Resolution, DPI, & PPI?

[Jeffrey Dewing]

Hello everyone... I have Corel Draw X7 & a Speedy 300 80 watt. I've been putting images on all sorts of sub-straights and have a couple of questions in this area I'm hoping some of you people can help with. One of the first things I do is resample my piv. to the size I want to engrave it. That is my DPI (Dots Per Inch.) So, when I get something that is say 76 DPI, and resample it to 333 or 500, I'm really changing the DPI? My next question is, I'm told that I should match my DPI & my PPI. That makes sense, but where do I do this! Is the only place to change the PPI in the power settings when you're sending it over to the engraver. That is the only place I can find mention of PPI anywhere through the entire process. Thanks for any helping clearing up some of these questions I have. Jeff...:D

[Richard Rumancik]

Hi Jeffrey . . . to answer this accurately would require a lot of words and would probably repeat some good info that you might be able to dig up on this forum by searching on ppi and dpi . . . also there is a lot of good info on CorelDraw on resampling and dealing with photos both here and other places on the Internet.

But briefly - when you get a photo file from someone it might be called a 72 dpi photo but in reality the photo has no "dots" or "inches" to it. Only the device that it will be printed to knows about dpi. So first thing try to get a handle on dpi (output devices - printers and lasers) and ppi (meaning pixels per inch here , not to be confused with another laser parameter called pulses per inch.)

There would be a "header" on the photograph file that says "72 dpi" but what it really means is that if you printed it to a 72 dpi printer you would get a photo of 3456 x 5154 pixels. That would be a photo of 48" x 72". You could collect all this info by opening the file in Corel Paint and clicking on file -> document properties.

Now if you tell PhotoPaint to resample the "72 dpi" photo to 300, what actually happens depends on one checkbox. It is titled "Maintain Original Size". But what it really means to me is "maintain the original size of the data file." That is, "don't throw away any data, and don't try to create any data by interpolating". So, if you check this box, it will re-label the photo file as "300 dpi" and now tell you that if it was printed on a 300 dpi printer (or laser) the output size would now be 11.52" x 17.28". It doesn't add, combine, or discard pixels. It will be instantaneous as it is not processing anything.

If you don't check the box, PhotoPaint will try to keep the 48"x72" image but fill in (interpolate) pixels so that there are 300 pixels per inch each way. The file size will increase by over 16 times (more than 4x as many pixels in each axis.) It will take some CPU time to do this so you will know it is calculating a lot. You probably don't want to do that . . .

I would suggest that you take your photo and do some experiments in a scratch folder and see what is happening with each operation.

In the above case, suppose you wanted to laser at 300 dpi, and the photo showed 11.52 x 17.28. This is too large for your workpiece. You could resample down to 50% in each axis, holding the 300 dpi and NOT checking the Maintain original size box. This would reduce data file size but still match the 300 dpi output for the laser.

The 300, 333 dpi or whatever is selected in your print driver for the laser.

[Jeffrey Dewing]

Thank you for the info Richard! Armed with this knowledge, I'll be able to better attack my quest for information in the field of laser engraving! Now I can ask myself more informed questions as to how to make a better product. This has helped me sort out a few things that have been haunting me, like having 2 different PPIs. As soon as I read that ",dawn started rising over Marble Head." The whole picture what I'm really doing when re sampling became a little clearer. Thanks again, Jeff....

[Dan Hintz]

Richard pretty much nailed it. If you start looking at images in terms of raw pixel count rather than dpi, you'll be better off. A 333x999 pixel image will give you a 1"x3" engraving when run at 333dpi, or a 1/2"x 1.5" image when run at 666 dpi. Same image, different final result size.

[Richard Rumancik]

Jeffrey, I didn't really address the "pulses per inch" issue in my other post. I can understand if you are confused about the "ppi" issue. In fact, a lot of us are . . . and it is partly because different machine vendors use different terminology.

I suggested that you search the forum but unfortunately you can't search on PPI or DPI as the search algorithm ignores short terms. Here is a thread from 2007. I hope it does not confuse you even more . . . but you will see that
it is a difficult topic to get a handle on.

http://www.sawmillcreek.org/showthread.php?68957-PPI-vs-DPI-the-difference-and-effect&highlight=

I would like to tell you that PPI (pulses per inch) is a cutting parameter, as Rodne said in the 2007 thread. I think it is true for my GCC laser. But when I read the Trotec info, I can understand why there is so much confusion between users, as it seems PPI means something a bit different in the Trotec world.

Also, I don't understand what they are saying in several places. The highlights are mine.


Here is a excerpt from a Trotec manual:
PPI/Hz (Basic, Advanced, Expert)
PPI = pulses per inch (laser pulses per inch). Only for the Engraving process.
This setting determines the number of laser pulses per inch emitted by the laser.
This occurs depending on the position, e.g. the pulses are always emitted at the
same interval irrespective the speed.

Adjustment range: 100 - 1000 PPI

The higher the selected PPI value is, the better is the quality of the engraving. To obtain a good result, the PPI value should be usually larger than or at least the same as the dpi setting of the printer driver (Process options – Resolution), e.g. a minimum value of 500 PPI should be selected for a resolution of 500 dpi. A low PPI value is only useful when processing some types of rubber or in cases where the contour is only to be perforated but not cut through. Unlike the resolution in dpi, increase in PPI does not have an effect on the engraving time.

Hz
Frequency of laser pulses during cutting. This setting determines the number of laser pulses per second emitted by the laser. Unlike engraving, with cutting it is not necessary to create fine contours by means of resolution. In this case, the position sensitive control used with the PPI can therefore be replaced by a set frequency.
Adjustment range: 1000 - 10000 Hz for CO2 systems,
10000 – 150000 Hz for YAG systems
Additionally YAG systems can use frequency mode for engraving.

Resolution
Selection of the required resolution in dpi (dots per inch). Equivalent to the lines to be engraved per inch (25.4 mm). A higher value improves the engraving quality, but results in increased engraving depth and longer engraving time using otherwise identical parameters as the laser has to process accordingly more lines.
Setting range: 125, 250, 333, 500, 600, or 1000 dpi
The setting for standard engraving should be 500 dpi. The resolution to be selected largely depends on the material used as well as on the focal length of the lens.

---------------------------------

Ok, they say for Trotec, PPI is "only for the engraving process" . Then they go on to say : "A low PPI value is only useful when processing some types of rubber or in cases where the contour is only to be perforated but not cut through." This doesn't sound like engraving to me . . . so the statement is contradictory.

Under HZ they say:
"Unlike engraving, with cutting it is not necessary to create fine contours by means of resolution. In this case, the position sensitive control used with the PPI can therefore be replaced by a set frequency."

I have no idea what this means. I always thought that for cutting you wanted to keep the pulse distance constant (as the vector speed can change on contours or corners.) So, does Trotec PPI parameter apply to cutting or doesn't it?

Under Resolution: they say " . . . dpi. Equivalent to the lines to be engraved per inch". "Lines" is a vertical measurement - so it makes me think that they are using the ULS terminology where the vertical direction (Y) is measured in DPI and the horizontal direction (X) is measured in PPI. That is an odd way of doing things to me. Most people think of dpi as the measurement unit in both axes.

The point is, until you can understand what terminology Trotec is using, and clarify what I see as inconsistencies in the above info, it will be hard to gain a thorough understanding of your machine and how to set the parameters. What I (or someone else) tells you might be inconsistent with Trotec terminology.

Maybe some of the Trotec users can help on this point, or else I suggest you challenge your Trotec rep to explain the information in the manual.