Fragment Color Image Processing

1. Original photograph. The fragments were photographed with a Sony DKC-ST5 digital camera, which produces 24-bit color images at 1030x1300 resolution.

2. Lens distortion correction. The raw images were processed with custom software which corrects for two types of lens distortion:
   • Lateral chromatic aberrations. The chromatic aberration correction spatially shifts the red and blue image channels separately by predetermined offsets and linear scaling. These shifts were on the order of one half pixel distance.
   • Geometric distortion. The geometric distortion correction warps the image to primarily address a pincushion-style distortion which was present in the original images; some points were shifted a distance on the order of several pixels.
   Because the lens distortion correction program requires resampling of the image, minor image degradation (blurring) can occur during this step.

3. Mosaicing. 227 of the fragments were photographed with multiple images that required mosaicing together (these images are labeled as such in the database). Most of the mosaics were made from only two or three source images and were interactively stitched together using Panavue's ImageAssembler software. This software reads the source images, performs registration, color equaliziation, and resampling of the images, and outputs a single large mosaiced image.

   44 of the fragments could not be stitched adequately using this software, and were instead completed using PTGui, a graphical interface for the PanoTools software suite for creating panoramas and stitching images. This software works similarly to ImageAssembler, but allows for many more control points in the stitching process and more flexibility in anchoring the adjustments made for alignment and color and brightness equalization.

   Mosaiced images may exhibit parallax artifacts due to the image being a composite of several different camera positions. These artifacts are usually manifested as blurring along the edges of the fragments, and were removed manually with an image painting program.

   Other minor mosiacing artifacts may be visible along the boundaries where sub-images were stitched together, as seen in the example below (Fig. 1). The image color, contrast, sharpness, or geometic alignment may vary slightly across the interface.

4. Rotation. Images were rotated so that the fragment orientation generally matches the orientation in the PM1960 plates, when possible. Note that rotation may require another image resampling step, and thus very slightly reduce the image quality.

5. Cropping. The images were cropped so as to have a semi-uniform width rectangular black border around the edges of the fragments.

6. Background Masking. The fragments were photographed on a black felt background, and a greycard and blue plastic ruler was included in most frames. To remove these background objects and other debris from the images, the fragment shapes are manually traced to create masks or mattes which correspond only to the foreground shape of the fragment. The portions of the background outside the mask is colored black.

7. Color Correction. Most of the original images are too dark for reasonable display, and required lightening via color remapping. Because the fragments were not photographed under consistent lighting and camera conditions, the specific color correction steps applied may vary depending on the image. Typically, the color correction involved a brightening by uniformly increasing the midtone color intensity using the "Color Balance" function in Adobe Photoshop.

8. Compression. The images are lossily compressed for web distribution in JPEG format.

Other possible artifacts remaining in the images:

• Scaling. The images were originally photographed at an approximate resolution of 100 pixels per inch, and are distributed in the database at an approximate resolution of 50 dpi. However, the precise resolution varied during the photography process, so the images do not have consistent scale and vary approximately 10% about the mean.

• Color Accuracy. The colors of the fragments depicted in the images do not necessarily accurately reflect the actual fragment coloration.

David Koller & Kevin Coletta