From overlapping geo-referenced aerial photo blocks, orthophoto is rectified from single photos against a DEM into a 2D constant-scale map-like image.
PurVIEW displays directly overlapping pairs of geo-referenced photos as stereoscopic image models, where imaged features can be digitized as 3D map objects—geodatabase-direct.
How does stereo display work?
The OpenGL video card used generates 120Hz video frames, where 60Hz frames are assigned to each eye in sync with the LCD shutter eyewear. Your brain does the rest.
Is the stereo display quality comparable to photogrammetric workstations?
Photogrammetric workstations also use CRT.
Common 96kHz CRT can support 1,024 x 768 @ 120Hz, easily matching many special workstation monitors of old. Higher resolution 130kHz CRT can support 1,280 x 1,024 @120Hz, achieving dot pitch of <1 arc minute, limits of human visual acuity.
In addition, PurVIEW dynamically re-samples each display refresh to monitor resolution. Unless extreme zoom-in is used, image pixels cannot be observed.
What about LCD or plasma displays?
The latest true 120Hz gaming monitors such as Samsung 2233RZ and Viewsonic VX2265wm FuHzion models work well. Both are 22” in diagonal and support the same 1,680 x 1,050 stereoscopic view resolution with negligible “cross talk”. Their effective stereoscopic dot pitch is 0.28mm, matching or better than special dual LCD or DLP stereo display monitors at a fraction of the price.
LCD or plasma displays rarely support refresh rate >75Hz, therefore unusable.
Other stereo LCD displays assign alternate columns of LCD pixels into left and right eye views for one or more view-points, reducing the horizontal resolution by half or worse, often failing to resolve even standard text.
Will my Brand A high-resolution CRT work?
A good indicator is a specification that manufacturers always quote, often even printed on the shipping carton: 1,600 x 1,200 @ 80Hz (lowest resolution usable)
Higher resolution monitors are better and might be specified as higher frequency or pixel count, or both: 1600 x 1200 @ 102Hz, or 2048 x 1536 @ 80Hz.
A CRT capable of sweeping horizontally (through 1600 pixels) 1200 lines per frame at 80 frames per second…has a horizontal bandwidth of 96kHz (1,200 x 80Hz). To function at 120Hz, the maximum sweep per frame will have to be reduced to 800 lines (96kHz/120Hz), supporting 1,024 x 768 (lines) with some headroom.
1,280 x 1,024 @ 120Hz can be supported by monitors with horizontal bandwidth >125kHz. Most OpenGL video cards also support intermediate resolutions.
Will my Brand B video card work?
PurVIEW standardizes on the latest VESA standard OpenGL stereo-ready video cards available from multiple vendors, in either AGP or PCIe buses. Retailers all tend to describe such product with the same nomenclature.
VESA – Video Electronics Standards Association
OpenGL – Open Graphics Library
Stereo-ready – equipped with a female 3-pin mini-DIN jack for direct, one-way only eyewear Emitter plug-in.
Most OpenGL video cards support dual monitor operations both at 1,280 x 1024 @ 120Hz…and also intermediate resolutions.
Will my LCD eyewear work?
StereoGraphics and NuVision, the competing vendors of higher quality products, offer LCD Eyewear/Emitter separately or as a kit. Both use the standard female 3-pin mini-DIN jack for emitter plug-in. Reuse or buy new, either brand.
What about anaglyph stereo display?
Anaglyph is displaying a composite of the left and right eye views each tinted the matching colour to the eyewear red/cyan lenses. Colour cross-filtering separates the left/right perspectives. This lens colour combination allows some appreciation of greenish colours but the image model is predominantly mono-chrome.
The composite image is a normal computer display at 60Hz, needing only standard video card and any monitor, including laptop LCD display and LCD projectors.
Card board mounted disposable “eyewear” is often distributed free. Durable polycarbonate lens spectacles or clip-on’s are available for ~$10 each in quantity.
Is PurVIEW stereo model display accurate?
PurVIEW displays stereo models rigorously.
Rigorous is a photogrammetric technical term meaning absolute adherence to the geo-referencing parameters…not best-mean-fit, not rubber-sheet, not approximation. In other words, if the available geo-referencing parameters are good—such as those generated by IMU or photogrammetric aerial triangulation—the PurVIEW display will be good.
What about epipolar resampling?
PurVIEW performs dynamic epipolar resampling for each model display refresh…without degrading the system performance. End-users will not even perceive its execution.
How accurate are Z measurements?
Virtual-Z accuracy is dependent on DEM accuracy.
Manual Z measurement accuracy is dependent on lens focal length (f) to image base (b) ratio, monitor stereo resolution and input device acuity…same as in photogrammetric workstation operations.
Why not digitize features from orthophotos?
Ortho-rectification corrects scale variances due to topographic relief…but at the DEM surface only. Un-modeled object micro-relief causes apparent displacements (leans) away from the photo centre.
Leaning object causes unknown X/Y positional errors for the object top and occludes surrounding lee-side features. Even if taller objects are absent, orthophoto features digitized can only be 2D.
