DPI and PPI with respect to power, speed and output

[James Terry]

I am doing some test learning tonight before I engrave my first plaque with the laser and I have found that when you select a higher DPI in the driver, the output burns more. Perhaps that makes no-brainer sense, but it isnt something I have thought about until now so I was hoping you guys might give me a nutshell brief on how this all inter-relates and works to provide best output.

In this example, the source image is 300dpi and the left output was selected in the driver as 300dpi, 40,100% and the right is 600dpi, 40,100%.




Plainly I see more burning at 600dpi, so is the driver interpolating the extra pixel points then?

Did I slow down the output by selecting a higher dpi?

Is there a benefit by selecting a higher dpi and then using a higher burn speed?

When does dpi matter? When does ppi matter and should I just be leaving mine on auto for the time being?

What should I be targeting for dpi of my artwork?

Should the art and the print always match?

In the examples above, I prefer the darker burn on the right but I could do without the overburn/surrounding marking. To correct, should I just speed this up, select a lower dpi, select a lower power...? How does one decide how to load all of the variables to get an optimal output? It seems to me that varying just one of the variables would have similar results as varying just one of the others.

[Niklas Bjornestal]

If you do double the amount of dots you'll get the double amount of power.
It doesn't interpolate the pixels, but uses the closest pixel. It works the same as for example a laser printer, they (usually) use at least 600 dpi when printing.
It will take twice the time to run the jobb, each line will engrave with the same speed, but you'll need to make twice the amount of lines.

The resolution of the art and engraving doesnt have to match (except maybe when using photograv).

PPI is the same as DPI but for cutting (at least on laserpro's machines).

[Richard Rumancik]

If you do double the amount of dots you'll get the double amount of power.

I agree with this statement.

. . .It doesn't interpolate the pixels, but uses the closest pixel. It works the same as for example a laser printer, they (usually) use at least 600 dpi when printing.
It will take twice the time to run the jobb, each line will engrave with the same speed, but you'll need to make twice the amount of lines.

Not so sure what you are saying here, though, Niklas. First you say that it uses the closest pixel as it won't interpolate. But then you say that it will require twice the number of lines. Isn't that interpolation?

If it only used the closes pixel, then plotting a 300 dpi artwork at 600 would result in hitting each pixel twice, which is equivalent to engraving the entire image at 300 dpi twice. I had in my mind that when an image was plotted at twice resolution, that it would be equivalent to resampling the artwork to twice the dpi in Corel. That "fills in" the missing pixels, although it may introduce some jaggies.

Can you clarify?

[Mark Winlund]

This doesn't entirely answer your question, but when viewed under a microscope, I have noticed an interesting thing: quite often, an engraving in wood that looks dark is the result of light getting "lost" in the vertical "canyons" created by the narrow laser beam. When examined very closely under a microscope, the surface color due to charring is actually the same from sample to sample.

Imagine a piece of wood engraved with a single line to a depth of 1/4". When looked at closely, the line looks very black. The light entering this "grand canyon" simply reflects off the walls multiple times and gets absorbed before it can get out again.

To create this effect, the laser must be sharply focused to create the "canyons".

To test this, I have done a series of engravings with rastered letters using different levels of "quality" (Universal's term for the distance between scan lines). In every case, the lower "quality" setting produced a darker looking image. Under the microscope, there was no difference in surface charring. The power and speed settings were the same throughout the tests.

So, a tentative conclusion would be that some of the differences in apparent "darkness" are due to optical effects and not charring of the wood.

Please note that I am not saying that the wood species has no effect, just that there may be other factors at work here. The difference in surface charring between maple and alder is quite obvious, for example.

Regards, Mark

[Rodne Gold]

More dpi = more overlapping of dots , the laser ALWAYS works by "dots" , even when cutting.
The reason you get a darker burn is cos by overlapping you dont just vaporise the wood , you create large heat affected zones which gives you the branded look. A high power laser will actually engrave wood a lot cleaner with almost no char as it vaporises more effectively.
You can go a teeny bit out of focus and use a lower DPI (for more speed) and you should get a shallower but darker "burn" Using less air assist when rastering wood could also give a darker "burn" - soft resin free woods (like balsa) wont engrave dark no matter what you do cos its the resins in the wood that burn and give you the darkness.
PPI matters when vector cutting , it sets the amount of pulses or "Dots" per inch of travel - too little and your cut looks like perforations , too much and you load too much energy into the cut resulting in either scorching or bad melting.
Artwok DPI and printing DPI are different things , in fact artwok DPI is actually NOT Dots per inch , but pixels per inch. A pixel is interpreted by the printer (laser)
To give an example , lets say a pixel was interpreted as a 3 x 3 dot (laser dots) element , then a pure black pixel will have all the 9 cells "coloured" in , a light grey might just be one cell coloured in. In reality the laser cannot possibly resolve more than 100-200 pixels per inch in the original picture and it is relatively useless to use more than that. DO NOT confuse the DPI a program like Corel says a picture is with the ACTUAL pixels per inch it contains.
If a picture is 50 pixels by 50 pixels , then a 50 dpi OUTPUT device will display/output it as 1" x 1"
A 200 DPI output device will output it as 1/4" x 1/4" at its native resolution.
I am being simplisitc here to illustrate the point , there is more to it like dither and 1/2 tone screens etc etc etc.

[Richard Rumancik]

. . . In reality the laser cannot possibly resolve more than 100-200 pixels per inch in the original picture and it is relatively useless to use more than that. . . .

Rodne, I'm confused here . . . you seem to be saying that you would not use a setting on the laser higher than 200? If the beam diameter is .003 and you wanted the dots touching, you would need 333 dots per inch of laser travel. But it looks better if the dots overlap. I don't see how you can get a good image at 200. You would have some serious jaggies on text at 200 dpi.

[Richard Rumancik]

Plainly I see more burning at 600dpi, so is the driver interpolating the extra pixel points then?

I thought it would, Nikolas says no; let's wait on this one . . .

Did I slow down the output by selecting a higher dpi?

Yes, it would have taken twice as long as it is plotting twice the vertical lines. It will not increase the time required for a horizontal line as it is just doubling number of laser pulses in that line.

Is there a benefit by selecting a higher dpi and then using a higher burn speed?

In my opinion it will smooth the edges, as there will be more overlapping "dots". Keep in mind that your dots will be likely .003" or larger (which is a limit on the laser technology and lens used) so there is a limit that can be achieved with this.

When does dpi matter? When does ppi matter and should I just be leaving mine on auto for the time being?

The required DPI (or dots per inch vertical & horizontal) is subjective. Basically, if the image looks good to your customer, it is good. Some of us tend to be more fussy than our customer. But there is no scientific measure for raster engraving, it is based on visual results. I don't use auto as it hides my settings. I don't think you will learn what the laser is doing by using auto. I'd rather tell it what to do, and if I was wrong at least I know. Some people prefer the "black box" approach - let the laser and software make all the decisions but I don't use that approach.

BTW - Does your laser measure dots in both vertical and horizontal directions? I assume so based on what you asked. Here's the problem - many lasers use the term ppi to mean pulses per inch and it is applied to vector travel only. It defines how many thimes the beam "flashes" along the curve that is being cut.

But some manufacturers like ULS seem to use a different terminology. For raster images, they call the horizontal spacing "ppi" and the vertical spacing "dpi". Newer ULS machines are even more confusing - they replaced the (vertical) dpi setting with a term called "image density". This makes it even harder to speak in general terms, as ULS seems to have their own language. So be careful when reading on this topic.

What should I be targeting for dpi of my artwork?

I would say a minimum of 300 dpi for plaques (text) but I often use 500 or 600 dpi for text and/or artwork. Rodne seems to imply this is too high but my eyes tell me otherwise.

Should the art and the print always match?

I don't think it has to, but if they are not multiples it will not plot well. ie. you can plot a 300 dpi graphic at 600, or a 600 dpi graphic at 300, but I would not plot a 600 dpi graphic at 500 as it will try to convert on-the-fly and you may end up with a poor image.

In the examples above, I prefer the darker burn on the right but I could do without the overburn/surrounding marking. To correct, should I just speed this up, select a lower dpi, select a lower power...? How does one decide how to load all of the variables to get an optimal output? It seems to me that varying just one of the variables would have similar results as varying just one of the others.

Is all the smoke residue cleaned off? It appears that there are stains. You need to use coated wood so you can wipe the smoke residue off. You could probably reduce power and go a bit lighter. I generally try to raster at 100% speed to reduce job time, and adjust other parameters accordingly. How do you decide? Well, you need to experiment some more and record your settings (on the sample) until you get a feel for things. There are more than one set of parameters that may work for a job.

On some woods you just can't get a dark contrast using power alone. It will look too charred before it darkens. In these cases, you need to use something to darken the image. That's another topic.

[Niklas Bjornestal]

Not so sure what you are saying here, though, Niklas. First you say that it uses the closest pixel as it won't interpolate. But then you say that it will require twice the number of lines. Isn't that interpolation?

It will make 2 identical lines. When interpolating you guess how the missing pixels would look.

For exampel:
original image: "300dpi"
xxxxx
xxx
x

image, 600dpi, not interpolated: (.="added pixel")

x.x.x.x.x.
..........
x.x.x.
......
x.
..

image, 600dpi,interpolated

x.x.x.x.x.
........
x.x.x.
....
x.
.

[Martin Boekers]

Here are a few pages from Epilog, visuals are sometimes easier to work with.

http://www.epiloglaser.com/tl_resolution.htm

http://www.epiloglaser.com/tl_dithering.htm

http://support.knowlton.ohio-state.e...log-manual.pdf
(see pages 72 & 73)


Marty

[James Terry]

Well I did some more testing on some waste veneer panel and decided to go with 600dpi, 50sp, 100po onto this basswood plaque. It came out pretty good. I'm trying to decide if I should seal it before I send it off or just keep it natural which I know she'll love.




A friends husband died last week so I decided to engrave his actual wedding proposal into something nice. He was an engineer type if that didnt come through in the design. They are simple and woodsy folk so the bark border really makes this perfect for her.

One thing I learned is that it is time consuming trying to clean up handwriting for engraving, especially when sourced from a low quality jpg.

The laser also plainly points out any stray dots. Even a dot just one number off of white that you dont notice will be lased. So cleaning up the art work goes a long way towards a better end product.

And working with high res art is very important.

[Martin Boekers]

A few tips if you seal it before you laser, then you can clean the "sappy" wood burn easily with a Magic Sponge and some Endust.

If you take the jpg into Photopaint, or Photoshop, convert to greyscale, resample at a higher resolution this may help.

Also after that you can open the levels drop down and work with getting a nice contrast range. I always boost the white balance a bit that takes care of the random dots most of the time.

Use the eye dropper tool and check various areas around the artwork to make sure you don't have a slight background that may not be visible.


Hope this helps.

Marty

[Rodne Gold]

Richard , the laser can resolve at most 300 dots per inch based on beam diameter and thats assuming there is no dot gain , ie the dot "printed" is actually 0.003" without any adjacent heat affected zone. The problem is that a pixel is hardly ever a dot , it is generally a shade of grey and a shade of grey cannot be interpreted by a single dot , it has to be a matrix. I said the laser cant really resolve more than 200 pixels per inch , it can print as many dots per inch as it is allowed , thus photos with more than 200 pixels per inch are a waste of time.
DPI is not the same as PPI - dpi always refers to an output device , its VERY rare to have a single colour printing device able to map a pixel on a 1:1 basis
Often clicking on a picture on screen in corel , it says its 72 or 96 dpi , but it can contain many more pixels per inch which is the only thing that counts in terms of resolution and the size it can be printed.
When scanning , the DPI term is misused , it is actually scanning at PPI , not DPI. It's always better to scan as high as possible as this will determine the max size of the output and will allow you to crop or use a smaller section of the image with no apparent loss of resolution.

[Richard Rumancik]

. . . The problem is that a pixel is hardly ever a dot , it is generally a shade of grey and a shade of grey cannot be interpreted by a single dot , it has to be a matrix. I said the laser cant really resolve more than 200 pixels per inch , it can print as many dots per inch as it is allowed , thus photos with more than 200 pixels per inch are a waste of time. . . .

I understand what you are getting at, but I don't think you can say that a pixel is hardly ever a dot. For photos, yes, but when laser engraving a lot of images are line art and text, a great majority of it is b/w, one-bit, binary, or whatever name you want to give it. In this case a pixel is a laser dot. Since the laser can only "print" one-bit images most files are either created as one-bit, or converted to one-bit, before lasering. The image James showed was basically one-bit line art, so I was not thinking about photos. I guess I was trying to figure out what you were recommending to James in terms of what "dpi" to use for his image. I didn't read your post carefully enough to realize you were only talking about processing photos - wasn't on my mind as he was displaying line art.

If I was scanning the artwork James used, I may have sampled it in color as I find it can be easier to do clean up on color images, but at some point I would have converted it to one-bit before sending it to the laser. I would not have scanned the text as grey scale and then try to let the laser process it into b/w - although it could.

Another question - you say a laser always cuts by making dots. It was my impression that the LaserPro machines automatically change mode to lase continuously at slow speeds for cutting. Do you agree that this is happening? I don't know how to prove or disprove it, but that is the way I read it.

[Martin Boekers]

Actually he was using a photo, he scanned an image making it a bitmap or raster image, that is unless he converted it to vector format.

Not sure about Laser Pro, but Epilog you specify frequency for cutting, I believe it is PPI (pulses per inch). I have "perforated" card stock and can see the spacing so it is not countinuos mode on Epilogs.

I have also seen that with Epilog that it does read grey scale as the denser the grey area the deeper it burns, if it was actually converted to a 1 bit image would that be true?

I may be wrong, so help me understand this a bit() better.


Marty

[Niklas Bjornestal]

It was my impression that the LaserPro machines automatically change mode to lase continuously at slow speeds for cutting. Do you agree that this is happening? I don't know how to prove or disprove it, but that is the way I read it.

At least laserPro mercury II doesnt change mode at slow speeds, you can specify in when printing if it should be continuously or a specific ppi.