Chuong V. Nguyen; David R. Lovell; Matt Adcock; John La Salle

“ Furthermore, insects can be complex in structure and pose many challenges to computer vision systems. We present a new prototype for a practical, cost-effective system of off-the-shelf components to acquire natural-colour 3D models of insects from around 3 mm to 30 mm in length. (“Natural-colour” is used to contrast with “false-colour”, i.e., colour generated from, or applied to, gray-scale data post-acquisition.) Colour images are captured from different angles and focal depths using a digital single lens reflex (DSLR) camera rig and two-axis turntable. ”

“ Holotypes - each the single specimen of a species that is used to define the characteristic features of that species. Holotypes exist as a physical object carefully protected from damage through handling. Digital colour 3D models of sufficient detail will enable collections managers to liberate these precious specimens for the research work they are intended to fulfill. ”

“ The 3D model of granary weevil was obtained from images captured in macro-mode, while 3D models of larger insects were obtained from images captured in normal-mode. The 3D visualisation of insect models is based on the open-source X3DOM framework [39] which uses WebGL for plug-in-less display within a web browser (such as Firefox and Chrome) . The file size of models, including 3D mesh and texture, depends on the desired visualisation quality and the complexity of the geometry and colour of the actual specimen. ”

“ We therefore note that the best mounting orientation is specimen dependent: visual hull reconstruction of geometry improves the more surface normals are captured in silhouette, while colour and texture improve the more surface normals are captured parallel to the camera viewing axis. ”

“ Pinning insects horizontally allows many insects to be stored in wide, flat display drawers but creates a few problems for our system:# The pin becomes part of the 3D model and must be edited or segmented out in post-reconstruction# Editing can often not fully remove evidence of the pin# Images of the underside of the specimen can be difficult or impossible to capture, leading to an incomplete 3D model. ”

“ Macro-mode. There are many ways to automate the acquisition process. The desire to use off-the-shelf components led us to use the GigaPan Panorama Robot EPIC 100 [23] for mounting the specimen. The GigaPan is designed for mounting and controlling a camera—and this led to the GigaPan robot also acting as the acquisition controller. In other words, it is the turntable that triggers the macro-rail. The macro-rail moves and triggers the camera which triggers its flash and takes an image. ”

“ By transforming this 2D data into a 3D model, the system further achieves a 30∶1 compression of data. This level of compression enables useful information about the specimen to be exchanged via email, presented in web pages and embedded in 3D PDF documents.
This work raises a number of research challenges and opportunities for further improvement, including:# Eliminating the need for the printed mat: 3DSOM requires this mat to estimate the camera pose of individual images. We have reached the lower size limit of what we can straightforwardly print and attach to specimens. ”

“ Next, a specially patterned mat (Figure 5B) , required by the reconstruction software (3DSOM [22] ) , is attached to provide information about camera pose and position relative to the specimen. Generally the suitable size of the pattern is about one to two times the length of the insect to be scanned. Scanning smaller insects requires smaller patterns to be printed. Currently, modern laser printers with 1200 dpi printing resolution can produce patterned mats as small as 5 mm in diameter. ”

“ In high-level terms, our system and work-flow involve three main steps (Figure 2) :
The three main steps to create a natural-colour 3D model of specimen.The steps are mounting the insect onto a pin, acquisition of 2D images of the specimen at different poses, then reconstruction of a single 3D model from those multiple images. Mounting. the physical specimen is pinned onto a pre-printed mat used later by the reconstruction software to estimate camera pose (viewing angle and position) . ”

“ At each position, the control box triggers the camera to capture an image.# A two-axis turntable to present views of the specimen from different angles of rotation# A macro-rail to vary the distance between the camera and specimen in macro-mode# A camera with macro lens and flash.# Two laser pointers for specimen alignment# A computer for 2D image processing and 3D reconstruction. ”

“ In this paper, we have shown that high resolution, natural-colour 3D digitization system for insects and other small specimens can be implemented using readily available components with hardware and software cost under AUD8000. ”

“ This approach does not recover concave surfaces, but photo-consistency can be used to correct this to an extent [29] . The extent of improvement by photo-consistency is limited for some insects due to strong speculiar reflections on the outer-surface and fine body structures such as legs, antennae, spikes and hairs. ”

“ A photograph of the real insect specimens of the 3D models captured. Figure 9 illustrates the effectiveness of macro-mode image acquisition as compared to normal-mode image acquisition when applied to very small insects such as the granary-weevil. A Canon EF-65 mm macro lens was employed in both cases. In normal-mode, a stencil with a File:Pone.0094346.e016.jpg2 mm hole had to be attached immediately in front of the camera (Figure 9A) to reduce the effective aperture and increase the depth of focus. ”

“ F) for this specimen.Impacts of mounting orientation and tilt on reconstruction quality.While additional images at tilting angles of File:Pone.0094346.e012.jpg, File:Pone.0094346.e013.jpg, File:Pone.0094346.e014.jpg and File:Pone.0094346.e015.jpg improve reconstruction quality in both horizontal and vertical mounting (in comparison with Figure 11) , vertical mounting leads to sharper model with more vivid colours and textures. Further surface geometry issues arise as the structures of specimens become more complex. ”