Image misalignment with multiple 90 degree passes

MakeFailRepeat · June 28, 2019, 10:25am

I have an Image I need to engrave with one horizontal pass and one vertical pass.

I copied the image onto a second layer, setup the new layer at 90 degrees to the first and aligned the images (using the align buttons).

The two passes are out of alignment though. At first I thought one of my axis had slipped on a fast move but even when I use the Preview tool you can see that they are out of alignment.

Does anyone know what I’m missing?!?

Blake · June 28, 2019, 11:34am

Can you post a screenshot of your layer settings and the preview?

MakeFailRepeat · June 28, 2019, 11:46am

As you can see on the preview they are very slightly out of alignment. It probably wouldn’t be noticable on most projects but i’m cutting a mask for a PCB

MakeFailRepeat · June 28, 2019, 11:46am

MakeFailRepeat · June 28, 2019, 11:47am

Blake · June 28, 2019, 11:50am

It looks to me like you need to adjust your scanning offsets. It’s in device settings, under “Scanning Offset Adjustment”.

github.com

LightBurnSoftware/Documentation/blob/master/ScanningOffsetAdjustment.md#scanning-offset-adjustment

[Return to main page](README.md)

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# Scanning Offset Adjustment

Modern lasers are capable of moving very fast, and with remarkable precision, however firing the beam still takes time.  Some power supplies and tubes may respond in less than a millisecond, but many take longer.

At 100 mm/second, 254 dots per inch means your dots are 0.1mm in length, fitting 1000 of them in 100mm. At 100 mm/sec, if your power supply and tube take 1 millisecond to fire, your engraving will be offset by a full dot width.

At 500 mm/second, that 1 millisecond delay means you'll be off by 5 dots, or 1/2 a millimeter. Still not very much, but visible. Many power supplies and tubes will take even longer to fire.

The result often looks like ghosted edges.  The image below is a 20mm square at 1000 mm/sec, with a 1ms delay, resulting in a full mm of skew between scans:

![](./img/ScanningOffset-Illustration.png)



LightBurn has a setting to counter this, called Scanning Offset Adjustment, in the [Device Settings](DeviceSettings.md):

This file has been truncated. show original

MakeFailRepeat · June 28, 2019, 12:19pm

Certainly seems like something to look into, but I cant see how the LightBurn software could have any idea how my laser performs and so shouldn’t be able to show an offset in the preview?? Surely LightBurn would create the g-code aligned, show it aligned in the preview and then depending on how the machine performs you should adjust the offset (assumingly using trial and error before getting it right).

Attached is a better example of what is seen in the preview window.

Blake · June 28, 2019, 12:30pm

Yeah, that makes sense, actually. Perhaps you’ve already changed those settings though? I know that if you do an offset, it’ll look like that in the preview window.

Let me get some more coffee in me here.

raykholo · June 28, 2019, 2:49pm

Going off on a tangent here, why use an image for something this precise? By its very nature, you have pixels to contend with.

I would think the better way to go about it, and what I once did to make solder paste stencils (before LightBurn though, so didn’t go so well, but I could see it working well now), was to export DXF of the mask layer from Eagle, import into design software, and Fill it. The previous software at the time did not have a vector fill function, so I had to resort to cutting these tiny outlines and it was a huge mess of flame ups. I think scanning vectors might be the way to go.

MakeFailRepeat · June 28, 2019, 2:56pm

You’re absolutely right and I have since made them using a vector fill (which I should have done in the first place )

The interesting thing is that this issue doesn’t appear with a vector fill done in the same way.

Obviously my immediate problem has been solved but it does beg the question whats going on with the image fill process?!? It can’t be anything to do with the offsets as that would apply to vector fills in the same way.

raykholo · June 28, 2019, 3:16pm

I’m still trying to understand and visualize what this all looks like - if you could do an screen capture video of the LightBurn job preview playing for each type (the image scan and then the new vector fill) that would help me.

system · July 12, 2019, 4:16pm

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