OBJ Files for PolyJet
In previous tutorials in this series, we explained the capabilities of STL and VRML files in 3D printing. In this tutorial, we will explain OBJ files and their expanded capabilities in addition to color, transparency, and bump.
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Step 1: Introduction
In previous tutorials in this series, we explained the capabilities of STL and VRML files in 3D printing. In this tutorial, we will explain OBJ files and their expanded capabilities in addition to color, transparency, and bump.
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Step 2: OBJ Files
Similar to VRML and STL, an OBJ file saves the geometry’s approximation in the form of tessellations. But unlike those two file types, OBJ has the ability to represent the surface geometry more precisely, in the form of mathermatically defined freeform curves of the models surface. This can provide a more accurate representation of the surface geometry.
Using simple mathematical functions to represent the surface geometry, results in a much smaller file size, even though the geometry approximations are more precise.
With OBJ files, geometry can also be specified by tiling the surface with freeform patches instead of polygons. These patches are good for describing surfaces that are organic in form like a car body (not planes, spheres, cylinders/cones as used with solid body modeling) and enable you to save precise NURBS (Non-Uniform Rational B Spline) freeform surfaces.
The advantages of using these freeform patches are similar to the advantages of using freeform curves. They are more precise than freeform curves, since they encode the surface exactly, not approximately. Additionally, these too lead to smaller file sizes.
In addition to thiese accuracy improvements, the OBJ file format lets you store color and texture information in a companion file, called Material Template Library (*.MTL). MTL files contain ASCII text that defines the light-reflecting and material properties of a model’s surface. Such as material type, color, ambient color, diffuse color, specular color, transparency, etc. Using the combined OBJ and MTL files, it is possible to render a multicolor, textured model. In addition to supporting these material properties, the MTL format also supports texture maps, which is a convenient method of assigning complex images and graphics. It does this by using a separate texture (2D image) file in the same way as VRML.
In order for the file to open as designed, it needs to contain all these componants in the same directory.
Although they are more precise and lighter than VRML files, both OBJ and VRML fall into the category of advanced file types that are capable of representing mesh and data. (STL files are only capable of representing a mesh). Our next tutorial in this series will address how to apply transparency to all three file types.
