| Multi-point imaging
of architectural and heritage structures
In
association with Skycell Ltd. (www.skycell.ltd.uk)
work is under way to develop airborne imaging systems suitable for the
controlled and repeatable imaging of architectural and heritage structures
at close range. Using a digital camera carried by a remotely controlled
airship, it is possible to acquire large quantities of digital imagery
from an assortment of views and ranges. But there are a range of major
research issues involved in the consolidation of the massive amounts of
data involved (10Gb per day of survey work) into a single, unified high-resolution
'seamless' archive.
Even though the images may
at first seem to be 'direct' rather than 'indirect', the data recorded
on disk in fact represents a geometrical projection of the region of interest
(dependent on the camera position and orientation) and distorted by lens
and camera-specific factors.
|
The basic concept of being able to construct
and interact with a massive composite high-resolution seamless image
of a heritage structure.
The composite here of the South Transept of York Minster is about 8,500x6,200
pixels in size.
(Click image for zoomed version)
|
Background:
Photogrammetry is an established
technique for the accurate recording of buildings and structures, but
has limitations. In general, a photograph offers an excellent way of recording
variations in contrast and colour, but is rather worse at providing 3D
and textural information – problems with parallax, perspective and
lighting effects can be introduced depending on the position of the camera
and the shape/structure of the region of interest.
Laser ranging systems
are a newer development which are still relatively expensive and harder
to use, but provide a superior way of obtaining quantitative range and
texture information.
Geomatics involves the
use of these methods in combination – the aim being to combine the
two different data sets to construct an interpretation which is true to
the original, in the sense that it not only contains high-resolution textural
and colour information, but also places this information in 3D space.
The natural next step
is to use multiple sources of information (such as many different images
and ranges taken from different angles) to construct a single master archive.
Most previous work in
this area has concentrated on fairly low resolution VR applications where
the camera and ranging system may be some distance from the surfaces of
interest. The aim of the York work is more ambitious - to move towards
the creation of extremely high resolution digital archives from close-range
images.
Approach:
The York work concentrates on four related aspects:
1) Data acquisition – to obtain high-resolution data, the imaging
system is flown on a remotely controlled airship, allowing extremely high
quality and close-range imaging.
2) Laser scanning – laser scanning equipment can be used to project
grid lines onto a region. These assist in step 3 below.
3) Data processing and data fusion – the images first need to be
corrected for their range, angle of view, lens distortions and exposure
effects. They can then be used to construct a new massive integrated data
set.
4) Content engineering – the ability to explore (change view, pan,
scan, zoom, and measure) the resultant massive (Gb scale) data set with
ease. |
The Skycell aerial imaging vehicle carrying
a digital camera in front of the Jesse Window in Wells Cathedral
(Click image for larger version)
|
Data Acquisition:
The vehicle, developed by Skycell
Ltd. at York, is soft and moves slowly - it cannot cause damage to surfaces
or structures. The equipment cradle, camera, control unit, video downlink
and power systems are hung below the main body of the balloon. The imaging
platform is stable, controllable and is entirely suitable for flight indoors
or outdoors in low winds. Larger systems are required for worse conditions,
or a series of tethers can be used to provide stability.
In short,
- The vehicle can be flown and
controlled by radio to desired imaging points.
- A real-time video downlink allows
instant analysis and a camera preview.
- The camera can be controlled by
a radio uplink.
- Individual images or groups of
images can be obtained from any point
- High-resolution close-range views can be taken
that would otherwise require expensive scaffolding or pneumatic lifts.
|
A composite (scaled down) of part of the
Jesse window in Wells Cathedral. The full size version is 2,600x10,000
pixels in size - seventy-five times larger.
|
Data Fusion:
If multiple images have
been obtained, representing overlapping views of the same area, then it
is possible to construct a vast high-quality composite data set from these
separate images. Construction of such high-quality seamless images is
difficult and processor intensive – matching points in overlapping
images can be used to determine unknown parameters such as the camera
angle and position, but additional information from ranging or scanning
systems would ease the process greatly.
The research at York is
aimed at attacking two key issues associated with this process:
- The massive nonlinear optimization
problem associated with estimating the unknown camera positions and
orientations (the EOP - Exterior Orientation Parameters) associated
with each image, and the unknown internal distortions due to the camera
optics and configuration (the IOP - Interior Orientation Parameters)
for each image. The York work is investigating the introduction a
priori information and the use of both hard and soft constraints
to increase the effectiveness of the nonlinear parameter estimation
algorithm.
- Automation of the registration
process between overlapping pairs or sets of images - this is most often
carried out by using pairs of user-specified control points chosen from
regions of overlap between images. This process is not only extremely
slow and labour-intensive, but is also sub-optimal in the sense that
only very limited amounts of the image data are ever actually exploited.
Instead a multi-resolution wavelet based approach is being investigated
which will allow automatic alignment of image sections with residual
mismatches being used to provide information about appropriate geometrical
corrections.
Delivery:
Delivery of the archive
to a wide (non-expert) user base is also a crucial part of the York work.
The aim is to exploit the use of ECW (Enhanced Compressed Wavelet) techniques
to allow both the exploration and navigation (panning, zooming, etc.)
of multi-gigabyte archives and the straightforward analysis of the image
content (overlays, comparisons, links to metadata and other relevant information,
etc.).
Click here
to see a video (5Mb - Windows Media Video) illustrating the extent to
which it is possible to zoom in and navigate the composite image (video
copyright the University of York and Skycell Ltd).
The ECW format means that access
to a large composite archive is quite feasible even for non-technical
users using a standard web browser over a normal internet link. Click
here
for an example of a 52 megapixel composite image of the South transept
of York Minster in compressed ECW format (1.3Mb - you will need the free
"ER Viewer" software, available from the ER Mapper web site
at www.ermapper.com, where free
ECW plugins for Photoshop and PaintShopPro are also available).
Using the ECW image format,
the user can navigate the image at varying levels of zoom in real time. |
