Maya Skies Internal Production Site

Unified Test A

Written by Administrator

Monday, 06 August 2007

Overview

In order to efficiently capture details for important epigraphic passages, we propose integrating high-frequency photometric stereo data with low-frequency stereo matching data. This first test is meant to familiarize the team with the problem space. A simulated Maya stele.

For this test we carved a small test subject in high-density foam. This particular foam was chosen for some superficial similarities to the limestone surfaces present in Chichén Itzá. Both have very little specular component in their BDRF, and feature significant fine-scale porosity throughout the surface. We planned to use a camera system optimized for a capture region sized 1.5m x 1m, in order to approximate the system we are building for the Autumn field work in Chichén Itzá. However, we were forced to use a system with a much smaller capture region, approximately 300mm x 230mm. As a result, the photometric stereo fiducials take up a much greater percentage of each input image than is intended for the production system.

Stereo Image Capture

All images were captured using the following system:

(2) Pentax K110D 6.1MP Digital SLR Cameras with 18-55mm f/3.5-5.6 Lens
Image size: 3008 x 2000
F4.5, Focal length: 43mm

Raw image data is seen below:

Left Eye / Right Eye images with structured light projection:

A structured light pattern is projected into the scene using a customized strobe system. This RGB pattern, shown below, helps our software identify matching points in the absence of features in the surface under study.

Detail showing 100% image size:

The resulting range data from stereo matching will be posted soon.

Photometric Image Data

Using one camera in the stereo pair, we acquired 8 images with a remote strobe in roughly compass point positions relative to the test surface. In the proposed hybrid system, it is unclear whether we should continue to use a manually controlled satellite strobe. The advantage of using fixed lighting (as Holly Rushmiere did with her MR16 sources for IBM's Pieta project) is that fiducials should not be required in order to accurately locate the light sources. This should accelerate capture in the field. The disadvantages are: 1. Each strobe source can have a slightly different output profile, which must then be calibrated for. 2. In order to be practical, only 4 or 5 strobes are possible, which may not be enough to achieve results for all pixels in the scene. 3. Having many strobes in outriggers adds to the weight and bulk of the capture system. As shown above, fiducials are inserted in the scene in order to locate the strobe source in the scene. One of eight input images for photometric stereo processing:

Download the image data in the "Jamie" directory on the insightdigital.org server: Unified_Test_A.zip

Limitations of the Image Data

Despite shooting at an "ideal" focal distance of ~70mm and highly uniform depth in the scene, some regions of the image are out of focus.
Since the test system we used is hard-wired to use ISO 800, grain is apparent in the images.
Comparatively high sampling rate on ground truth. Total ground truth samples, in fact, are approximately the same the total photometric image samples.

Ground truth range data for comparison.

In addition to the stereo matching and photometric stereo image capture, we used a commercial close-range scanner to create a "ground truth" record. A view of this model data is below. Note that an explicit mapping between the photometric image data and this ground truth model does not exist.

Three views of the ground truth data: shaded mesh, normal map, and Z-depth: