Confused about PPI

[John Noell]

I am unclear about the impact of changing the PPI value on my Epilog Mini. I can see that higher (5000) is better for acrylic edges but I am having trouble seeing just what difference it makes on some materials (e.g., coconut shell). In general, what is the effect of varying the PPI value on common materials (esp. woods)?

[Sarah Jones]

what is nice about engraving woods, is the fact that any pixel out of place is usually masked by the grain of the wood, so you can techniqually engrave a lower resolution image on wood and it still come out okay. I would not suggest the same for glass metal and acrlic engravings becuase Jpeg artifacting on those stick out like a sore thumb..

[Frank Corker]

John the 5000 that you see on the Epilog refers to the pulsing of the laser during vectoring. The laser is pulsing at 5000 is good for continuous cutting of materials like acrylic/corian - when cutting materials like wood, laserable plastic, leather, mat board it is reduced to 500. With a reduction like this there will be less charring.

Try cutting wood with the frequency at 5000 and then again at 500, I can't understand why you cannot see the difference. Same goes with leather, cuts much neater and smooth at 500 even though it can be cut with 5000.

[Sarah Jones]

Oh opps my bad! I was talking about raster engraving, the frequency is rather important when vectoring

[John Noell]

I have tried cutting coconut shell, Fiji mahogany (fairly hard), and kavula (fairly soft) at 500 PPI and 5000 PPI but I cannot see any significant difference. The cut surface appears to be equally blackened. I varied the power & speed from where I am just barely cutting through (i.e., pieces come out but mucst be pushed out) to where the pieces fall through. Maybe these materials are dofferent than most? (We have so many exotic hardwoods here and our own flavor of pine that it is not easy to get samples of typical woods available in places like the USA.)

[Frank Corker]

John for you and your exotic coconuts (whey hey!) experimentation is your only course of action. Try taking the frequency right down 50, 100. 150 etc until you find a desireable one. I've vector cut acrylic at 50 frequency and got a rather surprising result, dependent on the thickness, sometimes the finished edge can be smoother.

[Marc Myer]

Wow! I've got to try that!

[Gary Hair]

Marc,
I have used a very low PPI setting to perforate paper. Try something very low on paper and you can really see the results.

Gary

[John Noell]

Yes, I can see the difference with very low PP values on paper but not on the woods I have tried. Not enough time to experiment right now as we are still cutting and engraving coconut shells for a big resort. The biggest problem is the irregular shape and thickness which means every single piece must be hand arranged and the focus is a compromise. If I had the time I would have gotten a longer focal length lens but the resort is in a major hurry. Even DHL took 12 days to deliver a box from the USA. (And the box sent the same day via regular air mail only took 14 days.)

[Peck Sidara]

John,

PPI and DPI are essentially the same. It refers to the printing resolution. DPI is for raster engraving mode.

Frequency is adjustable from 100-5000 and applies to vector cutting only. As others have stated, wood is typically vector cut at 500 Frequency and acrylic at 5000. It's the pulse rate of the laser tube in vector mode.

Setting your frequency from 100-200 will result in preforation of your material. Where the laser beam is pulsed with spacing to allow for punching out the part later.

Reasons why you may not see a difference in the woods edge quality when adjusting the frequency from 500 to 5000 is possibly because:

1. You're using too much power for the particular wood which is causing the browning/charring of the edges.

2. You don't have enough power so the speed setting you're using for vector cutting results in too much dwell time causing the browning/charring regardless of your frequency setting.

HTH,

[Mike Null]

PPI and DPI are essentially the same. It refers to the printing resolution. DPI is for raster engraving mode.

You may be correct for Epilog but they are not the same for Universal and Trotec.

PPI refers to pulses per inch and differs from DPI. DPI refers to dots per inch and relates to resolution. I believe one of your manuals has an excellent explanation of dpi.

One thing i disagree with is the blanket statement that higher resolution produces better image quality. That is dependant on materials and, in fact. lower resolution may produce a better image with some substrates.

[Rodne Gold]

Peck , on other systems , PPI =Frequency.

[Peck Sidara]

One thing i disagree with is the blanket statement that higher resolution produces better image quality. That is dependant on materials and, in fact. lower resolution may produce a better image with some substrates.

Agreed Mike. I'm not sure who made this blanket statement but it wasn't me.

Mike & Rodne,

Thanks for the clarification on PPI vs. DPI. On your systems, are you able to adjust both PPI and resolution in raster mode? If so, please elaborate on how they affect raster engraving quality.

[Marc Myer]

...and it gets worse.
In the world of Graphics Dorks (me) working in Photoshop, ppi means Pixels Per Inch.

...and among fantasy writers, of course, it's Pixies Per Inch.

...and...

[John Noell]

Reasons why you may not see a difference in the woods edge quality when adjusting the frequency from 500 to 5000 is possibly because:

1. You're using too much power for the particular wood which is causing the browning/charring of the edges.

2. You don't have enough power so the speed setting you're using for vector cutting results in too much dwell time causing the browning/charring regardless of your frequency setting.

HTH,Well, it is not the former as I am barely cutting through (and in many cases not all the way). But 45 watts may not be enough for some of the coconut shells. They vary tremendously in both thickness and hardness. The ones that are both very thick and very hard are extremely difficult to cut. Adding the curvature in, and thus uneven focus, makes it even trickier. (But this justified a higher quote so I am still making decent money for this part of the world - where skilled workers may earn less than $3 USD/hour.)

[Mike Null]

Peck

The statement about the higher resolution equals a better image is on page 60 in the Epilog manual for the mini and Helix.

With both my old ULS and my current Trotec I can set both PPI and DPI in the raster mode.

[Chris J Drew]

Peck, am I correct in assuming that on Epilog''s lasers, PPI(pulses per inch) would depend on frequency AND speed?
ie: with frequency at, say, 1000 & speed at 20%, slowing to 10% speed would double the PPI...

[Peck Sidara]

Chris,

PPI is not an adjustable variable on the Epilog.

For raster engraving; users can adjust resolution (DPI) from 75-1200 for different engraving quality.

For vector cutting; users can adjust frequency to adjust the number of laser pulses per inch.

Frequency is independent of resolution. Running a vector job at 5K frequency at 600DPI will be the same as 5K frequency at 300DPI.

Additionally, regardless of speed, your frequency (amount of laser pulses per inch) remains the same as the distance traveled doesn't change.

Mike,

Our latest manual is downloadable at http://www.epiloglaser.com/laser_manuals.htm

I am unable to find a blanket statement saying "higher res = better quality" in our most recent manual.

Our manual contains a wealth of info. If there's a descrepancy, I'll ask to have it edited and/or omitted for the next release.

John's original inquiry was why does adjusting the Frequency have very little affect on the vector cutting edge quality.

I believe we've come to the conclusion that because coconut shells are very hard and not perfectly flat, using a 45W laser at maximum power, minimal speed results in a certain edge quality regardless of what frequency setting is used.

[Mike Null]

Peck

This manual is about two years old or a little more and I keep it because you have some very good info in there that we can all use regardless of brand.

[Rodne Gold]

In raster mode you only have one setting , and that is DPI , ppi (Freq) is irrelevant , same in vector mode , you only have PPI (Freq) and have NO DPI.
It cannot be any other way. (The only way I can see a DPI type value in vectoring is when you have a stepper motor and might be able to set the no of steps per inch in the X and Y axis?)

As to DPI and resolution , well any laser using stock optics can at best only deliver 300 discrete dots per inch , based on its spot size. Any increase in DPI over this will not increase resolution , it cannot if the laser and the material cannot resolve more than 300 discrete dots per inch.
What increasing DPI will do however is increase dot gain and dot overlap and , dependant on material , might give you a darker/deeper engraving , but this is NOT an increase in resolution at all.
Now in raster printing (bear in mind the laser is a single colour printer/plotter) you have PPI which is pixels per inch , HOWEVER 1 dot is not always equal to 1 pixel , so you might have a resolution of 100 pixels per inch but print at 300 DPI , with stuff like greyscale where a pixel with a shade of grey cannot be represented by a single dot.

When vector cutting , the laser acts as a trepanner , ie drills a series of holes and breaks out material between em. In an ideal world , you would have this: A laser pulse that thermally shocks the material to an extent that vaporisation takes place and a clean hole is drilled. However lower power lasers cannot do this so raising the PPI creates a lot of overlapping holes and thus heats up the surrounding material so the next pulse already has "hot" stuff to work with. But this also creates a big heat affected zone (charring , melt etc)
One also has to consider how the laser pulsing is handled on curves etc as the speed of the laser head drops and if the algorithm for PPI is not properly adjusted within the driver , it can "overburn" on corners.
Problem is , we with lower powered lasers do not use em correctly and rely on Heat affected zones .
What do I mean? Well if you correctly engraved wood with instant vaporisation , you would have no "branding" effect, so we have to fiddle with strategy to create a heat affected zone......

Using a low powered laser to cut wood by repeated "burning" of the cut area is also not correct.

[Mike Null]

Rodney

As previously noted, my former ULS and my current Trotec permit varying the PPI and the DPI in the raster mode. PPI is a useful setting in the raster mode if your machine offers the option.

As noted from your remarks and those of others it is not an option on some brands which obviously default to a standard setting.

[Rodne Gold]

How can you possibly set ppi and dpi , if ppi is pulses per inch?
300 dpi means the laser will lay down 300 dots per inch of travel in x and y directions.
If it enables different dot spacing in the X direction , then its not DPI but rather some "bastardised" setting.
So your laser can fire PPI dots in the X and will step down for the next line at your DPI setting?
Ie 5000 dots per inch as the laser traverses back and forth and if you set DPI at 300 , it will take 300 scan passes per inch in the Y axis?

Its amazing , these are terms that are WELL known , why should laser mnfgrs make a dogs breakfast of them and have their own "propietory" system , the laser is a single colour printer in the raster mode and a plotter in the vector mode .... nomenclature is well defined for both these devices.

[Gary Hair]

Rodney,
First - I am in agreement with you that PPI is for vectoring and DPI is for rastering.

That said, my GCC/Pinnacle driver appears to allow you to set Power, Speed, DPI and PPI for both raster and vector. Does the PPI have any effect on raster or does DPI have any effect on vector? No. But, it is still there in the driver. This may be how some of this confusion happens.

The reason it's that way in my drives is because of layout of the driver screen.

I have a screen snap of it attached so you can see what I mean.

Gary

[Mike Null]

This is from the current ULS web site.

PPI (Pulses Per Inch) Proportional Pulse Control (patented) – User-definable spacing between laser pulses can be optimized for cut penetration or smoothness of cut and allows for better engraving or cutting quality (even on materials like wood or acrylic).

[John Noell]

Peck

The statement about the higher resolution equals a better image is on page 60 in the Epilog manual for the mini and Helix.

With both my old ULS and my current Trotec I can set both PPI and DPI in the raster mode.
It's on page 69 in the new manual, but if you look at the context (it is in the "Using the Epilog Dashboard" section), I think it is not so outrageous. Yes, it is an over-simplification but in general, lower-res pictures are not as 'good' as higher-res pictures to the typical viewer. The key statement is "If low quality artwork is sent to the laser system, even the highest resolution will not improve it." 'Optimal resolution' is a complicated subject and the finer points are beyond the scope of that part of the Epilog manual.

[Mike Null]

John

All things being equal higher resolution should mean better image quality but it's the substrate that dictates whether you can use it or not.

If you use high dpi on laserable plastic you can lose detail; same is true on anodized aluminum.