The human visual system - uni-bielefeld.de · Realtime 3D Computer Graphics / V irtual Reality Ð WS 2005/2006 Ð Marc Erich Latoschik Simulating visual stimuli 3D CG rendering provides:

Realtime 3D Computer GraphicsVirtual Reality

Human Visual Perception

Realtime 3D Computer Graphics / Virtual Reality – WS 2005/2006 – Marc Erich Latoschik

The human visual system

• 2 eyes

• Optic nerve: 1.5 million fibers per eye (each fiber is the axon from a neuron)

• 125 million rods (achromatic low light sight)• most outside the fovea

• 6 million cones (high detail color sight)• most concentrated inside fovea

• 3 types: R, G, B sensitive to long, middle and short wavelengths (!) respectively

• Approximately 100:1 compression from the number of receptors to the number of fibers in the optic nerve.

• Quick transmission rate through the optic nerve

• Monocular visual field is 160° (w) x 135° (h)

• Binocular visual field is 200° (w) x 135° (h)

• Processing is not uniform across the visual field: • 25% of cortex is devoted to the central

5° of the field of view.

! (nm)

|350

|780Light

Radio Heat


Human visual perception• Human visual perception processes

• position/orientation and movement in 3 dimensions plus

• color.

• The third dimension depth is processed based on several physiological and psychological depth cues.

• Depth cues1 can be binocular or monocular.

Binocular depth cues:• Convergence

• Difference in the direction of the eyes.

• Our eyes point slightly inward for closer objects.

• Only effective on short distances (< 10 meters).

• Binocular Parallax

• Difference in the sensed images by our two eyes.

• Our eyes see the world from slightly different locations;

! images sensed are slightly different.

• Human visual system is very sensitive to these differences;

! most important depth cue for medium viewing distances.

• Can be used to achieve depth sense even if all other depth cues are removed. 1depth cues following (Okoshi, 1976)

different images (size, position and content) on the real retina as well as on the virtual projection plane.

l-eye

r-eye

c

l-eye

r-eye

convergence


Human visual perception

Monocular depth cues:

• Monocular Movement (motion) Parallax

• Depth perception by moving each of our eyes (head).

• Depth information is extracted from consecutive similar imagesin the same way as images from different eyes are combined.

• Retinal Image Size

• Brain compares the sensed size of an object to its “known” real size.

l-eye

t

l-eye


Human visual perception

Monocular depth cues continued:

• Linear Perspective • Straight parallel lines meet in the horizon.

• Important depth cue.

• Texture Gradient • Closer objects look more detailed.• Objects with smooth surface textures

are usually interpreted being farther away (especially true if the texture spans from near to far).

• Occlusion, Overlapping • Out of sight blocking of objects.

• Aerial Perspective • Distant objects (mountains in the horizon) look always slightly bluish

or hazy due to small water and dust particles in the air between.

• Shades and Shadows• Objects shadowing others are closer to light sources. • Useful to resolve ambiguities. • Bright objects seem to be closer to the observer than dark ones. • (Example: Three dimensional looking WIMP interfaces.)


Simulating visual stimuli3D CG rendering provides: Methods:

Retinal Image SizePerspective projection

Linear perspective

Texture GradientHigh tessellation, LOD, texturing (images, bump maps, normal maps, height maps…)

Occlusion Occlusion culling, z-buffer algorithm

Aerial Perspective fogging, atmospheric models

Shades and ShadowsSpecial lighting equations, shadow maps,

shadow casts

VR requires immersion and hence a simulation of visual stimuli which provides a mature depth perception:

ConvergenceStereoscopy, channel separation

Binocular Parallax

Monocular and binocular Motion ParallaxHead (motion) tracking, dynamic view frustum


Implementing additional depth cues

• Stereoscopy using stereo parallax:

• Render from two offset eye points (IPD) or center of projections (COPs).

• Feed each generated picture to the appropriate eye (channel separation)

binocularparallax

Right eye

p

p``

Left eyep`

Image plane/screen 1fixed w.r.t. (right) eye

Center OfScreen (1)

(COS)View PlaneNormal (VPN)



• Motion parallax:

• Track head (and hence eye movements) and calculate new perspective projection.

• Calculate dynamic view frustum in case of image plane fixed w.r.t. world.

translation

motionparallax

nearness:

leftness:

Left eye

p

p’t

p‘t-1

Image plane/screenfixed w.r.t. world

Left eye

COPt-1

COPt

qq‘t-1

q‘t



• Stereo and motion parallax require off-axis projection.

Right eye

Left eye

p

p``

p`

binocularparallax

Image plane/screenfixed w.r.t. world

COS

VPN"

off-axis projection" (and # in 3D):


StereoscopyFeatures of binocular parallax

• Negative: object in front of screen

• Zero: object on the screen

• Positive: object behind the screen

• Focus vs. convergence

• Focus is on image plane

• Convergence is on virtual object

! Large parallax puts strain on the eye.

Stereoscopy methods

Feed each channel and its rendered picture to one specific eye by

1. using one screen per eye (HMD).

2. time-multiplexing generated images (shutter glasses).

3. filter images through polarization filters.

4. filter images using color filters (anaglyph).

5. using auto stereoscopic displays.

• Shutter Technology

• Close left eye when right eye image is displayed and vice versa.

• Controlled through infrared or wired up.

• Usually connects to V-sync signal (vertical retrace of CRT).


Polarization

• Light: wave length and direction of polarization. Two components orthogonal to each other.

• Filters can block certain directions of polarization.

• See through linear polarization (use two projectors):

• Left view: vertical filter in front of the lens.

• Right view: horizontal filter in front of the lens.

• Wear glasses with polarizing filters.

• Left eye: vertical

• Right eye: horizontal

polarized light“normal” light

Stereoscopy


Stereoscopy• Linear polarization

• Can’t tilt head

• Little ghosting

• See through circular polarization (using two projectors):

• Left view: clockwise filter

• Right view: counter clockwise filter

• Allows arbitrary head orientations

• In general more ghosting than linear polarization

linear polarization

circular polarization

Anaglyph stereo

• Combine each channel’s R,G,B values by two complementing transformations to calculate an integrated channel.

• Several anaglyph version exist. Usually black/white images, color possible but filter and image colors may interfere.

• Example for a red/blue transformation:

Stereoscopy


Stereoscopy• Pulfrich effect

• At low light levels the eye-brain visual response is slower.

• Using a neutral (transparent gray) filter over one eye.

• Movement perception by that eye will lag behind perception by the unimpeded eye.

• Lag induces a difference in the images perceived by each eye.

• This induces a binocular vision illusion of depth.

• Auto stereoscopic displays

• Holographic displays, e.g. laser projection on gas or fluids.

• Modified LCDs

• Assign alternating pixel columns for each eye.

• Filter outgoing light by prisms or by two vertically striped masks located in front of the LCD.

• Slightly dislocate the masks in depth and displace them horizontally.

SeeReal Technologies C-ntres. 1600x1200 monores. 800x1200 stereosweetspot distance 650 mmsweetspot width/depth 50/150mm

• Autostereograms

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The human visual system - uni-bielefeld.de · Realtime 3D Computer Graphics / V irtual Reality Ð WS 2005/2006 Ð Marc Erich Latoschik Simulating visual stimuli 3D CG rendering provides: