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Head-Mounted Display. Sherman & Craig, pp. 151-159. Visually Coupled Systems . A system that integrates the natural visual and motor skills of an operator into the system he is controlling . Basic Components An immersive visual display (HMD, large screen projection (CAVE), dome projection)

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Head mounted display l.jpg

Head-Mounted Display

Sherman & Craig, pp. 151-159


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Visually Coupled Systems

A system that integrates the natural visual and motor skills of an operator into the system he is controlling.

Basic Components

  • An immersive visual display (HMD, large screen projection (CAVE), dome projection)

  • A means of tracking head and/or eye motion

  • A source of visual information that is dependent on the user's head/eye motion.


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Head-Mounted Displays

  • Optical System

  • Image Source (CRT or Flat Panel (LCD))

  • See–Through or Non–See–Through

  • Mounting Apparatus

    • What are some factors?

      • Eyeglasses

      • Weight

    • Solutions?

  • Earphones

  • Position Tracker


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Field of View

Monocular FOV is the angular subtense (usually expressed in degrees) of the displayed image as measured from the pupil of one eye.

Total FOV is the total angular size of the displayed image visible to both eyes.

Binocular(or stereoscopic) FOV refers to the part of the displayed image visible to both eyes.

FOV may be measured horizontally, vertically or diagonally.


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Focal Length & Diopter

Focal Length - The distance from the surface of a lens (or mirror) at which rays of light converge.

Diopter - The power of a lens is measured in diopters, where the number of diopters is equal to 1/(focal length of the lens measured in meters).


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Ocularity

Ocularity

  • Monocular - HMD image goes to only one eye.

  • Biocular - Same HMD image to both eyes.

  • Binocular (stereoscopic) - Different but matched images to each eye.


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IPD

Interpupillary Distance (IPD)

  • IPD is the horizontal distance between a user's eyes.

  • IPD is the distance between the two optical axes in a binocular view system.


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Vignetting and Eye Relief

Vignetting

  • The blocking or redirecting of light rays as they pass through the optical system.

    Eye Relief Distance

  • Distance from the last optical surface in the HMD optical system to the front surface of the eye.


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Cornea

Crystalline

Lens

Fovea

Optic

Nerve

Retina

Basic Eye


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The Eye

  • Accommodation - Term used to describe the altering of the curvature of the crystalline lens by means of the ciliary muscles. Expressed in diopters.

  • Retina - The sensory membrane that lines the back of the eye and receives the image formed by the lens of the eye.

  • Fovea - The part of the human retina that possesses the best spatial resolution or visual acuity.

Cornea

Crystalline

Lens

Fovea

Optic

Nerve

Retina


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Properties of the Eye

  • Approximate Field of View

    • 120 degrees vertical

    • 150 degrees horizontal (one eye)

    • 200 degrees horizontal (both eyes)

  • Acuity

    • 30 cycles per degree (20/20 Snellen acuity).


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Simple Formulas

  • Visual Resolution in Cycles per degree (Vres) = Number of pixels /2(FoV in degrees)

    Example: (1024 pixels per line)/(2*40 degrees) = Horizontal resolution of 12.8 cycles per degree

  • To convert to Snellen acuity (as in 20/xx)

    Vres = 600/xx (20/47)


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Optical System

  • Move image to a distance that can be easily accommodated by the eye.

  • Magnify the image


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Simple Magnifier HMD Design

q

p

f

Image

Eye

Eyepiece

(one or more lenses)

Display

(Image Source)

1/p + 1/q = 1/f where

p = object distance (distance from image source to eyepiece)

q = image distance (distance of image from the lens)

f = focal length of the lens


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Thin Lens Equation

1/p + 1/q = 1/f where

p = object distance (distance from image source to eyepiece)

q = image distance (distance of image from the lens)

f = focal length of the lens

Conventions:

  • If the incident light comes from the object, we say it is a real object, and define the distance from the lens to it as positive. Otherwise, it is virtual and the distance is negative.

  • If the emergent light goes toward the image, we say it is a real image, and define the distance from the lens to it as positive.

  • f = positive for a converging lens

  • A light ray through the center of the lens is undeflected.


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Virtual Image

Virtual

Image

Lens

Display


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Resolution

  • (low) 160 x 120 color pixels per eye

  • (high) 1000+ x 1000+

  • Note that resolution and FOV are independent

  • Another important factor: pixel density

    • Pixels per degree of FOV


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LEEP Optics

  • Large Expanse Extra Perspective

  • Give very wide field of view for stereoscopic images

  • Higher resolution (more pixels) in the middle of the field of view, lower resolution on the periphery

  • Pincushion distortion


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Fresnel Lens

  • A lens that has a surface consisting of a concentric series of simple lens sections so that a thin lens with a short focal length and large diameter is possible

  • More even resolution distribution

  • Less distortion

  • from lanternroom.com



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Distortion in LEEP Optics

A rectangle

Maps to this

How would you correct this?


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To correct for distortion

  • Must predistort image

  • This is a pixel-based distortion

  • Graphics rendering uses linear interpolation!

  • Too slow on most systems

  • Pixel shaders!

  • Render to Texture


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Distorted Field of View

  • Your computational model (computer graphics) assumes some field of view.

  • Scan converter may over or underscan, not all of your graphics image may appear on the screen.

  • Are the display screens aligned perpendicular to your optical axis?


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Distance along

z-axis

Distorted FoV (cont.)


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Collimated: p=f

  • 1/p + 1/q = 1/f q = , if p=f

  • If the image source is placed at the focal point of the lens, then the virtual image appears at optical infinity.

f


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Exit

Pupil

Intermediate

Real Image

Image

Relay Lens

Eyepiece

Compound Microscope HMD Design

Relay lens produces a real image of the display image source (screen) at some intermediate location in the optical train. The eyepiece is then used to produce an observable virtual image of this intermediate image.


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Exit

Pupil

Intermediate

Real Image

Image

Relay Lens

Eyepiece

Exit Pupil

  • The area in back of the optics from which the entire image can be seen. Important if IPD not adjustable.

  • Compound microscope optical systems have a real exit pupil.

  • Simple magnifier optical systems do not have an exit pupil.


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Virtual Research V8 HMD

  • Display

    • Dual 1.3” diagonal Active Matrix Liquid Crystal Displays

    • Resolution per eye: 640 x 480 (307,200 color elements)

    • focal length = 1m

  • Optical

    • Field of view: 60° diagonal

  • Solve

    • What is the cycles per degree?

    • What is its horizontal and vertical field of view?

  • Pros/Cons


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Characteristics of HMDs

  • Immersive

    • You are inside the computer world

    • Can interact with real world (mouse, keyboard, people)

    • Mask out real world

  • Ergonomics

    • Length of use is limited, cue conflict!!! (accomodation vs. parallax, perspective, etc.)

  • Resolution and field of view

  • Tethered


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Exercise (Part of Quiz grade)Due: October 20th (Thursday)

  • Fill in the following table through research on the Internet:


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