

(HW) View-frustum Culling: where?

...as usual, only from this step!

Final pixels

(18)

Software View-frustum Culling

• Idea: do it before pushing the triangles in the

rendering pipeline

• Divide the scene into blocks

– Most scenes are naturally organized in blocks

(19)

Forms of Culling

• Occlusion

culling

– Not visible ... because it is covered

• View-frustum

culling

– Not visible... because it is outside the view frustum

• Backface

culling

– Not visible... because it is in the inside of a closed figure

• Importance

culling

(20)

Occlusion Culling

• Very important form of culling

• Can discard lots of primitives

– Mostly the primitives composing the

– environment

• Where can we apply it ?

Final pixels

(21)

Hidden surfaces removal

• Closer objects must cover thode more far

away

(22)

Hidden surfaces removal

• _{Different from}

_{occlusion culling}

– Can only remove whole triangles (or groups of

triangles)

– It is only an optimization

• Back-face culling is not enough

– example:

non back-facing

surfaces, not totally,

(23)

Hidden surfaces removal

• _{Solution 1:}

– Split any triangle into a visible and an occluded

part?

• By performing a clipping

How to decide when a triangle occludes another one?

One triangle is clipped and what about the other?

(24)

Hidden surfaces removal

• Solution 2:

– Draw only whole triangles, but in the

right order

(25)

Painter’s algorithm (depth sorting)

• Given a scene (composed of primitives)

– Sort the primitives by depth

• From the farthest away to the closest one to the viewpoint

– Draw them all (according to the ordering)

(26)

Painter’s algorithm (depth sorting)

Where?

y

v

0

v

1

_v

0

v

1

Before feeding the triangles.

Executed by the CPU ! 

Final pixels

(27)

Painter’s algorithm (depth sorting)

• Given a scene (composed of primitives)

– Sort the primitives by depth

• From the farthest away to the closest one to the viewpoint

– Draw them all (according to the ordering)

Sorting

!

Pseudolinear

complexity wrt the

number of primitives.



_{   }

The primitive has more than

one depth value!

(28)

Painter’s algorithm (depth sorting)

• _{Should we draw C or D?}

• Does there exist a

“right order"?

D

ep

th

z

min

Primitives

B

z

max

A

E

_D

C

(29)

Painter’s algorithm (depth sorting)

• The primitive A can be drawn first

• Should we draw C or D?

• _{Does there exist a “right order"?}

– NO!

• intersections

(30)

Painter’s algorithm (depth sorting)

• Computationally expensive

– sorting: (n log (n))

– Made in software!

• Poorly integrated in the HW schema

– Works in eye coordinates

...but before the HW projection ?!

– Needs to work globally on the scene, not parallelizable

• Difficult to test

– When the intervals overlap

• It also uses clipping

(31)

Painter’s algorithm (depth sorting)

• Only used when the primitives are already

sorted

– During the preprocessing phase

• _{Example: ground represented as an}

(32)

Hidden surfaces removal

• Solution 3:

– Remove the hidden surfaces with a

FRAGMENT TEST

– "

Z-buffer

“ algorithm

(33)

z-buffer

Algorithm

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Depth

buffer

RGB values

(34)

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z-buffer

Algorithm

Transform.

Add the

resulting z

As an

attribute

Interpolate

s

the z

(likewise

other

attributes)

depth test

Fragment with screen coordinates

(x,y)

and interpolated z:

if ( z <= DepthBuffer[x,y] ) {

WriteFragment

(35)

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rasterizer

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rasterizer

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_buffer

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buffer

z-buffer

Algorithm

(36)

z-buffer

Algorithm

:

example

64 64 64 64 64 64 64 64

64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64

5 5

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5 5 5 5 5 5 5 64 5 5 5 5 5 5 64 64 5 5 5 5 5 64 64 64 5 5 5 5 64 64 64 64 5 5 5 64 64 64 64 64 5 5 64 64 64 64 64 64 5 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64

+

=

5 5 5 5 5 5 5 64 5 5 5 5 5 5 64 64 5 5 5 5 5 64 64 64 5 5 5 5 64 64 64 64 5 5 5 64 64 64 64 64 5 5 64 64 64 64 64 64 5 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64

7

2 7

5 5 5 5 5 5 5 64 5 5 5 5 5 5 64 64 5 5 5 5 5 64 64 64 5 5 5 5 64 64 64 64 4 5 5 7 64 64 64 64 3 4 5 6 7 64 64 64 2 3 4 5 6 7 64 64 64 64 64 64 64 64 64 64

+

=

5 5 5 5 5 5 5 5 5 5 5 5 5

(37)

z-buffer

Algorithm

:

properties

• Does not depend on the order of the primitives



• Works in all cases

– even:

• Fragment Operation

– Is executed in the

“fragment computations"

• Well suited for a HW implementation

5 5 5 5 5 5 5 64

(38)

z-buffer

Algorithm

:

problems

• Needs memory

– depth buffer = at least half of screen buffer

• Reads and writes shared data

– (the HW implementation should take care)

• Problems with

aliasing

– When precision is insufficient

– As an example, if we render two surfaces that

are parallel and very close

• Not suitable for transparent surfaces