Hello, 

I will just start by asking if this is even something that is possible to achieve in Inkscape; as I have very little experience with graphic editing software. I thought I was onto something with Floyd-Steinberg Dithering but can't seem to make that work. I am now trying to create this line effect from an actual image or picture for laser engraving. I know it's able to be done because the images I added will laser engrave with no problem and were made from stock images. Unfortunately for me, I have no idea how this effect is made and feel that it may need other software. My laser software wants it to be one single vector image if that helps at all. 

It may be possible to create a similar effect in Inkscape, but it may result in massive data that is unusable.

Please expand on your comments that other images added have laser engraved, but also your laser software wants a single vector image. These seem to be inconsistent.

Information on the laser and its software may also be helpful.

Some laser softwares can work with raster images directly. But if it's a must to replicate the effect, there are always options.

Representing a greyscale image with a black&white dithering is based on the raster format.

The calculations to determine wether a pixel is black or white is always based on a neighboring pixel, and not only the reference image. 

Svg filters don't have the toolset to replicate the effect and the scalability would also be an issue.

Leading to the second point: you'll need to start by using a raster image editor ("to do it properly").

In gimp, you can change the colour mode to strictly black&white with some options on dithering. 

Then, you'd need to scale up the image -like 200 or 300%-, and add some interlacing effect. 

After that, export to a raster image and centerline trace that in inkscape.

Needs some testing. Have doubts that the result would be optimal for laser engraving, although there is the arrange/z-order extension which may help it a bit.

@TylerDurden My laser will engrave the images I attached. They are from another customer and were supplied as a sample engraving. The software says it's converting the image to 1 vector image when imported. These files are also saved in the software for the laser, so it may be saying this because it's reading the .MCL format it was saved in for the laser software. When new files are imported to the MarkUS software they have to be in a .DXF/.DWG or a .AI format. I'm not able to ungroup the lines from each other in the software once it is imported. I can only scale its size, and the scaling gets to be a worse looking image the larger/smaller I make it. So somewhere, someone, was able to take a normal photograph and change it into this line effect. The lines are all the same color in the images, the grayscale look is determined by the spacing of the lines and their length. I'm not sure if that helps at all, like I said I'm new to this and all the new software/terminology. 

@Lazur I will try the instructions you laid out and report the results. My laser may read a raster format also if there is another way to achieve this effect.  However, I can only import a .DXF/.DWG and a .AI format file to the laser software I use. Not sure if that helps you at all either but figured its worth mentioning again. 

Thanks for the help so far!

Atom1223

the images I added will laser engrave with no problem and were made from stock images.

Please attach an example of one of these images. (The actual file(s), not a screenshot.)

Here is a quick example as a proof of concept:

https://openclipart.org/detail/346583/mountain

With some 180k+ nodes (didn't adjust the contrast before).

Here is an original image of the owl.