Virtual 3D Representational image

CAD computer software such as Adobe’s Photoshop LE® and Photoshop Elements® [54] have been able to stitch 2D digital photo images together for a number of years, creating panoramic views of city, sea or landscapes. More recently there were several software programs capable of stitching 150 or more, high resolution digital images together to form a 3D image [55].

Figure 1.20: Examples of 3D virtual textured point cloud data images.

1999 saw a breakthrough in the algorithm that launched photogrammetry software no longer requiring matched photographic scenes, but able to capture and create large point clouds from very large photo sets (Figure 1.20). The efficiency of the software which can convert digital images taken with a DSLR camera is so high, that it can turn billions of point cloud data points into 3D images within hours. This must now be a threat that the LIDAR world, with such disadvantages as the high cost of scanning and associated equipment, collecting data, slower speed of processing and the steep learning curve needed on the

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research and understanding involved in utilising the entire point cloud, must now consider [56].

1.20 3D Reliefs

1.20.1 Bas-relief, Sunken and Raised Relief

To the uninitiated or casual observer all of the above methods are the same, or similar; reliefs being an artistic method of expression hovering between a 3D sculpture and a 2D painting or drawing, often described as 2.5D art. Bas-relief, from the French meaning “lower”, and sunken reliefs are those where the drawing or pictorial image has been cut into a brick, stone, wood or metal surface.

In the main, raised relief is where the surrounding material has been cut away to lower the original surface and leave the subject matter to stand proud. All the aforementioned techniques are several thousand years old, with examples of bas-relief being found from around the world (Figures. 1.21 & 1.22), in all of the early civilisations such as Chinese, Egyptian, Persian, Greek and Indian, as well as from the Mayan period in South America.

Figure 1.21: Roman pot shard – Bas-relief.

Today these forms of relief can be found on coins, both ancient and modern, jewellery such as brooches and rings, and on the walls of buildings as bas or high-reliefs, where the art work can be protruding almost 100% from the surface.

Several modern techniques exist to copy or improve on the originals, where the different types of relief have been created using different starting criteria; examples cited here include restoration of brick and stone relief, an art form as practised by the Han Dynasty, (c206 BCE-220 AD). They recorded their

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individual personal achievement, local events or national military campaigns and victories [48].

In 2011, Wang et al. [57] proposed a method using 3D models as input data to generate sunken reliefs, in which the incised lines form the contours below the surface, whilst the heightened forms are on the surface level. By overlaying the feature lines, further refinement was achieved, enhancing the final visual impact on the viewer. The final digital 3D generated mesh was then used, via appropriate software, for machining real geospatial parts either using AM or computer numerical control (CNC) technologies.

Figure 1.22: Egyptian Horus raised-relief.

In a similar vein, in 2013, Yu-Wei Zhang et al. [58] looked at replication by computer generated 3D mesh of line-based sunken relief, as found largely in the Ancient Egyptian artwork. Their aim was to find a way of combining the contour incised lines, which form the first outline sculpture, with the remaining sunken, variable depth features, leading to changing shadow and thickness under strong lighting conditions, in which replication had been problematic.

A further interesting computer based method of reconstruction of bas-relief carving, is a system for semi-automatic creation of bas-relief sculpture as described by Weyrich et al., in 2007.

"the unique challenge of squeezing (3D) shapes into a nearly-flat surface while maintaining as much as possible the perception of the full 3D

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scene. Our solution to this problem adapts methods from the tone- mapping literature, which addresses the similar problem of squeezing a high dynamic range image into the (low) dynamic range available on typical display devices….. Given a 3D model, camera, and a few parameters describing the relative attenuation of different frequencies in the shape, our system creates a relief that gives the illusion of the 3D shape from a given vantage point while conforming to a greatly compressed height” [59].

Their problem was to maintain the centuries old artistic principles of bas-relief starting with a geospatial model, a camera view of the model and a selection of controls such as output height (relief height) and overall breadth and width, taking into account the amount of detail to use or reject and the final material to be used in presenting a semi-automatic computerised solution.

Working with more conventional oil paint based art works, were the paint on canvas forms a raised relief, Peng et al. [60], in 2015 were experimenting with UV Ink-Jet technology, whilst gaining some success they felt that many problems were still to be overcome before high value art work would be replicated by the use of 3D fabrication technology.

1.20.2 Image-Based Displacement

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Finally, the focus of this thesis is on image based displacement (IBD), which is seen as a relatively new original digital method, combining both bas and raised relief, whereby a 2D painting, drawing or relief (see Figure 1.22) is photographed by a digital camera and the data is transformed into an AM replicated object, the image being imprinted to appear both below and above the surface.

Figure 1.24: Top and bottom of block showing surface displacement according to grey scale colour intensity.

By converting the coloured image into a “grey scale” image (Figure 1.23), the points of the mesh created, move relatively to the grey colour intensity of the digital image, which displaces the surface geometrically (Figure 1.24). A coloured textured mapping system can be applied to the whole surface so that a digital computer file is produced which can be 3D AM fabricated as a raised or bas surface with the original colour picture printed on top of the relief (Figure 1.24).