DPI Confusion??

[Sylvia Mossbrucker]

Okay so this may be a very detailed question, however I really need clarrification from anyone who has information on the subject..!!

I am running a universal PLS 4.60 60 watt laser and an Epilog Legend EXT 36x24 120 watt laser for granite, marble, and ceramic tiles. I do images and photographs both. When the images are opened in corel the dpi (resolution) is usually 72 dpi?? so what kind of difference does it make to run my laser at 300 or even 200 dpi? If there are only 72 dots per inch in the image how does the laser add any running it higher dpi???? That is the main question that confuses me and how does adjusting the image dpi in corel relate or does it??? If anyone has an elaborate definition for dpi and how it works please let me know. I have read up on it but its still so confusing how the two (image dpi and laser dpi) work together!! Anything would be helpfull at this point.

Thanks,

SMoss :confused:

[Darren Null]

DPI is exactly that. Dots (or pixels) per inch. So you can have a 3-inch image at 100dpi and a 1-inch image at 300dpi and BOTH IMAGES CONTAIN THE SAME AMOUNT OF DOTS (or pixels).

A bitmap/raster picture is made up of a grid of coloured dots that collectively fool the eye into thinking it's seeing the complete image. The more dots you can lay down in a particular space the more information the picture will contain and the more real the image will appear to be.

For lasering purposes, 300dpi would seem to be the sensible upper limit for images- if you go ny higher than that you are burning dots over the top of other dots (your laser dot is a fixed size) and therefore losing quality. With some materials, you have to drop the dpi somewhat because the material cannot cope with the image density. I burn glass at 150dpi, for example, works for me.

If it's a 72dpi image, the laser will either just burn those 72 dots every inch (depends on your laser settings); or it will have a guess at what dots should be filled in the gaps.
For optimal quality, if you're burning at 300dpi and you want the end result to be -say- 5 inches across, your input image wants to be 1500 dots (pixels) across. It doesn't really matter what dpi the image says it is.

Images are commonly at 72 or 96 dpi for the internet- it is important to reduce the filesize of images on the web and 72dpi is sort of a minimum amount of dots to convince the eye that it's looking at a proper picture. So at 72dpi, you can more or less have a guess what size it's going to be on the net. Or you could, before everybody started using different screensizes.

For printing and lasering it's all about information density- how many dots can I lay down in that area to get the best image I can? Filesize is of secondary importance, if that. 300dpi is sort of the industry standard for both, for various reasons.

For lasering, for the best results, 300dpi is a good size to go for.

That's DPI

By 'Laser dpi', are you thinking of LPI or PPI? This is a setting for how often the laser pulses while doing VECTOR lines. You can either set it real low, so you get:
dot dot dot dot dot dot
...or set it a bit faster and get:
dotdotdotdotdot
...or set it faster yet and overlap the dots. That's difficult to demonstrate with text.

[Joe Pelonio]

By 'Laser dpi', are you thinking of LPI or PPI? This is a setting for how often the laser pulses while doing VECTOR lines. You can either set it real low, so you get:
dot dot dot dot dot dot
...or set it a bit faster and get:
dotdotdotdotdot
...or set it faster yet and overlap the dots. That's difficult to demonstrate with text.
With Epilogs it's dpi, the ppi is called "frequency".

I have done images that were given to me at 1200 dpi that came out better at 600 dpi than 1200 dpi. Usually 300-400 dpi is fine if the file is at the size to be engraved. Where you get problems with clarity (resolution) is when someone gives you a 300 dpi jpg at 1"x1" and you have to engrave it on a 12"x12" tile. As we used to say when I did some computer programming, "garbage in garbage out."

[Rodne Gold]

The dpi that corel shows is irrelevant , cos DOTS PER INCH only applies to an output device.
Its PIXELS PER INCH (PPI) that you need to be concerned with. IE what is the resolution of the graphic
A 300 x 300 Pixel graphic will output at 1" x 1" with a device that can resolve 300 pixels per inch , the same graphic will print or output at 6" x 6" on a device that prints at 50 pixels per inch.
However there is a further confusion cos a laser can only really print at 300 DOTS per inch (either burn or not burn - 1 colour only) but a greyscale pixel CANNOT be represented via 1 dot (it has to have a matrix of dots to represent it).
In essence , any greyscale picture above 150-200 PPI is useless on a laser assuming you want to laser it at the same size it is represented in the software. the laser can only really represent 100 ppi in terms of greyscale , and the factor of 1.5 to 2x to 200ppi comes from nyquists theorum which I am not going to get into here.

Where scanning at a higher PPI comes in is when you want to blow the picture up bigger than the original.
If you have no intention of making the picture bigger than the original (if you scanning) then the best is to scan at 200 PPI (often called DPI in scanning) , you will gian nothing and actually lose if you scan in higher and ask the print driver to "downsample" as you will introduce innacuracies and increase file size and processing time.
To repeat , if you want to blow a picture up , you need more PPI , if you want the same size or smaller , do not use more than 200 PPI.

Now what you set your lasers output to has an effect. Remember cos of spot size , a laser with a std 2" lens canniot resolve more than 300 dots per inch due to its spot size , however increasing this will give more "overlapping" dots , which can result in a deeper burn or an impression of more "detail" as one will get some edge sharpening this way , however the true resolution and tonal graduations of the picture and thus its dynamic range wont be preserved.
In most cases , you will get your best results with greyscale at 200 pixels per inch of output and lasered at 300dpi (and you should get a lot faster lasering than any higher dpi)

[Darren Null]

In essence , any greyscale picture above 150-200 PPI is useless on a laser
Sorry, I don't agree wholly with this. Now it's true that a colour or greyscale image is going to have to drop a whole lot of information to be processed by a laser (colour for one!), but the physical dimensions of the grid you're burning remain unchanged.

Therefore, if you feed a laser a 200dpi image and ask it to print you out a 300dpi image, you're asking the driver to UPSCALE....or spread the information out over 1.5 time the area the image has information for. With all the attendent losses/Moire effect/Nyquist–Shannon theorum shennanigans that that implies.

Also to make greyscale from either black or white (burn/not burnt), you need a grid around the pixel, if you're after a 1-for-1 correlation HOWEVER each pixel and it's grid are part of the neighbouring pixel's grid. That's why proper dithering is so important- it fakes the greyscale in a much smaller space by considering areas, and not just individual pixels. And that's why Photograv sells so well- that's what it does & it's good at it.

I'll agree that there's a (reasonably insignificant, with today's computers) time saving and a (significant) storage saving by doing things at 200dpi.

But.

I run a (raster) image through Photoshop to get the image I want, then through Photograv to get the dithering I want, then through Corel to get the positioning I want. By the time it gets to the printer, it knows that I want THESE dots HERE, and there is minimal interference from either the print driver or Corel, neither of which I'm particularly impressed with.

I start off at 300dpi if there's no change to the image. If there's work to be done on the image I'll quite often do the work at at least 600 dpi for 2 reasons:
1) If you make any mistakes with the manipulation, they'll likely be lost in the downscaling. (if you're working at 200dpi, you'll be magnifying any mistakes).
2) Downscaling the image sharpens the picture up. If you downscale as the last thing you do, you will end up with a better final result.

So.
Photoshop- image manipulation and downsizing, if necessary.
Photograv- dithering
Corel- position.

Each program is doing what it's good at, and only necessary changes are being made.

Yes, there are significant time savings to just bunging an image into Corel at 200dpi and asking it to output at 300dpi. I wouldn't trust Corel's resize routines to find their butts with both hands and a doctor standing by, to coin a phrase. Given the time any given job can spend on my (10W!) laser, I prefer to feed it the best information I can.
================================================== =======

Oh yeah, I knocked this little (to relative scale) comparison chart. You'll see that they all contain the same amount of information. When you're outputting on a printer or laser or other output device, you'll see that a 4-inch image at 72dpi would contain much less information than a 4-inch image at 300dpi.

The dpi is just an indication of what size your final result would be. But means nothing while it's in your computer.

If you're scanning an image in and the image is 4" and you want the result to be 4", then scan at 300dpi (or 200 if you like Rodne's theory). If you want your image to be double the size, then double the scan input (600 or 400 respectively).

[Rodne Gold]

Darren
The salient point is that 1 greyscale pixel cannot be represented by one laser dot , if you want to represent greyscale the laser has to use a matrix of dots , with the matrix not filled for white and totally filled for black and partially filled for anything inbetween.
If you take a 3 dot by 3 dot matrix , you will get a maximum of 100 pixels that can be represented by grey in a 1 inch traverse.

You use a figure of 200 ppi cos of nyquist - that anything can be represented 100% if sampled at 2x (to be simple)
You are actually not asking the print driver to upsample , you are asking it to downsample 200ppi to 100 ppi. You have to differentiate between pixels per inch in a graphic vs dots per inch in an output device. If you had the ability in your laser to represent 300 pixels per inch (which you dont on greyscale) then feeding it a 200ppi image would make it upsample , but this isnt the case in my previous post.

There is no difference between a colour greyscale and any other , greyscale is when colour has been stripped off the image. You cannot make greyscale from a black and white image , you cannot fake the info that is not at all there.

There is no problem working with a larger PPI image when scanning or manipulating barring some innacuracies and time.. the innacuracies , considering the grossness of a laser engraver are not that relevant. In fact , its better to overdo things in this dept , especially if you want to crop and so forth or perhaps need to blow it up in the future. Its far more accurates to downsample than to upsample , cos the infor for upsampling has to be guestimated.

The laser itself is a lousy printer , there are huge problems using it for greyscale or graduated images , the major one being dot gain (which is the propensity for one dot to bleed into another due to heat affected zones ..like wood engraving will have a dot that is a lot bigger than the laser spot sizes)
You have to remember that photograv is not engraving greyscale at all , all it does is render the greyscale image using various techniques to a 1 bit black and white image , ie it does what the laser driver does but lets you have more "control" over edge enhanching etc , in fact it makes the greyscale image "Worse" in absolute terms than any 1/2 toning driver would , however this "worsening" gives a better result using a laser.
At the end of it all , the object is to put out an acceptable image balancing time and quality , there is a point where you are wasting your time by trying to get the nth degree of quality that is not attainable.
As to engraving without photograv , it depends how good your lasers greyscale driver is compared to Corels which one you use , but by far the best way is to use neither and to do manually what photgrav does and end up with a 1 bit image.

[Darren Null]

The salient point is that 1 greyscale pixel cannot be represented by one laser dot , if you want to represent greyscale the laser has to use a matrix of dots , with the matrix not filled for white and totally filled for black and partially filled for anything inbetween.
Agree absolutely.

You are actually not asking the print driver to upsample , you are asking it to downsample 200ppi to 100 ppi. You have to differentiate between pixels per inch in a graphic vs dots per inch in an output device. If you had the ability in your laser to represent 300 pixels per inch (which you dont on greyscale) then feeding it a 200ppi image would make it upsample , but this isnt the case in my previous post.
This is the bit I was rambling on about in a disagreeing fashion. 300ppi is 300ppi; whether it's colour, a variety of greys or a simple choice of black-or-white.
Yes, you're dropping information at every stage; yes, lasers are lousy printers; yes, 1-bit. But if you have -say- a 5-inch image at 200dpi and you ask the laser to print it at 300dpi at the same size; then the software is going to have to make up 1/3 of the final image.

The way I do it is still dropping picture information all over the place, granted, but it's the least amount of information that I can drop and all of the image that ends up at the laser came from the original image.
I'm not really impressed with results from either Corel's handling of raster images or my GCC printer driver and prefer to present them with a fait accompli.

Whether the difference is worth the time, is another question entirely. :D

[Richard Rumancik]

. . . When the images are opened in corel the dpi (resolution) is usually 72 dpi?? so what kind of difference does it make to run my laser at 300 or even 200 dpi? If there are only 72 dots per inch in the image how does the laser add any running it higher dpi???? That is the main question that confuses me and how does adjusting the image dpi in corel relate or does it???

While others discuss Mr. Nyquist, let's try a different angle. You have an image from the web that is 72 dpi (like many web pics.) Let's use my Toy Car picture (Toy_Car.jpg) for this discussion as it's small. When I open in Paint, I click on resample. It tells me the image is 5" x 5" with a resolution of 72 dpi. (screenshot1.jpg) Notice the file size of 388,800 bytes and keep an eye on this as you make changes.

We are not really interested in linear size @ 72dpi. We really want to know how many pixels there are in the pic. So if you go to image size and change it to display in pixels, you get screenshot2.jpg. You will see that the image has 360 px. (Makes sense; 360/72 = 5" picture on a 72 dpi printer.)

Now if we wanted to laser this picture at 300 dpi, how large can we make it? (360px)/(300 pixels per inch) = 1.2" wide. In screenshot3.jpg I have changed the dpi to 300. Now some people say it doesn't matter but let's change it anyway. The size will probably try to change to some large number like 1500 and say 417% or such. The anticipated new file size will also jump. It is trying to increase the size (by the ratio 300/72 = 417%). Don't let it do this. Change it back to 100%. Then you will see screenshot3.jpg. (To avoid it trying to scale up 417%, you can change the Image Size to display pixels, then check off the box "Maintain Original Size". It will grey out the top section and only allow changing the "resolution" data. "Maintain Original Size" is a bit of a misleading command; it maintains the total number of pixels and maintains the file size but the original linear dimensions will NOT be maintained.)

Note that the file size did not change in screenshot3. After you click ok it will execute the resampling command and you can save the resulting image. I would save it as a tif or bmp rather than jpg. Then you can use CorelDraw, PhotoGrav, etc to take it further. I have always set the bitmap image to whatever dpi I intend to use on the laser. Not everybody seems to do this.

If you re-open the file it may revert back to inches on the size display (mine does.) It is showing 1.2" (as expected) at 300 dpi in screenshot4.

For photos, the camera is saving pixels. But the image software needs to see some declaration of dpi in an image file so the camera makers decided to use 72 dpi as the default. But at 72 dpi my photos are 32" x 24" (2304 px by 1728 px). The pixel size is what you need to know. You can change the dpi to 200 or 300 without adding or losing information from the file. Just make sure you are set to view pixels not inches (and check MAINTAIN ORIGINAL SIZE). If the file size did not change, you have not lost or "created" pixels. With my 2304 x 1728 px photo, I could print it at 300 dpi and get an image 7.68" x 5.76" (2304/300 = 7.68").


You might find this helpful
http://www.rideau-info.com/photos/changedpi.html
http://www.rideau-info.com/photos/mythdpi.html

[Rags Alan Ragland]

HUH !
Rags