Help on Grayscale and Dither

[Rod]

Hi
I need some serious help on grayscale and dither photo, as none of mine is turning out the way I would like. My problem is trying to determine the resolution.
What I know:
25.4 ppi is equal to 1"
So, I made a list of 25.4 /resolution(starting with 0.5 - 0.30)
25.4/0.05=508 etc.
25.4/0.1=254
25.4/0.2=127
25.4/0.3=84
Now do I use the original picture pixels or the dpi(ppi) to do the calculation to get the resolution  ex. I want do a 4x6" (101x152mm), 25.4x6=152.6
Question 2. Next is the feed rate, is there a calculation for that?
Currently trying the T2 logo @100x54, Res 0.06, Feed rate 800
Thanks for your help and understanding for a newie
Rod

[Zax]

Resolution is related to the output quality, but there's also a limit in the dot size of your laser. The 2500mW are typically 0.16~0.18, lower power lasers may be slightly smaller 0.12~0.18 and the higher power lasers more like 0.16~0.24.

So, let's assume your laser is focused at 0.16. You could pick a resolution of 0.15 for fine quality engraving (also slow), if the design is small and/or has a lot of detail this may be right, otherwise I normally de-focus slightly and use 0.2 for speed.

As you said, 0.2 would be 5 pixels per mm or 127 dots per inch. If you export an image at the size you want and select 127 DPI it will generate the correct pixels for the laser. If you use 254 DPI it would generate twice as many and be double the expected size, although you can and should set the output size in T2Laser to fix it.

So now you have resolution set, the feed rate depends greatly on the material. I would start at 800, if it's too dark increase to 1000, then 1200 up to 2500 or maybe a little higher and then if it's still too dark reduce the laser max power by 50. If it's too light just reduce the feed rate until it looks good.

As a reminder, if you are using my Grbl 1.1e (probably should be) then the laser min/max are 0 and 255.

[ggallant571]

Zax has the authoritative answer, I am lazy and create and etch all images at 254dpi. Makes it simpler to keep track of desired power and speed levels.

[Rod]

Ok, sorry if I am a pain and I apologized if I am, just trying to get my brain to understand.
My equipment is a couple of laser both 2500mw. The current one I am working with is a A3 Pro, flash both with G 1.1e.
The first picture is what my laser look like, The second a picture that I did with 0.16 on cardboard due to the fact I am almost out of wood.(i have a stack of nothing but black mark)
I have two picture I want to engrave 153x204 px and 599x599 px.
The first one is in the picture, here the setting I use:
Feed rate 900
size 100x134 (when I change it the output shown 631x842 px) not sure what that means.
Max (0-255)
Res 0.16
FR(vector) 2000
RFR3000
can you dumb it down a little more, that I may understand better.
again thank you for your patience 

[ianchia]

Ok, sorry if I am a pain and I apologized if I am, just trying to get my brain to understand.
I have two picture I want to engrave 153x204 px and 599x599 px.
The first one is in the picture, here the setting I use:
Feed rate 900
size 100x134 (when I change it the output shown 631x842 px) not sure what that means.
Max (0-255)
Res 0.16
FR(vector) 2000
RFR3000
can you dumb it down a little more, that I may understand better.
again thank you for your patience

Here is my take with my graphic designer hat on. For best results, you should *not* focus on pixel dimensions but on the actual millimeter dimensions. Then calculate what you need for your source files from there.

Let's take your example of a source bitmap image with a pixel dimension of 153x204 px. I create a blank bitmap image (I'm using Photoshop, but you can use the bitmap editing tool of your choice). If I create an image at 153x204 pixels, at 127 ppi (which equals a resolution of 0.2 in T2 Laser), that gives me a physical size image of 30.6mm x 40.8mm. That's a very small postal stamp sized image. You can resize it to a larger image using T2L or your image editor, but the rule-of-thumb of garbage-in->garbage-out applies. You will get a blurry photo if you resize that little image to a larger image, simply because the details are not present within your small 153x204 source file.

Let's say you're targeting an photo to be lasered onto wood at 7 x 5 inches. That equals 177.8mm x 127mm. At the 0.2 T2 resolution setting, you should have a source image of *at least* 889 pixels x 635 pixels, because that's the number of pixels required for a 127 pixels per inch resolution. If you're aiming for a 0.1 resolution = 254 ppi, then your original source image should be at least 1778 pixels x 1270 pixels.

Does that make sense?

- Ian

[Zax]

Ian has given great advice.

The only other thing I noticed on that first picture is the laser looks like a line rather than a dot, are both like that? I have seen this on the cheap 5500mW lasers but usually the 2500mW should be much more round. You may want to check the lens.

[ggallant571]

I would start with a "known" test pattern on a "known" material. For testing I prefer "Neon Colored" index cards available at Walmart for $2.50US per 100. They react very nicely at low power.

[BigAlienRobot]

That looks like a poorly focused laser...

[Rod]

Ok guys
Here the result of all your guys hard work. I went into Gimp and change the resolution to 254, change to grayscale. Set the resolution the the following (see picture)
First I adjusted the laser and engrave the first one. It just came out as a n outline, I wonder WTF. Then I notice the light was on and check the low light (I left it on lolo) Then  I re-engrave and this is the results. You are great teachers for this hard headed guy.
Thanks again for your help.

[Zax]

That's a huge transformation, it's nice to see the improvement with the correct settings.

[ggallant571]

We ALL went thru the same learning process.

[ianchia]

Yup. Still learning stuff all the time and I've made a decent amount over 2 years. I'm about to embark on actually cutting 3mm material with proper air assist, instead of just focusing on engraving on stuff, or working with paper and cardstock

Back on the original topic of your photo, the grain of the wood will also affect the fineness or resolution of the result. If your grain is coarse, a larger picture would appear more effective as the wood grain wouldn't impact on your photo's tonal quality and you could probably get away with a 0.2 resolution.

One of the big traps I first stumbled on was trying to engrave fine details into wood. There's a certain point where it no longer works, unless you do a great deal of prep work on the wood beforehand to obtain a smooth, consistent surface. (Like this...) Once you've settled on a resolution you're happy with, then as Zax pointed out, trial different feed rates - this will give you lighter or darker results, since the laser moves quicker or slower over the wood. To avoid wasting too much stock, you should go with @ggallant571's tip about using the neon card stock to get a feel for how the settings affect the end result, and then when you're ready to move onto the wood stock, I would personally crop a 30x30mm section of your photo where there's a full range of light and dark shades within the crop. Then use this 30x30mm test image and burn a series onto the wood with different feed rates until you're happy with the maximum range of dark and light tones. THEN I would do the whole piece for real at full size.

- Ian

[Rod]

@Ianchia, thank for the tips, yes I went to Walmart and brought some 4x6, 5x8, 8x11 card stock (after I ran out of wood lol)also did look at the post you gave me, a lot of information there. In the future, (about yr) I would like to get a CO2 laser engraver( the more inexpensive one) and start up a very small business, but that depend on how I learn. I was doing 3d printing and like that , so I though I try this, a lot harder than I thought it would be. But being on this forum, has help a lot for me to understand. I may have to start keeping notes, that I read here for future reference.
Thanks again
Rod