Obj files and Exocad

[Jabel]

Hello everybody,

In Exocad website they say that obj files are more optimized for design with Exocad. What makes obj files different from ply file ? They both have mesh with texture or vertex color, and while working with both it seems that there is no difference.

Thanks.

 

[Jussi Roivanen]

Err.. wierd statement.

Files are defined - not optimized. Its program that can be optimized (to certain extend) to read/write/operate with specific files.

File types are defined, usually by RFC document (or smthing similar) released by inventor (author) of file type. That defines what directives file may contain and in what format. Then various program developers can make their software to read and write such files. Definitions may change, but they do not change a lot because changes may break compatability.

Objs latest definition (v3) was released.. dunno, 95 or smthing, as far as i remember. By that definition, .obj does not store texture nor color - it contains information about the shape and possible reference to separate material file: .mtl (wavefront material file) -file which contains textures and stuff. Obj files can have something called Vendor Specific Extensions: extra things that can be added to a file - which may contain information about texture and color. These extensions are program specific: other programs might not be able read those extra thingies - nor write such information to obj files.

.ply stores texture and color information as well, it does not need separate material file.

 

[CoolHandLuke]

Hello everybody,

In Exocad website they say that obj files are more optimized for design with Exocad. What makes obj files different from ply file ? They both have mesh with texture or vertex color, and while working with both it seems that there is no difference.

Thanks.

you must compare obj and ply with STL data, not with each other.

STL data is a linked series of triangles that form a curved surface; OBJ and PLY files are not limited to triangles, they can use polygon shapes as part of the methodology of linking vertices to form a pointcloud.

this means obj and ply files can be encoded with colour data if needed. obj as a file format is much older, and has been in use industrially for animation and industrial 3d modeling for decades, ply is a relatively new file format in that respect but it is available as a scan format not just a design format.

 

[zero_zero]

Here's a brief write up about the popular formats used in dentistry, each encodes data differently requiring unique algorithms to perform calculations while handling them. If exo is more optimized code wise to work with obj's... I wouldn't think will stand out performance wise on these fast PC's we use, given the relatively low poly count of the models.

 

[Jabel]

In response to one message above, as far as I know, not only obj has surface normals to indicate the orientation of the 3d object. Stl and ply also has this information embeded, the orientation is determined by cross vector or using the right hand rule. So it's not specific to obj.

Obj the wavefront format is generally an end product of the CAD. It can support not only meshes but also curves and NURBS. But when it is derived from a scan ( point cloud),I think obj is just a simple mesh like stl. The only difference is that it supports other attributes like color and texture, in a separate MTL file of course. When I open an ASCII obj file in a text editor, I didn't see something different that could make obj more suitable for use with Exocad, it' s the same structure as an stl, same vertices and facets counts when I export the same model to different formats.

When we scan,I think the only difference with stl is color and texture. So this optimization must be something related to color and texture.

 

[erzdaemon]

JFYI

Yes, the Surface normals in stl/ply are interpreted by "looking" at a the triangle. From one side the 3 "Corner Points" are "numbered" clockwise, if you look at the triangle from the other side, these "Numbers" then read counter-clockwise.

The software then throws a perpendicular "Line" that makes the surface normal. That's how it knows the "Triangle's inside" and "the triangles outside" ( the red coloring in some software shows this "inside" of the Object, eg. Asiga Composer.