These results are really interesting to me and not what I’d expect to see based on my understanding of how these two processes differ.
Here is what I think i know. The dithering process takes a continuous tone image and converts it into pixels that are either black or white. The different dithering options are just different calculations or methodologies for making that conversion from continuous tone to black and white dots.
Grayscale is exactly what it sounds like. The image, instead of being converted to black or white pixels, is made up of pixels that are white, black, or some shade of gray. The number of shades of gray depend on the bit depth of the image. An 8 bit image would have 254 shades of gray plus black plus white.
The way the laser engraves each is different. For a dithered images, the laser scans back and forth and turns the laser on or off rapidly at a constant power (whatever max power is set to). Everywhere there is a black dot the laser is on and where there is a white dot the laser is off. For multiple black dots in a row the laser stays on and actually creates a line. The y axis then moves by the amount of the interval (inverse of the dpi) and burns the next row of dots/lines.
For a grayscale image the laser scans back and forth with the laser constantly on however it varies the power of the laser according to the level of gray for each pixel (between min and max).
We all know, particularly with a non-binary material such as wood, how dark the burn appears depends on the power of the laser. For a given speed, a lower power results in a “less black” burn. If the relationship were totally linear you’d see something like a very light brown for a low power progressing to an increasingly darker brown approaching black for a higher power.
Because of this, when trying to an engrave an image, I’ve always used dithering because I want everything that’s burned to be a consistent tone (black ideally) with the appearance of “grey” coming from how tightly the dots are clustered. With an area of relatively few black dots the brain perceives a lighter tone, high concentration of dots the brain perceives a dark tone. But in reality all the dots themselves are the same level of “black.”
Greyscale on the other hand I’ve always thought was used specifically when trying to do 3D depth engraving. You set a combination of speed/power such that the lightest shades of gray are burned only at the surface where the darker shades are burned further down into the material.
So what’s puzzling to me is that Jim’s greyscale example doesn’t look like what I’d expect to see with greyscale. To me it also looks dithered. I certainly can’t argue with results.
Maybe my understanding is wrong? Maybe it’s different with a diode laser? Jim’s greyscale image seems to have blacker blacks or more contrast but I believe simply increasing the power or slowing down on the dither would have produced the same black level. But it still looks to me like his greyscale image is still a bunch of dots burned at the same power. I’d expect to see more browns in the greyscale where the power was reduced.