View Full Version : A single, fine horizontal line every .200" ONLY when photo engraving.

Andrew Stow
12-05-2015, 1:29 AM
I've been playing around with photoengraving on anodized aluminum on my machine and it works great, except that I get one, single horizontal "ghost" line every .200" And it doesn't seem to be a problem with the image file itself, as I've tried it with several so far and they all do the same thing.

But it doesn't do this when rastering simple on/off images (like for data plates and such). It only does it for photoengraving.

Machine is an epilog legend 32, "30 watt."

Example 1, run at 100 speed, 35% power, 1200 DPI


Example 2, run at 100 speed, 30% power, 1200 DPI


And this is an example of the simple rastering I've been doing up to now. 100 speed, 65% power, 1200 DPI


It's hard to tell, but the effect seems to diminish the lower the power setting.

Any ideas? I thought about the tickle voltage issue that I've seen mentioned around here, but it doesn't seem to fit what is happening.

Any help at all would be greatly appreciated.

Kev Williams
12-05-2015, 3:52 PM
I'm thinking (uh oh) that the photo engraving data is segmented into 'slices' to save time, or memory, or whatever, and the lines are the borders of those slices. It could be a result of the high 1200 dpi setting-- I usually engrave photos at 300 dpi, sometimes 400 or 500, but 1200 seems extreme (to me anyway)... Best advice I have is to run some tests at 500 dpi with a snick more power and see what happens...

Andrew Stow
12-05-2015, 4:53 PM
Thanks for the suggestion. I've got that dinosaur pic running right now at 100spd/21pwr right now, but I'll try it again at 600 DPI and maybe 100spd/35 power and see what it does.

Been playing around with the image settings and I can kinda mitigate the problem somewhat depending on what power, how I adjust the contrast of the image, etc... it doesn't eliminate or change the frequency of those lines. But it makes them stand out less.

I'm pretty sure the issue is with the machine. Or with me (maybe I'm demanding too much of it?)

Regarding memory... I wonder if the motherboard would support more?

Epilog legend 32. The memory stick on it looks kind of like a PC memory module. No idea what speed or form factor, tho.

Matt McCoy
12-05-2015, 7:08 PM
is the image resolution the same as the engraving resolution?

Andrew Stow
12-05-2015, 7:10 PM
First image is 100 spd, 21 pwr, 1200 DPI. The line halfway up the flag is from the job being done twice on that spot (stopped halfway through and restarted at new position).

Second image is 100 spd, 40 pwr, 600 DPI


The 600 DPI still has a couple of visible lines, but they are far less noticeable. So hard to distinguish that I can't tell if they are appearing with less frequency or if I just can't tell if they're there. I *think* they're appearing with less frequency. If so, I think that would support your theory about memory or job segmenting, Kev.

I'll try another image with these settings and see what happens at 600 DPI.

Andrew Stow
12-05-2015, 7:12 PM
is the image resolution the same as the engraving resolution?

No, when I resampled (resized the image smaller) in corel, I just chose a high enough DPI resolution to ensure no data loss.

Matt McCoy
12-05-2015, 7:28 PM
Try engraving at the same DPI as your image.

Andrew Stow
12-05-2015, 7:54 PM
Try engraving at the same DPI as your image.

So when I import the image into corel, and go to the resample menu, choose 600 dpi for both horizontal and vertical, right? Sorry for the amateur questions, I'm new to the photoengraving thing.

Any theories on why not having matching DPI between the image and the print settings would make a horizontal line like that?

Glen Monaghan
12-05-2015, 10:54 PM
If that's the problem, it's known as an alias error, due to the mismatch or misalignment between the original rows/cols of dots in the original image and the newly created rows/cols of dots in the resampled image.

Andrew Stow
12-06-2015, 1:06 AM
It's still doing it, but in a very, very minor way. I don't know why, but it seems to do it much less and/or is much less noticeable at 600 dpi. Instead of evenly spaced sharp lines, there will be only one or two faint horizontal lines.

Matching the image resolution to the print resolution didn't seem to make it any better or worse. So I suspect I'm just not resampling properly. If that alias error is the issue, that is.

My procedure so far has been to:

1. Import jpg or bmp file
2. resample (set size and DPI of image)
3. Convert to greyscale
4. Adjust contrast
5. Sharpen (unsharp mask)

Is there something in this procedure that jumps out as incorrect?

Thanks for all the help, guys. We're at least making forward progress! :D

Kev Williams
12-06-2015, 12:20 PM
In the 600 pic the one line that stands out is in exactly the same place as in the 1200 pic, the one the passes thru his lower finger. I copied and resampled the 600 pic about 300% larger, and a couple of faint 'secondary' lines aren't in the same place as the 1200 pic--

To me this indicates an issue with how Corel, your machine's driver, or your machine's processor (or any combination of the them) is dealing with the data the laser is trying to reproduce...

About 10 years ago my NH rep brought me 2 18 x 12" beveled mirrors, one engraved, one blank, and a file on a CD to engrave on the mirror. The reason was because a guy with an identical machine to mine (my 25w ULS New Hermes Optima) couldn't get the photo (a wedding portrait) to engrave correctly. He'd never had a problem before, engraving these mirrors was a 'specialty' of his, and he wanted to know if his machine was messed up. Much to our surprise, and chagrin, my machine engraved the mirror exactly as his machine did-- what happened was, at about the 2/3 done point, the halftoning, which was perfect to that point, started going nuts for about 1/4" or so. Then it 'cleared its throat' and finished the mirror perfectly. Other than the 1/4" racing stripe down the middle (the photo was sideways to the machine) it was beautiful. BUT- my Corel, my machine driver, and my machine produced virtually identical engraving results. That issue was never figured out.

I realize this is of no help! Short version, I think your problem is simply a form of banding that we all seem to suffer with at times...

Back when we had the photo contest, I engraved this photo of our boats in our harbor-- pic turned out pretty good except for the terrible banding at the top part of the pic...


Here's a closeup of the top-- note the slice of pic on top, the banding started out horrible so I changed the resolution and ran the full pic below it---
Banding is still there and just as bad, but the frequency of the bands changed........ Just like yours...


-- I think it's just a matter of playing with dots per inch in both directions, speed and power until you find the sweet spot...