stereo – in theory and in practiceprojection calculation in computer graphics ... perspective...
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Stereo – In Theory and in PracticeMartin Ericsson
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3D?
● How do we perceive “3D” or depth in a picture?● 3D or stereo? (as in stereoscopic)● Content for today
– Depth Cues
– Theory
– Display Techniques
– Demos
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Monocular Depth Cues
● Secondary depth cues● All depth cues help the user to form a mental
image of the data displayed● Occlusion● Perspective● Relative size● Motion parallax● Shading and shadow● Atmosphere● Details
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Occlusion
● When two objects are at the same line of sight the object closest to the viewer should occlude the other object
● This gives information about the relative position of the two objects
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Occlusion cont.
● Objects that the user have no prior information about is hard to approximate
● Relates to both (relative) size and distance● Use overlap to help viewer● Depth buffer in computer graphics
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Relative Size
● As an object gets closer to the viewer it fills up more of our view space and by this looks larger
● So larger objects are perceived as being closer to us
● The type of object also makes a difference here, i.e., if the user has prior knowledge of the object
● Projection calculation in computer graphics● Also relates to perspective
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Perspective
● Orthographic projection where two parallel lines never meet can be practical in some applications
● But to give depth feedback to users a perspective projection should be used
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Perspective
● Put things on different depth to show inter relationship
● Lots of and details gives by perspective much depth cues
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Detail
● The amount of detail an object have also gives a hint on how close it could be
● More detail means that an object is closer● Compare this to a landscape. By your feet you
can see grass and individual stones. A bit further away they are not visible anymore only bigger thing and at a distance it is all “a blur”
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Detail cont.
● Much detail gives cue for closeness● Use texture maps for example to add details● Also geometric detail is important● Adds much spatial information● Also adds more motion parallax
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Motion Parallax
● Object (edge) speed gives hint on depth● Closer object is perceived as moving faster than
distant objects● Compare to what you see outside the window
when riding on a train for example● Related to detail, lots of geometric detail gives
more motion parallax● That is that the users sees many edges moving
at different speed
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Shading and shadow
● Shading gives a sense of curvature of an object● Shadows gives information about a relationship
between objects● One of the most important cues!
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Shading and shadow
● More complex objects need these cues to make sense
● Help the user to form a mental image
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Atmospheric scattering
● Light traveling through the atmosphere is scattered and absorbed
● This makes objects that are at a greater distance to the viewer to have more of a bluish tone.
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Atmospheric scattering cont.
● This is often implemented as some kind of fog effect
● Related to this is also to add focal blur
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An example
● Adding depth cues● Method
– Molecule model
– Imported a database file into Paraview (an application build upon VTK)
– Create geometry and export to Blender (www.blender.org)
– Render in Blender with different settings
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Example (no shading)
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Example (shading)
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Example (shadows)
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Example (ambient occlusion)
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Example (edges = contrast)
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Binocular depth cues
● Primary depth cues● All the secondary can we have in any picture● How come reality has more depth than a photo?● Three primary depth cues
– Accommodation
– Convergence
– Retinal disparity
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Accommodation
● To focus on something we apply pressure to deform the eye lens
● The amount of pressure is a depth cue● Works with one healthy eye and only at closer
range ● This depth cue is not something that we have
equipment for today
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Convergence
● When we look at something we rotate our eyes so the line of sight intersects at the point we are looking at
● The amount of rotation is also a depth cue for us
● Most people cannot diverge their eyes● We can use this with for example a stereoscopic
display
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Retinal disparity
● Majority of us have two functional eyes.● We get two inputs to our brain but we only have
mental image.● The two images are compared and the
difference between them also gives a strong depth cues
● Stereopsis!● Also supported in a stereoscopic display
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Display Techniques
● Four common techniques for single display plane– Polarization
– Anaglyph “Red-green”
– Time multiplexing
– Spectral Multiplexing
● Other types do exists, e.g., volumetric display, autostereoscopic displays etc.
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Display Techniques cont.
● Holographic Optical Element
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Display Techniques cont.
● Ground principle for stereoscopic image– Two images with a bit different point of view
– Make sure that “right” image reaches “right”
● Two ways to do this practically– Side by side rendering
● Compatible● Some side effect (later)
– Quad buffer● Needs special hardware
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Display Techniques cont.
● Projections– Need a way to project the world onto a planar
display
Parallel Toe-in Off-axis
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Display Techniques cont.
● Screen space vs. View space
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Display Techniques cont.
● Screen space vs. View space
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Display Techniques cont.
● Screen space vs. View space
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Anaglyph
● “Red-green” stereo● Both sources are weaved into one● Filtered based on color channels● Works on computer screens, printed form etc.● Support in VTK
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Anaglyph explained
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Time multiplexing
● Uses time to weave images together● One eye is occluded while the other sees the
image.● Uses active glasses that are a bit heavier than
other techniques
Frame n Frame n+1
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Polarizing
● Separating by polarization of light● Two sources with different polarized filters● Glasses with different polarized lenses● Requires a special screen that preserves
polarization
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Spectral Multiplexing
● Technique used here is called INFITEC and it is spectral multiplexing
● The visible light spectra is divided into six parts, three for each eye
● Each eye gets half of the light in the blue range, half from the green and half from the red
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Spectral multiplexing
Signal out
Left eye Right eye
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Method
● Computer with two video out● A box that enhances the signal● Two projectors with filters
– Notice the shift in color
● Screen– Regular screen
● Glasses with filters– Light weight
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Demonstration
● Questions?
Ewert Ingela Bo