Altium "Use software arcs" option - what does it do? - Page 1
Forum section: electronics / general

author: faststoff
author_rank: Contributor
timestamp: August 06, 2019, 04:07:46 pm
content: Hi all!In the "Advanced" tab of the Gerber Setup menu in Altium, one can tick the "Use software arcs" option (JLCPCB seem to be recommending this when creating Gerber files:https://support.jlcpcb.com/article/42-how-to-export-altium-pcb-to-gerber-files). Could anyone explain what this option actually does and why it is needed?With this box ticked, the Gerber files output by Altium are altered. I notice this especially on small arcs on my board outline mechanical layer. With the option ticked, arcs seem to be converted to a discrete set of lines. The picture attached shows a side-by-side comparison of the Gerbers without/with this option ticked. The rendering of the Gerber is done using the Ucamco reference viewer, so I guess it is not a rendering problem (I observe the same in gerbv and ViewMate).While it probably is OK, I must say I do wonder about the specific details here. I have little knowledge of the Gerber format. Gerber software arcs are only briefly mentioned around the internet, sometimes together with hardware arcs. Any help here on what this option actually does and why it is needed would be very helpful!

author: Pseudobyte
author_rank: Frequent Contributor
timestamp: August 06, 2019, 07:32:24 pm
content: When you use the software arcs functionality in the CAM processor, you are telling altium to generate curves and arcs by approximating them with straight lines. This is useful for ensuring compatibility with older (outdated) CAM software. The latest standards of the gerber X2 format support "hardware arcs" which is a gcode command that tells the machine to interpolate the arc given two points. The specifics of the command I am not super familiar with, but i believe you specify one point, give the interpolation command, then give the second point and the angle to rotate through.A line in a gerber file is. Move to one point (light off/not exposing), move to another point (light on/exposing).Software arcs gives you reverse compatibility at the cost of data accuracy and file size, depending on the minimum line length when you approximate curves.Hardware arcs are a perfect representation of your data.Hope that helps, and if I was wrong somewhere in there I apologize and someone should correct me.~Bryan

author: faststoff
author_rank: Contributor
timestamp: August 08, 2019, 07:43:57 am
content: Bryan,Thanks for the nice and clear explanation! I understand it better now.Max

author: Nominal Animal
author_rank: Super Contributor
timestamp: August 08, 2019, 09:18:21 am
content: The Gerber file format is documentedhere(download page); see page 9 (section 2.10), Table of Commands. When using software arcs, the application approximates the arc with line segments, as older Gerber utilities may not support real arcs.Gerber arcs are always circular, never elliptic. The underlying format is such that if one tries to draw an elliptic arc, the centerpoint is adjusted, so that the arc itself will be a circular arc. This makes mathematical sense, and basically all implementations will do it the same way (because math); this means that gerber viewers' renderings should match what you get as a result in real life.