The Tank Head Mantis concept, originally designed by Emerson Tung (http://www.emersontung.com/), was an interesting piece in and of itself to model. The challenge was in creating something of this scale to improve skill sets and push boundaries.
To begin, the proportions were estimated based on the assumed given scale. To do this, the cab in the reference image was examined, and it was assumed how big a person would have to be to fit in there. Taking that, assuming the operator was about 6ft tall, the mass of the machine was mapped out. Since there was only one view to work from, several other possible views were sketched up based on the given mass. This part was done in Manga Studio. This also helped create a vision for the back of the machine, for which there was no view given.
After analyzing this gigantic mech, the many individual pieces were unique, but not impossible to make. There were just a lot of them. The vast majority of the blocked shapes on this piece were created with subdivided primitives, but a good amount were spline generated as well. For example, the wheels were tori with soft-selection created shaping to them, while the wheel cover was created with a spline initially, but worked with in a subdivision manner once extruded and converted into an editable poly. The initial goals for getting this blocked in were just wrapping my head around the dimensions of this machine, and getting the basic shapes blocked in well.
One way the scale was kept in check was by importing a 6ft tall human figure into the scene to help with visualizing. After several weeks of blocking the right leg and claw section, it was time to mirror it over to check width. It ended up being too widely spaced apart, and the tires needed to be wider as to bear the weight of the machine better.
Here are some images to demonstrate how some of the more interesting shapes in the scene were created:
After several more weeks of refinement, it was time to do some comparisons to the reference. Taking the images in Photoshop, the two were stacked on top of one another this time, and the top layer was turned visible and invisible to compare back and forth between the reference and how the model currently looked. Then, notes were written on a higher layer in order to keep track of all the things that needed adjusting.
A render with the previous notes adjusted:
Finally, it got to the point where it was time to move on to texturing. Here are some grey shader renders of the Mantis at that point:
For the texturing of this mech, there were only a couple of main base textures that needed to be created: the painted metal, the rubber and grit of the tires, the glass, and the tarnished metals. To get some real-life reference, a day was taken to visit a construction site here in Dallas to take some photos of the equipment and get some insight. Here's a link to my Pinterest board that contains all of those reference photos: Mantis Reference Board
Going out and seeing that material firsthand helped me gauge the correct specular and glossiness values when creating the metal textures. Seeing all the caked up dirt in between the wheels also inspired the extra bumpiness around the wheels on the model. It was an interesting experience.
For the most part, the textures were all procedural textures that I put together, save for a couple of custom grunge and scratch maps that were created specifically for the task. Working with procedurals is a preference of mine, as the same texture can be used as a base coat for many similar objects in a scene, rather than having to unwrap every one of them. Sometimes it can be advantageous to work with these time-wise, and one can get astounding results with just a few procedural textures and some carefully placed vertex painting. Here is an example of a material that I generated to create a lightly scratched, worn looking metal surface: