The skin was created and inspired from HR Gigers' artwork which was used for the movie Alien. It's a morbid blend of organic elements combined with other-worldly mechanical parts - a fusion of human and alien parts. The rendering style derives from the airbrush which produces graduated blends, and simulate diffused lighting. Bones, sinew, translucent membranes, and mechanical parts are carefully placed on the weapon exterior like hieroglyphics telling an untold story of a race of people fallen to an outside invader of ultimate potency and ruthless terror. The unwrapped vertices for the texture was created by scanning sketches and original drawings into Photoshop, where the art was refined and modified.
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In creating the Xenomorph (the Alien), Giger actually used a human skull around which he built the Alien head using other bones and construction materials.
A hallmark of Giger's work is the repetition of textures which are actually objects when you look at them close up. This was before Photoshop and digital imaging, so each had to be carefully rendered individually.
For the "Alien K-47", each object was first sketched by hand using pencil, then brought into Photoshop as a layer over the Unwrapped Vertices Map. In Photoshop, multiple tools were used to generate the Airbrush-effect. Some hard masking was done, but each surface was ultimately redone freehand to recreate the Giger-esque diffuse lighting and rendering.