Image Resolution

A digital camera captures an image with an electronic sensor that records millions of minute points of light called pixels.  The number of recorded pixels determines the size and quality at which an image may be reproduced.  The resolution of a photograph is defined by the density of its pixels, usually expressed as pixels per inch (ppi).  The higher the resolution, or the more pixels per inch, the sharper it will appear to the human eye.  Conversely, the more pixels per inch, the smaller the reproduced image will be.  A resolution of at least 240 ppi is required to print a high quality photograph.  The professional photographic and publication standard is 300 ppi.

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Improvements to the First Photomosaic

Several shortcomings had been identified in the first photomosaic:

  • Some of the constituent photos had been improperly sized or placed.
  • The scale was unknown.
  • The accuracy of the colors was uncertain.

Both the photo placement and scale issues were related to the absence of any dimensional information about the plane of the facade walls.  Photogrammetric control could address both of these issues.  It was recognized that field photographic techniques should be improved for future projects.  However, it was determined that the photographs that had been taken along via Consolare were of sufficient quality to be reused to create a new photomosaic utilizing improved control techniques.

First a site visit was performed during the summer of 2003 to determine which of the standing vertical walls and columns formed an approximate flat plane along the length of the street.  Each of the vertical walls identified on this imaginary plane was then sketched on tracing paper that had been placed over the original photomosaic, which was used as a drawing guide.  Each individual wall and the distances between the walls were hand measured and the drawing annotated with the data.  Doorways and windows were also measured and sketched.  The curbstones along the street in front of the plane of the vertical walls were counted and noted on the drawing.  This figure shows part of the dimensioned field sketch.[1]

730_surg_sketch

Via Consolare traverses a moderately sloping hill.  Therefore the walls and columns that had been identified as being on the plane of the facade were not necessarily on the same horizontal level.  A separate field sketch was made of the frontages of the buildings, and fifty-five elevation points were measured with a dumpy optical level and leveling rod.  These elevation control points were taken along the base of the vertical walls, tops of curbstones and other locations such as doorway thresholds.  Each was recorded on the sketch.  The elevations were determined relative to the project-defined datum point located at the south end the street, which had been defined as 100.00 meters.

500_levels_sketch172_elev_list

The new control information showed that the original photomosaic had been produced at a scale of about 1:90.  In the fall of 2003 a new photomosaic of Insula VI, 1 was created at the exact scale of 1:100 utilizing the original photographs.  A base layer was made in Photoshop with a resolution of 300 ppi, and the control measurements and elevations obtained in the field were used to size and place the photos.  The work began at the north end and proceeded to the south, or left to right on the image. The dimensioned field sketch was used to calculate, measure and draw vertical guidelines on a new Photoshop layer representing the location of walls, doors and columns.  Horizontal guidelines representing various elevation levels were calculated, measured and drawn on the same layer.  These guidelines created a background layer that indicated where building features on the plane of the facade should be located. This allowed the photographs to be adjusted and moved as necessary.  All features on the plane were matched to the background as well as to adjacent images.  Features not located on the plane were left at the size at which they were captured.  Any photographs of upper levels that were taken with the camera tilted were modified with the perspective correction tool to match the lower photographs and control dimensions.  Finally, a 10 meter-long segmented scale bar was placed on the image.

730 surgeon photom d

The color accuracy of the photomosaic was then addressed.  Photogrammetry is primarily concerned with obtaining precise geometric information from images, and work products are usually in the form of maps or drawings.  Precise color realism is not a high priority, even for orthophotos.  However, the photogrammetric principle of obtaining control information about a subject suggested a potential solution.  Test photos were taken of several of the facade walls, printed on photo paper and compared to the buildings on site.  The images that best matched the true colors were used as guides to adjust the colors of the other photographs as they were placed into the Photoshop document.


Click image below to enlarge

730 vi1-final dThe improved photomosaic was reproduced and shared with the project archaeologists.   The consensus was that the incorporation of dimensional and color control techniques had significantly improved its accuracy and quality.  The scale of the image was now known and constant.  Comparisons with published site plans, with the dimensioned field sketch and with the appearance of the buildings themselves did not identify any discernible errors.  The hues and tints of the image were much more consistent with the colors of the buildings themselves.  Although further improvement could still be made, significant progress had been made toward the original goal of creating a single photo-realistic and dimensionally accurate image of the facade of the 110 meter-long city block.

A number of lessons had been learned during the photography and processing experiments as well as well as research in the literature:

  • The methodology works best on relatively flat, low relief facades and walls that form a linear plane.
  • The plane of the facade of the buildings that will be photographed should be defined in advance.
  • If possible, a high quality digital camera and lens should be used to take the photographs.
  • The resolution of the photographs (number of pixels) determines the amount of detail that is recorded.
  • Camera settings (focal length, aperture and exposure) should be identical for all photographs.
  • Photographs should be taken normal (at right angle) to the plane of the facade.
  • All photographs should be taken at the same distance from the plane of the facade.
  • Photographs should be taken in portrait format (tall rather than wide) in order to record the maximum height.
  • Photographs should be overlapped at least 50% so that any distortion at the periphery of the images can be excluded from the final photomosaic.
  • If upper level photos are required, the camera should be raised, if possible, and kept normal to the plane of the facade.
  • The total facade should be in sunlight, and photographs taken quickly to avoid large shifts in shadows.
  • Color reference images are required to adjust the color accuracy and consistency of the photographs.
  • Original photographs should always be preserved, and alterations only made to copies.
  • Control points located on the plane of the facade should be carefully measured and recorded in the field.
  • A base Photoshop layer should be created at the desired resolution on which all photographs are placed.
  • Control point data at the desired scale should be transferred onto a Photoshop layer forming a control background on which the photographs are sequentially sized and located.
  • The color of all of the photographs should be adjusted before incorporation into the photomosaic using the color control reference.
  • Photoshop tools (Opacity, Transform/Size, and Perspective Correction) are used to match new photographs with the control background and visible details from the previous photo.
  • Photoshop tools (Opacity and Eraser) are used to eliminate image overlaps and blend edges.
  • An appropriately sized scale bar should be placed on the image to facilitate measurement along the plane of the facade.
  • Titles and dates should be added to the image.
  • The photomosaic file should be saved with all layers intact for possible future changes or editing.
  • The layers on a saved copy of the photomosaic can be flattened into one in order to decrease the image file size after all other work is complete.
  • Copies of the photomosaic can be reproduced in other file formats (TIFF, JPEG, etc.) for distribution and use.

The improved photomosaic of the building frontages along via Consolare in Insula VI, 1 proved to be useful for both the archaeologists and conservation architects.  The images were utilized by the project to summarize excavation data, to propose architectural reconstructions, to study building construction and for general educational and public relations.

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[1] Thank you Sarah Jacobson and Craig Leyland of the Anglo-American Project in Pompeii for your hours in the hot sun to produce the field sketches.