Resolution 1 - Basics - Christie · Resolution 1 - Basics Gord Harris, Program Manager, ... Explain the basics of image resolution in simulators. 2. ... Laser Theodolite

Resolution 1 - Basics

Gord Harris, Program Manager, VisSim R&D, Christie

SIM University 2014

Learning Objectives

• By the end of the session you should be able to: 1. Explain the basics of image resolution in simulators. 2. Recognize some factors and tradeoffs affecting dynamic

and static resolution. 3. Identify a few simple tools and tips for measuring and

specifying resolution in requirements.

Optional SIM University 2014

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PART 1 - BASICS OF IMAGE RESOLUTION

Big idea: derive angle from distances


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Part 1 – What is resolution?

• What does “resolve” or “resolution” imply to you? • Typically it comes down to

distinguishing some kind of difference

• Differences are some kind of noticeable steps or changes you can discriminate

• For example, if we can only distinguish between two steps our discrimination is very coarse


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“heavy” or “light”?

Example – cutting a pie into equal slices


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The resolution of our pie cutting increases as the number of slices goes up. The size of slices goes down.

360◦ 180◦ 120◦ 90◦

Resolution – steps or differences you can distinguish in some quantity


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Lower Higher

Resolution in distance – simple 6” rule limits.


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Two lengths at right angles define an angle in degrees. From latin de=“down” + gradus = “step or grade”


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http://wiki.answers.com/Q/What_is_the_origin_of_the_word_degree#slideshow_catimages=callout_0.0 and http://www.edenics.net/english-word-origins.aspx?word=DEGREE

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D

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http://wiki.answers.com/Q/What_is_the_origin_of_the_word_degree

http://wiki.answers.com/Q/What_is_the_origin_of_the_word_degree

http://www.edenics.net/english-word-origins.aspx?word=DEGREE

Why is a circle divided into 360 “degrees”?


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http://mathforum.org/library/drmath/view/59075.html

• Babylonians used base 60 number system

• Perimeter of hexagon = 6 times radius r

• 6x60 = 360◦ • “convenient” to divide into

360 units called degrees • 360◦ also divides integrally

by many factors: 2,3,4,5,6,8,9,10,12,15 etc

60

60

60

r

r

r

r

r

r

r



Resolution is hard to specify and can have many meanings. For our purposes it is “how close objects can be while still being distinguishable”.


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Source: Wikipedia

Spatial resolution allows two point sources like double stars to be distinguished from one.


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Resolution starts with our eye’s capabilities.

Eye resolution is not constant – it varies over the retina widely: 6 million cones, 100 million rods.

Lens resolution ~ 1 arcmin. for wavelength 550nm, pupil diameter 2mm, focal length 20mm

Our eye can resolve around 1/60th of a degree.

1 minute of arc

Remember one Arc Minute is 1/60th of a degree.

• A unit of angular measure equal to 60 arc seconds, or 1/60 of a degree

• The arc minute is commonly denoted ‘ or AM

• Not to confused with the symbol for feet


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http://www.wolframalpha.com/input/?i=arc+minute

http://www.wolframalpha.com/input/?i=arc+minute

Snellen defined “standard vision” as the ability to recognize a letter when it subtended 5 minutes of arc. 20/20 is the smallest line a person with normal acuity can read at 20 feet.

2 AM/OLP

1 Optical Line Pair (OLP)

Snellen Chart

20/20 image on retina

• Our Cone spacing determines resolution limits

• Normal vision resolves a letter E twenty feet away that is 8.9mm high


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Some pilots have better than 20/20 vision!


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www. Wolframalpha.com

http://www.wolframalpha.com/

Height equation: Tan ϴ = Height/Distance


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D

H

Tan ϴ = H/D so ϴ= arctan(H/D)

ϴ

Inverse trig function

Practical resolution considerations - DORI • Johnson’s Criteria for minimum

dimension of a military target, per dimension: • Detect: 1.0 lp(line pairs) • Orient: 1.4 lp • Aim: 2.5 lp • Recognize: 4.0 lp • Identify: 6-8 lp • Recognize 50% accuracy: 7.5

lp/height • Recognize 90% accuracy: 12

lp/height


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Source: W. Smith: Modern Optical Engineering, pg 376

Example calculation – what system resolution is needed to clearly identify a 11 meter ship at 3km?

1. Calculate the angle ϴ subtended by the ship at this distance: • Tan ϴ = opposite/adjacent = Height/Range = 11m/3000m = 0.0037 • So arctan (0.0037) = 0.2101 degrees x 60 arcmin/deg = 12.605 AM

2. Now using Johnson criteria compute the system resolution: • 12.605 AM/ 6.4 OLP = 1.9695 AM/OLP • So we will need about 2 AM/OLP or eye limited resolution at about 10%

contrast modulation in our simulator to do this task!


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Remember: Different thresholds of resolution may be req’d for different tasks, various targets.

• We can’t tell you what you need for your training task…

• You should clearly specify resolution in arcmin/OLP similar to above


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Usually we apply some kind of “Kell Factor” or degradation fudge factor around 0.8 to allow for other losses and calculate ave. system resolution.


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Spatial Frequency -> % M

odul

atio

n

0

100

JND Limiting resolution

Modulation Transfer Function (MTF)


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Limiting resolution


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MTF is more informative than a single “resolution” spec, but is much harder to measure.


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USAF 1951 Resolution Chart is very useful.


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Practical resolution considerations - text

• Photography of Text (eg: lowercase e): • Excellent: 8 lp (line pairs) per letter height • Legible: 5 lp/h • Decipherable: 3 lp/h

• Point sizes P are: • Upper case: 0.22P mm • Lower case: 0.15P mm


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Source: W. Smith: Modern Optical Engineering, pg 376

26 px or 13 lp

The bigger the area under the MTF curve, the better – sometimes called “MTFA”.


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The final performance is limited by many factors – the MTF’s multiply together…


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DMD

Lens

Screen etc

IG & Database

Limiting Resolution

Once we know what you want in AM/OLP we can design the best system to optimize MTFA & resolution.


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Class Exercise 1 – paper handout – 10 minutes


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PART 2 – FACTORS AND TRADEOFFS AFFECTING RESOLUTION

Big idea: Work hard to minimize losses through the image chain


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Major Factors affecting perceived Resolution

• Display Resolution • Number of Projectors • Projector Addressable

Resolution • Projector ANSI Contrast • Bit depth (bits/colour) &

Gamma • Projector/Lens Mount Stability • Lens MTF & aberrations • Lens Depth of Field & Focus • Lens Scheimpflug Adjust

• System Resolution • Field of View from DEP • Magnification • Screen curvature • Relative Motion • Update rate • On time (smear reduction) • IG antialiasing • Data Base resolution • Warping & blending • Temperature Stability


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Our biggest decision is number of projectors which determines blending & overall # pixels.

(8) WU EGG – 18 Mpixels (4) WU DualView – 9 Mpixels


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source: Wikipedia

Addressable resolution – you might want more pixels/device but more is not always better.

• Depth of Field on curved screens is worse for big formats

• Tiling smaller frustrums is more flexible

• Harder to drive 4K then 2K & blurs faster with motion

Less is more? Not always when it comes to number of channels and projectors…

Single large projector limits DOF and placement

Multiple small projectors have better focus & flexibility

Wide aspect ratio 4K projectors hit Depth of Focus limits faster than squarer aspect ratios.

(S. Black, Fundamentals of Display Systems for Visual Simulations, IMAGE 2003)

The greater the contrast between projected white and black images, the better the modulation, MTF and perceived resolution.

Contrast affects perceived resolution. More is better generally up to a limit of ~ 1000:1 perceived.

Sturdy projector and lens mounts are essential to not lose resolution to vibration or backlash especially on motion base platforms.

Rugged StIM mount (3G) Stable 3DOF aimable mount

Lens quality in general affects resolution


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• Optics close to diffraction limit are ideal

• Image of a star with great mirror or lens gives an Airy pattern for point spread function

• Lenses should resolve single pixel edges clearly

Lenses with low spherical aberrations are good.


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Sharp optics should clearly resolve pixel edges and DMD dimple if critically focused.

Lens Scheimpflug adjustment (tilt or “boresight”) is required if projector not perpendicular and image not tangent to screen.

• Geometric Modeling by Hesham Khairy.

Resolution depends on accurate focus and Scheimpflug adjustment (“Boresight”)

IMAX® Camera Hres = 69.6mm x 70cycles/mm x 2 pixels/cycle = 9.74 Kpixels across!

~10,000

~7,000

~ 70 Mpixels/frame!

Tradeoff 1 – Cost: Wide fields of view like IMAX require ~70Mpixels which costs a lot.

10,800 arc minutes wide/ 2AM/OLP = 5400 OLP = 10,800 pixels wide

This is > 42 HD projectors for eye limited res!

Tradeoff 2 – FOV. Wide FOV (Field of View) is still costly to achieve & narrower is cheaper.

• Tip: establish desired viewing distance first, then FOV, then choose no more pixels than eye limit

• (if you can afford it!)

Tradeoff 3 – Resolution & contrast are degraded by RP screens.

Thermal Shock & even gradual temperature changes can affect focus and hence resolution.


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Dynamic Resolution – fast motion degrades sharpness

• Commonly known as “smearing” or “motion blur”

• Interdependence between spatial/temporal resolution

• Interaction between eye/scene movement, blur, resolution, aliasing

• Improvements in static resolution will highlight the issues with ‘dynamic resolution

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Source: Dave Kanahele

Unlike real objects, some motion blur is unavoidable with frame based displays @ 60 fps

Dynamic Resolution in Level D simulators – use moving bars at 10 deg/sec.


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Motion-Induced Blur 6 amin Resolution, 60 Hz

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Source: Dr Barbara Sweet, 2012

Motion-Induced Blur

6 amin Resolution, 120 Hz

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Quality of warping and blending affects resolution in seam areas – AutoCal 3.0 helps!


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Resolution Error Budget • Upper limit set by Display Resolution of projector • Minus losses from IG database, antialiasing, motion smear • Minus losses from shake, vibration, poor mounts etc • Minus losses from lens MTF • Minus errors from limited depth of field at near/far screen limits • Minus MTF losses from screen itself (diffusion, specularity etc) • Minus losses in blend zones (overlaps, filter MTF etc)

Paper Class Exercise 2 - Measuring dynamic resolution - Play 772 video


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C:\Users\Gharris\Documents\2013 GH CDS Documents\Videos 2013\Sim Tools 2013\Video 2 - EGG - P1140772.MOV

PART 3 – TOOLS & TIPS FOR MEASURING RESOLUTION

Big idea: Even a ruler, stopwatch and some trig can get you answers


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Typical static measurement instruments Laser Theodolite Luminance meter Digital camera Tape measures Plumb bob & laser level Test patterns, generators

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http://upload.wikimedia.org/wikipedia/commons/2/2e/Sokkia_1.JPG



Tools and tips for measuring resolution

If you have ability to program your own IG database: 1. Generate your own large white bars separated by black bars (for

example maybe 1 meter wide by 10 meters high. Say ~11 white bars separated by 10 black bars maybe)

2. Drive or fly IG eyepoint to a distance in meters where you observe a JND from proper DEP in simulator (just barely count cycles)

3. Calculate the angular resolution via the height/distance equation and assume that modulation to be ~10-20%

4. Translate into arc minutes per optical line pair (AM/OLP) as your limiting resolution


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Source: Werner Kraemer, private communication

Step 1 – generate Sample test geometry with 1m x 10m bars.


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Step 2 – fly out to where you can just barely resolve bars and measure distance from DEP


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Step 3 – Calculate angle ϴ from height equation

• Say one bar pair = H = 2m = 1 OLP

• Say you can just barely resolve these from D = 200m (say out of focus)

• Then tan ϴ = H/D • = 2/200 = 0.0100 • So ϴ =arctan(0.0100)

= 0.5729 degrees


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Step 4 – translate into arc min/OLP

1. Calculate the angle ϴ in AM subtended by one OLP at this distance: • One degree = 60 AM (Arc Minutes) • Therefore 0.5729 degrees = 0.5729 deg x 60 AM/deg =34.3763 AM

2. So our measured system resolution in the simulator is: • 34.4 AM/ 1 OLP = 34 AM/OLP • This is really bad compared to eye limit of 2 AM/OLP so you

would have to go back and figure out what was wrong (eg focus)


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Summary – Key ideas to remember 1. Please specify limiting resolution in

AM/OLP (Arc Minutes per Optical Line Pair) needed for your task.

2. Temporal Resolution (Dynamic Resolution - Update rate and smear reduction) are just as important as static resolution.

3. Height equation: Tan ϴ = Height/Range may be used to convert distances into angles and vice versa to calculate Arc Minutes.

4. Minimize MTF losses at every stage of simulator.

SIM University 2014

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Reference 1 – practical info on real optics tests


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Reference 2 – using HVS to inform design


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Reference 3 – great overall book on displays! Hainich/Bimber, 2011 www.displaybooks.info


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http://www.displaybooks.info/

Reference 4 – more advanced MTF testing


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Reference 5 – for the real optical keeners (big) Smith, Warren. Modern Optical Engineering, McGraw Hill, 2000


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Other Sources/References 6. Hubel, David. Eye, Brain and Vision, Scientific American, 1988 7. Howard, IP. Seeing in Depth: Depth perception. Toronto: Porteous , 2002 8. Keller, Peter, Electronic Display Measurement. Wiley SID, 1997 9. Maeda, John. The Laws of Simplicity, MIT 2006 10. Preim, Berhard, Visualization in Medicine , Morgan Kaufmann Elsevier 2007. 11. Duree, Galen, Optics for Dummies, Wiley, 2011.

Thank you.

Questions? <[email protected]>