The camera was used in two basic ways to capture the data images of the artifacts: in the first method, the camera was moved around the artifact on a tripod, in a 360° circle.
Figure 3.13: Close-up of turn table showing degree markings.
In the second method the camera was stationary on a tripod and the object to be photographed was rotated on a turntable in a 360° circle (Figures 3.13 and 3.14). According to how much detail was required to be captured from the subject, so the number of digital images required to captured and processed, increased or decreased. The object was placed on the turn table which was rotated either through 10° or 15° per photograph. The turntable and supporting
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pot, on which the prime object was placed, were also painted to match the Chrome Key covering. A rotation of 10° produced 36 images per rotation whereas a rotation of 15° produced 24 images per rotation.
Figure 3.14: Turn table as in use.
Figure 3.15: Use of cord to control radial distance.
For another indoor photo-shoot, this time of a relief painting (Figure 3.15), the object painting was left hanging on the wall, directly facing indirect light
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coming from a window on the left of the photograph. In this series of image capture, the camera was moved in a semi-circle around the hanging picture. Note the cord (arrowed in photograph) tied to the tripod leg and the top of the central heating radiator, thus keeping the cord tight maintained the radial distance from lens to subject.
Figure 3.16: Use of pole to control radial distance of camera position.
In another series of photographs, the camera is kept at the same distance from the subject by the use of a simple pole (arrowed in photograph) attached to the tripod leg (Figure 3.16). By keeping the camera at the same distance the size of the digital images and DoF was kept as near constant as possible.
3.5 Compact Camera Test - Canon IXUS 100 IS®
Although the Nikon D3100® was extensively used throughout, and because of the failure of the three compact digital camera frame (see Chapter 3.4.1), a small pocket digital compact Canon IXUS 100 IS® camera was used to take images of a few objects. It came with 12.1 megapixels, 3.0x Optical Zoom and an optical image stabilizer. This camera could also take high resolution images but could be set at a lower picture resolution of 1600 x 1200 pixels, equating to just under 2 megapixels. This camera was ideal to compare a low resolution point and shoot to the more complex D3100®. Although this camera has since been superseded
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by a new model, it can still be purchased for under £120. These images and results were used as comparison data in a later chapter (see Chapter 7).
Two methods of digital data capture were employed by the use of the Nikon D3100® and these form the bulk of the discussion in future chapters, together with the specific operating conditions in which objects were photographed. The first part of the process was the acquisition of the digital data images using the single DSLR camera. According to the type of processing software employed, some 60 – 150 images were taken from different angles, for the digital data sets needed for processing, ensuring that there was an image overlap of about 10- 20%. The images were taken using a mid-range resolution of 4608 x 3074 pixels. The final part of the process was the conversion of the 2D digital images to 3D CAD models, using the appropriate software as recorded in the examples cited, and transferring this data to the AM machines which were used in the fabrication of the artifacts to produce the geometric models.
The advantage of this single digital camera process was that 3D scanners were not required to capture the data necessary to produce 3D CAD images, and experienced technicians were no longer required to operate this equipment. By using a relatively modest DSLR camera, good results were obtained. A comparison between photogrammetry and laser scanning, their techniques and characteristics has been shown by Barsantia et al [74].
Details and results of the three artifacts that were used for a set of trials with different camera settings are found in Chapter 7. A processed 3D digital image for each artifact is compared with one taken with the Nikon D3100.
3.6 Lighting
The approach employed for lighting and camera positioning for the artifacts was different in each method used, the common factor being that shadowless, flat lighting was required to illuminate all the artifacts, as any shadow distorted the image captured and processed by the software. Highlights or reflections that the lighting might have created coming off the objects could also cause distortion to the digital image. In most cases, if the lighting level was low, the camera was able to compensate by the combination of shutter speed or aperture opening. Where small apertures were required, because a greater DoF was required to
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obtain maximum sharpness, the lighting level if raised too much could cause reflective flare. In such an instance the shutter speed would be lowered.
For future research, it might be useful to measure both the level of brightness (lumens) emitted by the light source in relation to the reflective spectral brightness (flare) of the light, to ascertain the point at which the digital image is distorted by this spectral interference. As there are so many different reflective surfaces, the outcome for such research may show that there is very little, if any, common ground to substantiate a given set of rules, thus proving that such an idea is unworkable and each object photographed must be tested to find the optimum level at which the flare results in the degradation of the digital image.