Skip to content The Concise RDWorks Learning Lab Series
In Session 22, Russ steps away from the machine to explain the digital-image concepts needed for bitmap and photographic laser engraving.
This session introduces raster images, pixels, image dimensions, resolution metadata, greyscale and dithering, and explains why a laser engraving image needs enough real pixel information for the detail you want to reproduce — without assuming that a higher PPI number automatically creates more detail.
What You Will Learn in This Session
Before photographic engraving makes sense, it helps to separate two very different ways of representing an image. Vector graphics describe shapes mathematically. Raster graphics describe an image as a rectangular array of picture elements — pixels.
A Raster Image Is a Grid of Pixels
Each pixel has a position within the image and stores one or more numerical values describing its colour or intensity. When enough pixels are viewed at an appropriate size and distance, the individual elements merge perceptually into a continuous-looking picture.
The word bitmap is often used loosely for raster images. In some software, including Photoshop, “Bitmap” can also mean a specific 1-bit image mode in which every pixel is either black or white. It is useful to keep those two meanings separate.
Pixel Dimensions Are the Primary Measure of Image Detail
An image that is 2677 × 2677 pixels contains about 7.17 million pixels. Those pixel dimensions tell you how much discrete spatial information the raster actually contains.
Changing only the PPI or DPI metadata without resampling does not add or remove pixels. It simply changes the nominal physical size at which software says those existing pixels should be reproduced. This is the key point behind Russ’s one-pixel-per-inch demonstration.
72 PPI Is Not Automatically a Low-Quality Image
The transcript describes 72 pixels per inch as “pretty crude”. That is too broad. A PPI figure only becomes meaningful when combined with the image’s pixel dimensions and intended physical output size.
A 2700-pixel-wide image can be excellent at one output size and visibly coarse at another. If no resampling takes place, changing its resolution metadata from 72 PPI to 300 PPI does not improve the underlying image; it only changes the nominal print or engraving size associated with those pixels.
Resampling Is Different from Changing Resolution Metadata
When an image is genuinely resampled, software creates or removes pixels. Downsampling discards information. Upsampling creates new pixels by interpolation, but it cannot recover detail that was not present in the source image.
For laser work, this distinction matters. Enlarging a small source photograph to a very high PPI does not magically create engraving detail. Start with enough genuine source pixels for the physical size and detail you want to reproduce.
Laser Engraving Resolution Has a Physical Limit
A laser cannot reproduce arbitrarily small pixels. The effective optical spot size, scan-line interval, mechanical accuracy, material response and process settings place a practical limit on useful resolution.
Sending far more image pixels than the machine/material combination can resolve does not necessarily improve the engraving and can increase processing time. The useful image resolution should therefore be matched to the physical engraving process rather than chosen simply because a larger number sounds better.
Greyscale Is an Intensity Scale
A conventional 8-bit greyscale image can represent 256 numerical intensity levels, commonly numbered 0 to 255. In the usual convention, 0 is black and 255 is white, with intermediate values representing progressively lighter grey levels.
These values are not literally “mixtures of black and white”; they are encoded intensity or tone values. Also, greyscale imaging long predates digital photography. Digital imaging simply represents those tones numerically.
A Laser Can Represent Tone in More Than One Way
A photographic laser engraving can create apparent tonal variation in several ways. One approach is to modulate laser energy according to greyscale values. Another is to convert the photograph to a pattern of black and white marks and use dot density to create the illusion of intermediate tones.
The most appropriate approach depends on the material, controller, laser source and engraving method. Wood, anodised aluminium, acrylic, coated metals and stone do not all respond to power modulation in the same way.
Dithering Converts Continuous Tone into a Dot Pattern
Dithering converts an image with many tone levels into a restricted set of output states — often just black and white — while preserving the visual impression of intermediate tones through the spatial distribution of dots.
Floyd–Steinberg error diffusion is one well-known dithering algorithm. It distributes quantisation error to neighbouring pixels and can work very well for laser engraving, but it is not universally the best choice. Other error-diffusion, ordered-dither and halftone methods can outperform it on particular materials, resolutions or image types.
Display Scaling Can Make Dithered Images Look Strange
Russ demonstrates a dithered image appearing poor at one zoom level and much better at another. The underlying issue is that the image pixels are being resampled onto the monitor’s display pixels. At non-integer zoom ratios, the screen preview can introduce moiré, aliasing or other scaling artefacts that are not necessarily present in the source image.
For critical inspection of a dither pattern, use a 100% or other integer-related zoom where possible, and judge the final result at the intended engraving size rather than relying solely on an arbitrary on-screen zoom level.
Photoshop and GIMP Are Both Capable Raster Editors
Russ uses Photoshop and recommends GIMP as a free alternative. Both can perform the core tasks needed here — resizing, greyscale conversion, tonal adjustment and dithering — although they are not identical products and their tools, workflows and colour-management capabilities differ.
Why This Matters to a Laser User
- raster detail is fundamentally determined by real pixel dimensions, not a PPI label on its own;
- changing PPI without resampling changes nominal physical size, not image detail;
- upsampling cannot recreate detail missing from the original image;
- the laser’s spot size, scan interval and material response determine how much raster detail can actually be reproduced;
- 8-bit greyscale provides 256 encoded tone levels;
- dithering uses patterns of discrete marks to simulate intermediate tones;
- and Floyd–Steinberg is a useful algorithm, but not the only valid choice.
Key takeaway: prepare images for the physical engraving process. Start with sufficient genuine pixel information, choose an output size and resolution the laser can actually reproduce, and use greyscale or dithering according to how the target material responds.
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Transcript for Essential Graphics Techniques for your Laser
The original transcript remains part of the source lesson. It records Russ’s explanation as presented in the video; the lesson notes above clarify the distinction between pixel dimensions, PPI metadata, resampling, greyscale and dithering.
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Last updated August 26, 2021
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