Reading binary STL files in Java

-
I liked the source code from MaskedRetriever user on github and I assumed it would work ok. It seems it was an attempt to create a self-contained tool for 3D printing but somehow it was left at an early stage. But I liked the fact that source code was documented.

My needs were to read STL files and to slice them. As usual you need to get familiar with the different classes involved but it took me not much to get it going. I stripped most of the classes as I was not interested on 3D printing nor on having a GUI for such a tool. However, as soon as I draw the output of the slice process I noticed something was wrong.

While the proper slice looked smooth like the image below:

What I was obtaining was a bit of a mess, though the general ideal was correct. As it was my first attempt I was not sure where the error might be.
After a few tests I realized that the same code would work ok if STL file was ASCII-based but it will fail as above if a binary STL was used instead. 

I had a look at the code and I could see reading a binary STL file involved obtaining float values out of 32-bit IEEE 754 encoded values on the file. It seemed it was there when error was made in the conversion.

Googling around I realized there was a library function in Float class called intBitsToFloat that will turn the 32 bit number into the proper float value. I only needed to get that 32 bit number out of the four bytes read from the file, easy peasy ... or not?

Well, if that were C, and you have b[0..3] unsigned chars, the expression could be something like 
 b[3]<<24 | b[2]<<16 | b[1]<<8 | b[0]

However Java language considers byte data-type as a signed 8-bit field (-128 to 127). And there is a nasty effect with the sign extension that may make you waste a fair amount of time. In summary, the C version won't work if negative values are involved, and they are most of the time (half of the time I would say). So one way of obtaining the proper 32 bit number in Java would be a bit longer : 

 (b[3]<<24)&0xff000000 | (b[2]<<16)&0xff0000 | (b[1]<<8)&0xff00 | b[0]&0xff

Comments

Post a Comment

Popular posts from this blog

VFD control with Arduino using RS485 link

-
Image
Variable Frequency Drives ( VFD ) allow the control of spindles so speed can accurately be controlled and a detailed acceleration profile for the spindle and reverse rotation can all be handled. In essence a VFD is an three phase inverter for three-phase AC motor. I am using a popular (I mean cheap) Chinese VFD and though the reference manual is not great, I could see there is a built-in RS-485 port. I usually control the start, stop and speed selection using the keyboard on the unit but I thought it will be more useful if I could control everything from the same Arduino is doing our CNC table control. Some cheap RS-485 off eBay and some lines of code later, I can start, stop and change the speed from an Arduino. What a cool thing to have! Some configuration of your VFD are needed before you can use it like that.  You need to set PD163=1 (I am using address 1 in the code). PD=164=1 (for setting serial to 9600bps) and PD165=0 (for using ASCII and 8N1 character format)
Read more

How to get sinusoidal s-curve for a stepper motor

-
Image
After some experimentation, and delving a bit into the math of motion, I realized there can be a significant difference on the motion of a machine by just making small changes. A common and popular approach to smooth one axis of motion is to use a trapezoidal speed pattern, where acceleration and deceleration are uniformly accelerated movements (aka constant acceleration motion) separated by a portion happening at cruising speed. The problem comes with the fact that, while that approach is simple to understand and to code, it does contain sudden changes in the acceleration, that show as spikes on the acceleration derivative, called jerk. These sudden changes in the acceleration translate into undesired vibration in our machines. Most of that can be eliminated by selecting a motion pattern that does not contain sudden changes of acceleration. If we chose a mathematical function for our speed whose second derivative is continuous, then we reduce system vibration quite a lot. One ma
Read more

Importing OpenSCAD designs into Onshape

-
Image
After using Onshape CAD software for a while I am really fond of it. So one natural thing to do, specially once they have released the Instructor's kit is to start using it in the classroom too. But one thing I was not so sure how to do was to use old designs I made using OpenSCAD. It does not mean I am completely quitting using OpenSCAD but suddenly I can see I can do assemblies in Onshape that will look quite nice for showing and documentation purposes if only I could easily import from OpenSCAD. I am glad to report that the route OpenSCAD --> FreeCAD --> Onshape worked brilliantly. Even more when a few tricks were applied. While some people suggest to import CSG files from OpenSCAD to FreeCAD, I have had more success by directly importing SCAD files into FreeCAD. One trick that worked well for me was to get rid of $fn references, so now circles become real circles (or cylinders) in FreeCAD. If a part keeps some $fn references then your cylinders or circles wi
Read more