The key to a good photo engraving is a linear projection of pixel brightness values to the surface of the target material. A laser engraving process is in most cases a heat treatment of the surface. The spot where the laser beam hits the surface heats up until the material shows a visible mark. The amount of absorbed energy is responsible for the intensity of the mark. The mark is more deep or dark or other way intense the longer the laser stays on a particular spot and the higher its intensity. Unfortunately the mark's intensity is not linear to the absorbed power. Wood e.g. needs a minimum amount of absorbed energy to get affected at all. Afterwards the affection is getting quite fast very strong. In addition there is some chain reaction effect observable: If there is already a mark on the surface under or close to the laser beam the darker color causes a more efficient energy absorbtion. These effects are leading to a non-linear projection of pixel brightness to the material surface. This introduction shows how to set the engraving parameters to get close to a linear transformation.
First it is important to figure out how the material gets affected by a laser beam. The easiest way is to check that with a greyscale image:
Laser the greyscale with the suggested default parameters on a piece of test material. If there are no marks at the material at all increase the laser's intensity for black.
If the laser intensity is already at maximum, reduce the speed for black. Repeat that until you get a proper representation of the black pixels. The result may look like this (Plywood):
Proper representation of black but brighter pixels are just not represented.
To improve the brighter pixel performace decrease the speed for white pixels. This additionally has an effect of all pixels as their speed parameter is calculated proportionally to their brightness value.
After some tests your result should look like this (Cardboard):
Now you are ready to engrave your first picture.
There are materials or types of wood which are just not able to show fine grained greyscale marks. In that case you need to activate the advanced settings.
Some materials (e.g. bright or humid plywood) are not able to represent fine grained greyscales at all. Then these tricks might help:
If the brighter pixels are not represented properly though the speed for white is already quite low the material probably needs some initial heating up period until the affection is visible. By setting a pierce time you can force the laser to dwell the value in milliseconds each time it is switched on. Too high pierce time values are causing ugly looking edges.
https://en.wikipedia.org/wiki/Dither converts the image into a bitmap of only black and white pixels. Greyscales are simulated by the density of black pixels. This is the fallback to get proper results if the material itself is not able to be affected in a fine grained way.
Besides all the options above the source picture is responsible for a proper looking result as well. Not every photo was shot under perfect conditions in terms of illumination and focus. In case the engraving looks like a big burned blob instead of showing details use the options to preprocess the photo. Contrast and sharpness preprocessing can be combined.
Increases the photos contrast like shown in the examples:
Emphasizes edges like shown in the examples: