3D Model: Green Crab [Carcinus Maenas]

3D Model: Green Crab

The below article overviews the creation of above pictured 3D model, from reference image capture, polygonal modeling, and final texturing.

Dissection and Photography
Having discovered a freshly deceased Green Crab [carcinus maenas] in the surf of the Acadia Maine coast, we took the opportunity to photograph and dissect the creature.

Green Crab, orthographic images of dissection.
Green Crab, orthographic images of dissection.

Once having captured imagery of the complete crab from a satisfactory number of angles, we proceeded to lift the dorsal carapace and remove each limb at the joint where it met the main body. Orthographic images of each limb where then captured: the photo of the anterior side of the crab’s left legs below provides an example. These photos severed as structural reference when creating the polygonal mesh and also as the basis for generating the model’s textures maps.

Green Crab, orthographic images of dissection.
Green Crab, orthographic images of dissection, included anterior side of left legs.

The Green Crab is indigenous to the Baltic and North East Atlantic. However it is one of the most invasive species on the planet, and is now quite common on the North East coast of North America, where it was reported observed in Massachusetts in 1817. Today it can also be found as far a flung from it’s native habitat as Australia, South Africa, South America and both and also Pacific coasts of North America.

Green Crab, orthographic images of dissection, underbelly.
Green Crab, orthographic images of dissection, underbelly.

The shape and coloration of the underbelly of this crab indicates that it is a male specimen, of full maturity.

Green Crab, dissection of carapace.
Green Crab, dissection of carapace.

Upon removing the carapace of the specimen shell, much of the interior anatomy of the crab was plainly visible, although starting decompose, now several hours postmortem. Here the large brown gills are clearly visible, bearing a resemblance to mammalia lungs. In the lower center cavity, the cartilage which supported the heart and, below it, that which supported the intestines is still visible. The stomach and intestines, which would have been found in the upper area of the center cavity, had broken down and presumably washed away before we discovered the specimen.

Modeling
After the images were processed, we began the process of creating the poly mesh. Orthographic photos of each major section of the crab were used with in the 3D authoring environment to assist in the development of mesh, as shown images below.

Modeling of crab carapace process documentation.
Modeling of crab carapace process documentation.
Modeling of crab left leg anterior process documentation.
Modeling of crab left leg anterior process documentation.

Each section of one side of the crab was modeled independently, then mirrored and sewed together along the center of carapace and abdomen. The each half of the model was ultimately divided into eight discreet poly-meshes: the main body, each of the four legs, the forward arm, the upper mandible of the claw, and the eye. Once sewn together, there 15 models resulted – the two halves of the main body resulting in one one poly-mesh.

3D Model: Green Crab, wireframe.
3D Model: Green Crab, wireframe.

Once the model was complete, the faces were checked and conformed to have only three or four vertices, known as Tris and Quads respectively. Polygons with more than four vertices are known as Ngons. Ngons are considered bad practice in 3D modeling because they behave unpredictably in rendering and physics engines.

3D Model: Green Crab, wireframe.
3D Model: Green Crab, wireframe.

The final model has 6520 polygons, 6278 vertices, and 8418 UV vertices: a relatively low poly model.

Texturing
Then a UV map – or 2D representation of the polygons’ surfaces were created an arranged with in a square space.

Crab 3D Model: UV set
Crab 3D Model: UV set

The photo reference was then used to create he diffuse, or color, map for the model; the original images being stretched to fit the distorted surfaces of the flattened forms of the UV map. Three more maps were then made. First a normals map, the three channels (Red, Green, and Blue) of which are used as directional data in a rendering engine to determine the way rays (simulated light) reflects off a surface. Normal maps are used create the appearance of more geometry, when in actuality is a trompe-l’œil. The next map created is known as a specular map, which effects the reflectivity, or specularity, of a surface. The diffuse, normals, and specular maps are pictured in the below image. Additionally, a transparency, also known as an alpha map, was created; this map was nearly totally opaque, except for section of the UV map for the eyes.

Green Crab model texture set: diffuse, normals, and specular maps shown
Green Crab model texture set: diffuse, normals, and specular maps shown

Final Model

3D Model Green Crab, 36 frame 360° full textured render.
3D Model Green Crab, 36 frame 360° full textured render.
3D Model: Green Crab, textured render.
3D Model: Green Crab, textured render.

There are some elements of the final model which are not anatomically correct, notably the rear legs, which are flap and flipper like. Furthermore, the red hairs which line parts of the crabs shell are absent, as are some of the antenna on the in the center of crab’s face and around the mouth. The eyes also diverge from complete accuracy, lacking the thing calcified covering upon their shaft. This model could be further improved by the breaking apart of the various sections of the legs and arms and the addition of a skeletal rig for animating.

3D Model: Green Crab, textured render, orthographic view: side.
3D Model: Green Crab, textured render, orthographic view: side.
3D Model: Green Crab, textured render, orthographic view: front.
3D Model: Green Crab, textured render, orthographic view: front.
3D Model: Green Crab, textured render, orthographic view: top.
3D Model: Green Crab, textured render, orthographic view: top.
3D Model: Green Crab, textured render with wireframe overlay.
3D Model: Green Crab, textured render with wireframe overlay.

Model Stats
Geometry: Polygonal Quads/Tris
Polygons: 6,520
Vertices: 6,278
UV Vertices: 8418
Textures: Yes
Materials: No
Rigged: No Animated: No
UV Mapped: Yes
Unwrapped UVs: Yes, non-overlapping

Keywords: crab, fish, ocean, sea, beach, sand, shore, coast, seafood, creature, crustacean, shellfish, low, poly, animal, water, bottom, coral, under, reef, claw, tropical.

1 thought on “3D Model: Green Crab [Carcinus Maenas]”

Leave a Reply

Your email address will not be published. Required fields are marked *