Question on Engraving Granite with Epilog Machine

[David Rust]

When Processing a picture for engraving on Granite I use a Dithering setting of "Floyd Steinberg" or "Stucki" or "Jarvis". After I am happy with the picture processing I set up for engraving. In the Epilog Dashboard there is also an option to choose Dithering ("Floyd Steinberg" or "Stucki" or "Jarvis", etc...).

Question:
Is there an advantage to "re-applying" dithering since I already applied dithering while processing the photo? Will "double dithering" make the picture better or does it "dilute" the pre-processed results".

I've done some comparisons with mixed results. I am curious as to what others do and your opinion on it.

Thanks,
Dave

[Frank Corker]

I don't think double dithering will help. Normally the standard setting for granite will be sufficient, I have found very little difference between Stucki jarvis etc. If you have a decent image and you prepare it in the normal way with a good even black and white GREY the Epilog will deliver a good result. Photograv is a great program for that, search for the Gold Method here on the creek and you will find a really good alternative processing method which Rodney Gold developed. Average speed your should be looking at is around 33% power to 100% speed, use more power and it will obliterate the surface too much.

[Aaron Koehl]

Dithering twice won't yield any consistent results; but checking to see that your dithered graphic resolution matches your engraving resolution will be very important. The process if dithering "breaks apart" solid areas and makes them more sparse to reduce the amount of engraving/ink/data/etc is used for a particular graphic, but is keyed to the resolution. The various dithering algorithms affect the "patterns" used when dithering, ideally no dithering pattern is visible in the finished dithered image. Brightness and contrast will have the greatest impact.

[David Rust]

Hi Aaron,

Here is an example, I processed a picture using a manual process recommended on a SMC post a while ago. IMO the process gives great results.
Process:
1. Convert to 8bit Gray scale
2. Adjust Contrast
3. Adjust Gamma
4. Adjustments with "Unsharpen" tool
5. Convert to 1-bit black and white, Apply Stucki (or what you like) Dithering
6. Convert back to RGB (for some reason the original RGB gets transposed to CMYK in the process)
7. Invert for black granite
When the file is rastered on black granite the solid areas (black in the file, white on granite) has a noticeable pattern/grid look to it. If I apply the Epilog Dashboard dithering the pattern/grid effect disappears.
Therefore I believe that the dithering in the file processing (steps above) makes the picture “recognizable” when lasered, a good thing! Then the dithering in the Epilog Dashboard breaks up the pattern/grid of the black areas in the file, resulting in a very good lasered picture.

Thoughts? I can’t argue with good results. I am just trying to understand a little better.

[Scott Shepherd]

David, what happens when you skip the "Apply Stucki" in step 5 and just do it at the laser? That, technically, should give you the best result. Real world, it might not, but from a technical standpoint, that's your best option.

[David Rust]

Scott,

In CorelDrawx6 to convert to 1-bit I use "Bitmaps", "Mode", "Black and White (1-bit)". Within that window you have to make a dithering choice, I can't "opt out" of it. however you can use "Bitmaps", "Convert to Bitmap" and then select "Black and White (1-bit), at this point you can un-click the "Dithered". Following the path of skipping the dithering yields very poor results.

It appears that using dithering both when pre-processing the file and again on the Epilog dashboard results in some pretty good results.

Here is a pic (I struggled trying to get a decent pic... I think the camera is focusing on the reflection)

258651

[Scott Shepherd]

Well, with results like that, I'd keep doing what you're doing. Looks great!

[Dan Hintz]

In CorelDrawx6 to convert to 1-bit I use "Bitmaps", "Mode", "Black and White (1-bit)".

Reduce your image to greyscale, not B&W... this will allow you to use the dithering of the Epilog without dipping into the pot twice.