Custom PC - UK (2020-02)

f all the graphics settings
that confront you when
tweaking a graphics card
driver or setting up a new
game, perhaps the most common option

• after choosing the resolution – is
  whether you enable anti-aliasing (AA).
  It’s so commonplace, and it’s been
  around for so long, that the proverbial
  rock under which you’d have to be living
  in order to not be aware of AA would give
  the moon a run for its money.
  However, for all its ubiquity, many of us
  struggle to understand all the many ways
  in which it can work, and their impact on
  image quality and performance. So that’s
  what we’re here to find out.

Whatisanti-aliasing?

At its broadest definition, aliasing refers to

any situation where distortion is introduced

when a signal is reconstructed from

samples, making it fundamentally different

from the original signal. In audio processing,

the signal is the original sound, and the

sampling is the chopping up into time slices

that’s performed when converting sound

to a digital signal (and performing other

calculations such as compression).

In the case of computer graphics, the

signal is the original data that describes the

objects making up a scene or image, and the

sampling refers to compressing, resizing

or other manipulation of that scene that’s

needed to make it appear on screen. Most

obviously, this refers to the resolution at

which you’re trying to render a scene. You’re

samplingthescene/imageateachpixel.

Thisaliasingcanresultinseveral

undesirablevisualeffects,themostobvious

onebeingthedreadedjaggededges(jaggies),

whereyouhavea stepped,unnaturallooking

edgetothebordersofobjects.Thisisn’tjust

a problemin3Dgraphics– it’salsoa factor

ineverydaycomputing.Technologiessuch

asMicrosoftClearType,forinstance,provide

anti-aliasingfortext,ensuringthattextlooks

smoothandeasytoread,nomatterwhat

screen resolution and text size you’re using.

However, you can also get other visual

artefacts, such as the moiré (or ripple) effect

that can distort regular patterns, such as in

cobblestone roads or brick walls. There’s

also the effect where, as objects or patterns

get more distant from the viewpoint of the

screen, the pattern can break down in an

unnatural-looking way.

These effects all occur because the

sampling involved doesn’t have the

granularity to correctly display what the scene

is trying to convey. Going back to the example

of the jagged edges, if you consider a diagonal

line created by the edge of a brick wall against

a blue sky. For each pixel that intersects this

line, some of the pixels will be covered more

by the brick and others more by the sky.

With normal rendering, the system simply

checksforwhetherthebrickortheskycovers

thecentreofthepixel(itssamplepoint)and

thenshowsonlyoneofthetwocolours(the

brickorthe sky),creating a jaggedlooktothe

linewheretheyintersect.If we’rerenderingata

highenoughresolution,thejaggednessofthe

linebecomesimperceptible(althoughyoucan

oftenstillseea shimmeringpatternaspixels

flipsharplybetweenonecolourortheother)

butatrealisticreal-timerenderingresolutions,

thepixelsandjaggedlinesareeasilyvisible.

WE’VE ALL HEARD OF

IT, AND PROBABLY ALL

USED IT, BUT WHAT

EXACTLY IS ANTI

ALIASING AND WHAT’S

THE DIFFERENCE

BETWEEN ALL THE

VARIOUS TECHNIQUES?

EDWARD CHESTER

INVESTIGATES

Technologies such as Microsoft ClearType provide anti-aliasing

for text, ensuring that text looks smooth and easy to read

ALIASING REFERS TO ANY SITUATION WHERE

DISTORTION IS INTRODUCED WHEN A SIGNAL IS

RECONSTRUCTED FROM SAMPLES