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Head-Mounted Display

Head-Mounted Display. Reading: Chapter 3 – Bowman et al. p 29 – 59. Visually Coupled Systems. A system that integrates the natural visual and motor skills of an operator into the system he is controlling . Basic Components

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Head-Mounted Display

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  1. Head-Mounted Display Reading: Chapter 3 – Bowman et al. p 29 – 59. Hodges and Babu 2008

  2. 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. Hodges and Babu 2008

  3. Head-Mounted Displays • Optical System • Image Source (CRT or Flat Panel (LCD)) • See–Through or Non–See–Through • Mounting Apparatus • Earphones • Tracker (Pos & Ori) Hodges and Babu 2008

  4. 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 at the same time. FOV may be measured horizontally, vertically or diagonally. Hodges and Babu 2008

  5. Field of View (FoV) vs. Field of Regard (FoR) A display’s FoR refers to the amount of physical space surrounding the user in which visual images are displayed. FoV refers to the maximum number of degrees of visual angle that can be seen instantaneously on a display. For Instance; In a HMD, the user may have 50 degree horizontal FoV, but 360 degree FoR. Hodges and Babu 2008

  6. 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). Hodges and Babu 2008

  7. 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. 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. Ocularity and IPD Hodges and Babu 2008

  8. 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. Hodges and Babu 2008

  9. Basic Eye Cornea Crystalline Lens Fovea Optic Nerve Retina Hodges and Babu 2008

  10. 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. Hodges and Babu 2008

  11. 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). Hodges and Babu 2008

  12. 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/x) Vres = 600/x (20/47) Hodges and Babu 2008

  13. Optical System • Move image to a distance that can be easily accommodated by the eye. • Magnify the image Hodges and Babu 2008

  14. Simple Magnifier HMD Design q p f Image Eye Eyepiece (one or more lenses) Display (Image Source) Hodges and Babu 2008

  15. 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. Hodges and Babu 2008

  16. Virtual Image Virtual Image Lens Display Hodges and Babu 2008

  17. 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 Hodges and Babu 2008

  18. 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 Hodges and Babu 2008

  19. Distortion in LEEP Optics A rectangle Maps to this Hodges and Babu 2008

  20. To correct for distortion • Must predistort image • This is a pixel-based distortion • Graphics rendering uses linear interpolation! • Too slow on most systems Hodges and Babu 2008

  21. 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? Hodges and Babu 2008

  22. Distance along z-axis Distorted FoV (cont.) Hodges and Babu 2008

  23. 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 Hodges and Babu 2008

  24. 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. Hodges and Babu 2008

  25. Exit Pupil • The area in back of the optics from which the entire image can be seen. Important if IPD not adjustable, mount not secure. • Compound microscope optical systems have a real exit pupil. • Simple magnifier optical systems do not have an exit pupil. Hodges and Babu 2008

  26. Characteristics of HMDs • Immersive • You are inside the computer world • Can interact with real world (mouse, keyboard, people) • Ergonomics • Resolution and field of view • Tethered Hodges and Babu 2008

  27. In Class Assignment • Go to http://www.virtualresearch.com/ • Figure out for the VR1280 HMD • Vertical and Horizontal Resolution • Vertical and Horizontal FoV • Vertical and Horizontal Equivalent Snellen acuity Hodges and Babu 2008

  28. OSG Tips • Loading objects and adding to scene hierarchy: • osg::Node *bicycle_node = (osg::Group*)osgDB::readNodeFile("bike.obj"); • bicycle_node->setName("bike"); • osg::PositionAttitudeTransform* bicyclePAT = new osg::PositionAttitudeTransform; • bicyclePAT->addChild(bicycle_node); • bicyclePAT->setPosition(osg::Vec3(0.2, 0.0, 0.38)); //Position • bicyclePAT->setScale(osg::Vec3(0.0148, 0.0148, 0.0148));//0.015 //Scaling • bicycle_shadow = (osg::Group*)osgDB::readNodeFile("C:/bike_shadow.obj"); • bicycle_shadow->setName("bike_shadow"); • bicyclePAT->addChild(bicycle_shadow); //Adding Shadow to the Bicycle object • osg::Quatquat = bicyclePAT->getAttitude(); //set rotation of the bicycle quat.makeRotate(90.0*3.1415926/180.0,osg::Matrixf::value_type(0.0f),osg::Matrixf::value_type(0.0f),osg::Matrixf::value_type(1.0f)); • bicyclePAT->setAttitude(quat); Hodges and Babu 2008

  29. OSG Tips Continued… • Loading objects and adding to scene hierarchy: • osg::Group *root; • root->addChild(bicyclePAT); • viewer.setSceneData(root); Hodges and Babu 2008

  30. OSG Lights // Lighting code osg::ref_ptr<osg::Group> lightGroup (new osg::Group); osg::ref_ptr<osg::LightSource> lightSource1 = new osg::LightSource; //light pos osg::Vec4f lightPos1 (osg::Vec4f(-2.3147,-0.3335,1.6764,1.0f)); //light parameters osg::ref_ptr<osg::Light> myLight1 = new osg::Light; myLight1->setLightNum(1); myLight1->setPosition(lightPos1); myLight1->setAmbient(osg::Vec4(0.5f,.5f,.5f,1.0f)); myLight1->setDiffuse(osg::Vec4(.5f,.5f,.5f,1.0f)); myLight1->setConstantAttenuation(1.0f); lightSource1->setLight(myLight1.get()); lightSource1->setLocalStateSetModes(osg::StateAttribute::ON); lightSource1->setStateSetModes(*lightSS,osg::StateAttribute::ON); //add to scene graph lightGroup->addChild(lightSource1.get()); root->addChild(lightGroup.get()); Hodges and Babu 2008

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