Preparing a High-Contrast Height Map for STL

Step-by-step

1. 1

   Start from 16-bit if you can

   A higher bit-depth height map reduces banding. If you must use 8-bit, keep gradients smooth and avoid aggressive contrast jumps. An 8-bit greyscale gives you 256 discrete levels. Across 4 mm of relief that is 15.6 microns per step, which sounds generous until you print a slow gradient — a sky, a cheek, a background wash — and every level boundary shows up as a terrace running across the surface. 16-bit gives 65,536 levels and pushes those steps well below anything the layer height can resolve. Photoshop, Krita and GIMP 2.10+ all handle 16-bit greyscale PNG; the file is twice the size and that is the entire cost.

2. 2

   Normalize to clean grayscale

   Convert to a single-channel grayscale image. Remove color noise and keep the background uniform. If the source has JPEG blocks, switch to PNG early to avoid amplifying artifacts. Actually drop the image to one channel rather than leaving an RGB file that merely looks grey; a converter that averages three channels will happily pick up chroma noise you cannot see on screen. Then read the histogram. If the background is meant to be the base plane it should sit at a single value — pure black at 0 for the lowest point — not drift between 3 and 11 across the frame, because that drift turns into a gently sloping floor that ruins how the piece seats.

3. 3

   Clean noise and compressional artifacts

   Run light denoise or a tiny Gaussian blur. The goal is to remove speckles that would become spikes in the relief mesh. A 0.5–1.0 px Gaussian is usually enough on a clean scan; a photo shot at high ISO wants a proper denoiser first. Judge the amount by what one stray pixel would become in Z. On a 60 mm map rendered at 1800 px, a single pixel is 33 microns in X — but a full-range white speck is millimetres tall in Z, a needle sticking out of the surface for the nozzle to knock off and drag across the print. Blur is cheap insurance. Past about 2 px, though, you start rounding off the edges you wanted to keep.

4. 4

   Control contrast without clipping

   Use levels/curves to set the dynamic range, but avoid crushing blacks/whites into flat plateaus unless that’s intentional. Clipping produces terraces and visible banding in the printed relief. Treat the greyscale range as your relief depth budget. If the model is 4 mm tall in Z and the artwork only uses values 60–190, you are throwing away half the available depth and the print comes out flat and mushy. Stretch the range so the deepest point sits at or near 0 and the highest near 255 (or 65535), but leave a little headroom at both ends — a handful of clipped white pixels become a plateau exactly where a nose or a hairline should still have detail.

5. 5

   Convert to STL and validate relief

   Upload the PNG to the PNG / SVG → STL converter, then inspect the surface in your slicer preview. If you see stepping/banding, reduce contrast or increase source bit-depth. Match the map’s pixel count to the size you are printing, not to whatever the camera produced. One sample every 0.1 mm is plenty for FDM: a 100 mm plaque needs about 1000 px across, and a 6000 px source only buys triangles. Every pixel in a height field becomes two triangles, so 6000 × 4000 is 48 million of them — a file most slicers open slowly and some refuse outright. Downsample with Lanczos or bicubic before conversion; it averages out residual noise for free.

6. 6

   Sanity-check thickness and scale

   Relief prints still need a base and a print-safe thickness. Verify X/Y/Z in the slicer and use Scaling & thickness as a reference for mm sizing. Give the piece a real base under the relief. 1.5–2 mm is a sensible floor for a plaque you intend to hang; below about 1 mm a flat panel curls off the bed as it cools and takes the relief with it. Base thickness is separate from relief depth — a 3 mm total height with a 2 mm base leaves only 1 mm of modulation, which is often exactly right for a subtle carved look and far too little for a portrait.

7. 7

   Invert when you want intaglio instead of relief

   Height maps assume white is high. Invert the image and white becomes the deepest cut, which is what you want for stamps, moulds and press tooling where the printed part is the negative of the finished object. Two things change when you invert. The background that used to be your base plane becomes the top surface and now has to be a clean solid value. And the finest details point into the material, where the nozzle has to reach them — a detail that prints happily as a 0.6 mm ridge often fails as a 0.6 mm groove, because a slot narrower than about 1 mm never gets a clean perimeter on either wall. Widen those strokes before you invert, not after.

8. 8

   Dial in lithophane thickness and backlight

   Lithophanes are height maps read in transmission, so the numbers change. Thin means bright: 0.6–0.8 mm for the whitest highlights, 2.5–3.0 mm for solid black, with the grey range mapped in between. Print them standing on the short edge at 0.1 mm layers, 100% infill, no top or bottom shell beyond the walls, in white or natural PLA — pigmented filament kills contrast. Slow to 30–40 mm/s so the perimeters fuse properly; any gap between them reads as a bright pinhole once the panel is lit. And remember to invert the source, because in a lithophane the darkest part of the picture is the thickest plastic.

9. 9

   Preview the relief before you commit an hour to it

   Open the STL and look at it from a raking angle with a light, not straight on. Grazing light exposes terraces, the ringing halos left by oversharpening, and the sharp cliffs where a levels adjustment clipped. Then slice it and step through the layer preview around quarter and half height: if a large area of the model appears or disappears between two adjacent layers, you have a plateau that will print as a visible shelf. Fixing that in the image takes a minute. Finding it after a four-hour print costs four hours.