3D laser engraving: depth maps and how grey becomes depth

A 3D laser engraving is a carving with real depth: the laser removes more material in some places than others, and the picture stands up in relief. The file for it is a depth map, a greyscale image in which each shade of grey stands for a depth. In laser software, darker means deeper: white is left alone and black is cut deepest. The laser reads the greys either as power, firing harder over darker pixels, or as a number of passes, going over darker pixels more times.

This is a different thing from engraving a photo, and a photo is the wrong file for it. It also needs the right kind of laser. Both points are explained below, from the documentation of LightBurn, Trotec and Glowforge.

What a depth map looks like

A depth map of a coin looks like a soft, foggy picture of the coin: the highest parts of the design are palest, the background is darkest, and rounded forms shade smoothly from one to the other. There are no shadows or highlights in it, because nothing in it is lit. Each grey is a height and nothing else.

The makers describe the mapping in the same way:

• LightBurn. Its Grayscale mode: “Varies power output as a percentage between Min and Max Power, using Min Power for the lightest shades and Max Power for the darkest.”
• Trotec. In its Relief mode “white is processed without power, light areas at low power, dark areas at higher power and black at full power”.
• Glowforge. With Vary Power on, “the chosen power level controls the depth of the darkest parts of your artwork, and Min Power controls the depth of the lightest parts”, and pure white pixels are ignored.

Check which way round a downloaded file runs. For a laser, the raised parts must be pale and the deep parts dark. If a file has a pale background and a dark subject, invert it before use; LightBurn’s documentation notes that its Negative Image setting “inverts brightness, leading to lighter areas engraving deeper than darker areas”.

Why a photo is not a depth map

A photo records light. A pale cheek and a dark eyebrow are different brightnesses, not different heights, and a shadow under a chin is not a hole. Engrave a photo as depth and the dark hair becomes a pit and the bright forehead a plateau. LightBurn’s documentation says of depth engraving: “Images typically need to be specifically created for this use.” Trotec says that high-quality relief data “should be created in a 3D program”, which it calls “a complex process”.

To put a photo on wood, engrave it flat as a pattern of dots. That is what dithering for laser engraving covers, and what Plotcaster makes. Plotcaster does not make depth maps.

Where depth maps come from

• Drawn by hand. Trotec notes that relief data can be made “by using grayscale fillings and gradients” in an ordinary graphics program. Flat greys give steps, like a stack of terraces. Gradients give slopes. Lettering, borders and simple emblems can be built this way: white for the raised parts, a mid grey for a lower level, black for the deepest.
• Rendered from a 3D model. Trotec says high-quality relief data should come from a 3D program, “because such programs generate extremely fine gradations” of grey. LightBurn’s documentation names STL2PNG as a tool that makes a depth map from an STL model, and xTool’s guide links to an online converter from STL to a PNG height map.
• Bought or downloaded. Check the licence before selling anything made from someone else’s file, and check which way round the greys run.

Two properties of the file matter:

• Bit depth. An 8-bit greyscale image has 256 shades, so at most 256 depth levels. Trotec’s Relief mode expects 8-bit. LightBurn supports 16-bit depth maps from version 2.1, which its documentation recommends if you plan more than 256 passes.
• Smoothness. A depth map is engraved literally, so the marks of JPEG compression and the pixels of a small image come out in the material. Use a lossless format such as PNG, at the size you will engrave.

Two ways a laser makes depth

By power, in one sweep. The laser scans the picture row by row and changes its power from pixel to pixel. This is LightBurn’s Grayscale mode, Trotec’s Relief process and Glowforge’s Vary Power. The power follows the grey, but how deep a given power cuts depends on the material, which is why a test comes first.

By passes, in slices. The software cuts the depth map into layers and engraves each layer as a separate pass, so dark areas are passed over many times and pale areas few. This is LightBurn’s 3D Sliced mode: “The darkest shades in your depth map will be engraved with the full Number of Passes, and all lighter shades will be engraved with progressively fewer passes, down to zero passes for pure white shades.” More passes mean finer steps: raising the number of passes “splits the depth map into more layers, increasing the effective resolution of the engraving”. With 10 passes there are 10 levels below the surface, so a smooth slope becomes 10 small steps.

Which lasers can do it

Laser	What the makers say
CO2	The usual tool. LightBurn: “With a CO2 laser, grayscale mode can achieve variable depth (3D) engraving rather than shading.” Trotec and Glowforge describe depth engraving on their CO2 machines
Diode	LightBurn says of its Grayscale mode: “With a diode laser this can provide great shading but is harder to get right than plain dithering.” That is tone, not depth. xTool’s guide does describe 3D engraving on its diode machines, with a greyscale bitmap mode for the blue laser. Follow your machine maker’s guidance and test first
Galvo fibre and similar	LightBurn’s 3D Sliced mode “is only available when using a Galvo laser”
UV	LightBurn’s Grayscale mode “is not compatible with UV lasers”, which cannot vary their power directly

Materials

Depth engraving needs a material that is the same all the way through. Trotec lists solid wood, MDF, clear and coloured acrylic, and Delrin.

For wood, Trotec recommends “relatively hard woods”: cherry, maple, walnut, and also MDF. It finds oak less suitable, because its growth rings differ in density and engrave unevenly, and alder too soft and too quick to discolour. Plywood is not on Trotec’s list.

A first test

Do not start with a large file. Depth engraving is slow and makes a lot of smoke and soot, so the fire and ventilation rules matter more than usual. Stay with the machine.

1. Make a test strip. Draw a row of ten squares filled with 10%, 20% and so on up to 100% black, and beside it a strip that fades from white to black.
2. Engrave it on an offcut of the real material, with your software’s power-by-grey mode. Trotec’s own starting point for wood on an 80 watt CO2 machine is three or four passes at full power and 30% speed. That is a figure for that machine, and yours will differ.
3. Measure the squares. Note which grey gives which depth. If the first few squares are all untouched, the minimum power is too low. If the last few are all the same depth or badly charred, the maximum is too high or the speed too low.
4. Clean and look. Trotec finishes a wood relief with one or two light, fast passes to clear the smoke residue, then scrubs it with a toothbrush, soap and a little water, and dries it well so the wood does not warp.
5. Then run a small real file, a few centimetres across, before a large one.

In LightBurn’s preview, turn on “Shade according to power”, or a Grayscale engraving shows as solid black.

Questions

Where can I get free 3D engraving files? Wherever you find them, check three things: that the licence allows your use, that it is a true depth map and not a photo, and that white is the high part. A quick check is to look for cast shadows. A depth map has none.

Can I turn a photo into a depth map? Not by converting it to greyscale. The documentation read for this guide describes depth maps drawn by hand or made from 3D models, not from photos. A portrait is better engraved flat, as dots.

Is this the same as a 3D crystal engraving? No. Crystal engraving marks points inside a block of glass. This guide is about carving the surface.

How deep can it go? It depends on the machine, the material and how many passes you are willing to run. The makers give no general figure. Your test strip gives yours.

Does the picture need to be mirrored or inverted? Not mirrored. Inverted only if the file has dark for high.

Sources

• LightBurn documentation (2.1), Image Mode and 3D Sliced Engravings
• Trotec Laser, Relief engraving, Relief mode and Tips for laser cutting and engraving wood
• Glowforge Support, Working with manual mode
• xTool, 3D laser engraving