Motion blur : an explanation to subjective quality differences between CRT & LCD ? VQEG Boston meeting April 2006 S. Tourancheau, S. Péchard, P. Le Callet,

Motion blur : an explanation to subjective quality differences between CRT & LCD ?

VQEG Boston meeting April 2006

S. Tourancheau, S. Péchard, P. Le Callet, M. Carnec, A. Tirel, D. Barba

Motion blur : an explanation to subjective quality differencies between CRT & LCD ?

- Uncompressed HDTV (1080 50i)

- 7 BITRATES H.264 (JM reference encoder) main profile

- SAMVIQ Methodology (Random Access, explicit reference & hidden reference)

- Room: Rec. ITU BT 500-10

- Observation distance : 3 H

MOBCAL PARKRUN

SHIELDS STOCKHOLM

4 CONTENTS from SVT (10s)

2.25 - 2.5 - 3.15 - 4 - 5 – 7 - 10 8 - 12 - 16 - 18 - 20 - 24 - 28

2.25 - 3 - 4 - 5 - 6 - 7 - 8 1.63 - 1.88 - 2.25 - 3 - 3.6 - 4 - 6

TEST SPECIFICATIONS


CRT

Studio Monitor JVC DTV1910CG 19’’

Interlaced scan

Picture height : 20 cm

DISPLAYS

LCDPhilips (prototype) 37’’ (native resolution 1920x1080)

Progressive scan

NO POST-PROCESSINGS

1 test per display with different observers (naïve)

15 valid observers for each test


VIDEO RENDERING CHAIN

SDI :1920x1080 50iYUV 4:2:2 8bits

DVI-D :1920x1080 50pRGB 8bits

HD PLAYERDOREMI V1-UHD

- real-time playback of uncompressed HDTV

SDI/DVI converterDOREMI HDVI-20d

- de-interlace - convert color spaces- top of the art quality conv

CRT DISPLAY

LCD DISPLAY

Full digital video rendering chain No lossy scaling from source to display



TEST RESULTS

Correlation between CRT and LCD MOS : 0,938


HDTV

CONTENT

MOBCAL PARKRUN SHIELDS STOCKHOLM

MOS

(CRT-LCD)

14.56 11.68 15.99 10.28

TEST RESULTS

Mean IC on CRT 5.34Mean IC on LCD 6.15


• Motion blur is induced by the type of response of LC display which is different from CRT’s one:

– Hold type display : LCD emitted light is sustained all the frame period (b) when CRT’s one consists in pulses (a) ;

– Response time : LCD response time is long with regards to classic frame rate when CRT’s one can be considered almost instantaneous.

MOTION BLUR


• ISO 13406-2 : response time is the sum of rise time (from black to white) and fall time (from white to black).

• Manufacturers specify this response time or even only the rise time from black to white.

• But gray-to-gray transitions are very slowest.

• For the majority of displays sold before 2005, the average rise time (over gray-to-gray transitions) was superior to the frame period which leaded to annoying motion artifacts.

LCD RESPONSE TIME


• Results (rise + fall times) :• Overdrive (OD) :

RESPONSE TIME COMPENSATION

without overdrive

with overdrive

(Xbit Labs, december 2005)


• With overdrive, the rise time is less than the frame period

… But even if response time was instantaneous, motion blur artifacts would be present

• LCD are therefore called « hold type » displays : emitted light is sustained on the screen during all the frame period …

=> when tracking

the eyes anticipate the object motion while the image is sustained. Edges of the object are then spatially integrated on the retina during the whole frame period, resulting in a blur.

HOLD TYPE DISPLAY


CRT

LCD

t = 0

MOTION BLUR PERCEPTION

smooth pursuit eye movement

eye

image on the retina

eye

image on the retina



t = T / 4image on the retina

image on the retina

CRT

LCD


eye

eye


CRT

LCD


t = T / 2


eye

image on the retina

eye

image on the retina


CRT

LCD


t = 3T / 4


eye

image on the retina

eye

image on the retina


CRT

LCD


t = T


eye

image on the retina

eye

image on the retina


CRT

LCD


t = T + T / 4


eye

image on the retina

eye

image on the retina


CRT

LCD


t = T + T / 2


eye

image on the retina

eye

image on the retina


CRT

LCD


t = T + 3T / 4


eye

image on the retina

eye

image on the retina


CRT

LCD


t = 2T


eye

image on the retina

eye

image on the retina


CRT

LCD

t = 2T + T / 4


eye

image on the retina

eye

image on the retina



CRT

LCD


t = 2T + T / 2


eye

image on the retina

eye

image on the retina


CRT

LCD


t = 2T + 3T / 4


eye

image on the retina

eye

image on the retina


• We set an experiment in order to measure the blur width of a moving edge, changing its velocity.

• The stimuli used was a periodic structure of vertical (resp. horizontal) bars moving from left to right (resp. from down to up) on a black background.

• Displayed and perceived stimuli :

BLUR WIDTH PERCEPTION


• The scrolling of the bars was continuous.• Using the arrow keys of a keyboard, the observer can increase or

decrease, in real-time, the space between the bars. • He can operate as many times as he wants, until he considers that

the two blurred areas are just merging. When he is satisfied, he validates his measure and then the next presentation is displayed.

EXPERIMENT : TASK


• 7 observers :– all of them were familiar with the procedure and have a perfectly

corrected sight.

• Protocol :– 1 session consists in a set of 17 presentations, concerning four types of

stimuli (vertical/horizontal and white/red) for different velocities,

– the sequence is totally random,

– each observers repeated the test twice, on a different day

– average lenght of a session was between 10 and 15 minutes.

EXPERIMENT : PROTOCOL


• Mean Blur Width

• Curve fitting give us the following model : avTw

aVw

047.1a

RESULTS

with

where :- w is the blur width in pixels- v is the velocity in pel / seconds- T is the frame period- V = vT is the velocity in pel / frames


• Results and model for a white stimulus moving with an horizontal movement :

RESULTS AND MODEL


• Pan, H., Feng, X.-F. et Daly, S. (2005). LCD motion blur modeling and analysis. IEEE International Conference on Image Processing, II:21–24.

• Mathematical model to predict the blur width in function of velocity and in which the temporal response of LCD is a parameter.

• They studied for examples two types of response, linear and sinusoidal one, the model give the following results :

• As we can see, the sinusoidal response gives model wich are very close to our results. ( 1.047(vT) )

)(1.1 vTwL )(044.1 vTws

PAN’s MODEL


• When a ideal instant temporal response (a) of LCD is used, a blur width of 0.8vT is obtained, only 25% motion blur is caused by slow response on a standard LCD panel.

• Hold-type is responsible for 75% of motion blur. • In the same way, reduction methods can be evaluated by modifying

the temporal response of LCD :– Black data insertion (b)

– Back light flashing (c)

– Frame rate doubling method and motion compensated inverse filtering

PAN’s MODEL





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Motion blur : an explanation to subjective quality differences between CRT & LCD ? VQEG Boston meeting April 2006 S. Tourancheau, S. Péchard, P. Le Callet,