Histograms

Histograms and their use can be useful in showing graphically the tonal range of pixels in a photograph and can be used to edit with great precision the digital image [171]. These histograms are shown in the right hand side of each image in Figure 4.35 (screenshot from Adobe PhotoShop CS6®) and Figure 4.36 (screenshot from Adobe PhotoShop Elements 11®). Both programs show similar

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information but CS6 is more spread-out. Some image editing software may show a very basic graphic image in which only Red, Green and Blue (RGB) channels are shown [172]. Most software also has the facility to show the dark and light tones - the highlights, midtones and shadows - whilst the more advanced software will have additional colour channels such as Cyan, Magenta and Yellow [145].

Figure 4.35:

Histogram - PhotoShop CS6®.

Figure 4.36:

Histogram - PhotoShop Elements 11®. Many digital cameras also have a histogram facility in the “playback” mode and this is useful to determine and assess how an image has been exposed [142]. Depending on the camera make, this may differ from digital editing software histograms. Figure 4.37 has been taken from the Nikon User Manual and explains the symbolic images and information that appear in the screen on the back of the Nikon camera. In the histogram diagram taken from the Nikon manual (Figure 4.37) the colour distribution in the Red, Green and Blue scale as well as other information is shown. In Figure 4.34 the brightness of each image is also shown (lower histogram) and at first glance all seem similar. However, on closer examination it can be observed that each histogram is different and there are subtle differences between the colour distribution scales as well as the white scale. These scales give a visual guide to the change in tonal values when editing an image.

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Figure 4.37: Nikon User Manual explanation of RGB Histogram [143]. It has been shown despite the potential problem of diffraction caused by stopping down the aperture too far, lens diffraction should not cause undue problems. This is on condition that a series of tests are undertaken to determine at what point image distortion occurs and how small the f/stop can be. This must be determined for each camera and lens combination, before a full dataset of digital images are taken.

As with many of the photographic processes and editing tools, it must be understood that the objective is not to produce a beautiful coloured printed image which is to appear in a glossy, expensive magazine, or hang in a national photographic gallery or museum, or a family grouping to be displayed on the domestic living room wall. Such utilisation seeks to perfect the visual image and as such may come under greater scrutiny. In such cases, if the original image is enlarged, many image defects may be visible to the viewer. However, photogrammetry in relation to reverse engineering is about achieving a set of images which satisfy the requirements of the software, software which can scan the image and reject unsuitable images, and which transforms the digital pixels into a point cloud image. This in turn produces a 3D model of the original artifact that is within the fabrication limitations of the AM machines being considered. 4.9 Software control – semi automatic

In this method, the software was considered semi-automatic. It allowed far more control, by the operator, over how the data was processed. Instead of processing

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the data via the internet, as with the examples in the first part of this chapter, it could be processed on the same machine, without the use of the internet, as long as the host computer had an i5 or preferably i7 CPU with a minimum 12GB memory.

Figure 4.38: PhotoScan Pro® Data processing Flow Chart.

The author upgraded the computer to 32GB of memory before Ultra High scanned data could be processed. In Figure 4.38, the Flow chart is very similar to that in Method 1 (Figure 4.5) but unfortunately the Agisoft software did not support the creation of an independent video, although there was no requirement for internet connectivity for data processing with PhotoScan Pro®. The software was more demanding to use than 123D Catch®, because it was not fully automatic, but the interface and icons employed were straightforward and easily learned. Unlike 123D Catch® it only used the subject image and not the background to stitch the digital images together to produce a data point cloud image.

134 4.10 Pre-Process Masking

Editing by the user began at this early stage of the process. But first, for best results, before processing the data, each image was masked from the surrounding background. This masking was possible using a built in tool in the software, as can be seen in Figures 4.39 and 4.40, the Egyptian Vase.

Figure 4.39: Complex masking line without Chroma Key backdrop.

Figure 4.40: Simplified masking operation when backdrop is used.

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Experimentation with inter-changeable Chroma Key backdrops was undertaken; this type of backdrop provided a very good contrast between the main subject matter and its surroundings. Before the data set could be processed, each individual photo frame was masked so as to outline the subject.

It was found that the time taken to mask each digital image was considerably quicker with the use of a single plain coloured Chroma Key background. The photographic frame in Figure 4.39 is without the Chroma Key backdrop, and masking became a much more complex operation than in Figure 4.40 which used the backdrop. In both frames the masking line is arrowed.

Figure. 4.41: Non-masked images of the Egyptian Vase – pre-processing. The more RAM that was available, the faster the digital data could be processed, and the more detail that was forthcoming.

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Figure 4.41 shows 10 non-masked images which formed part of a 130 digital data set that were shot for the Egyptian Vase, whilst Figure 4.42 shows part of the masked set, which the software uses to stitch the images together.PhotoScan Pro® was considered as being semi-automatic software, that is, it allowed the user some freedom to manipulate the digital image as it was being processed.