uneven dpi/ppi

[reaction]

why is it printers have uneven dpi?
ie I was looking at photo prints, and the lab printer prints at 300x600ppi. Why not 300x300? Why is one direction more?
I think home printers are also uneven.

[Lucas]

Where are you looking? That doesn't make sense. That means a photo with a resolution of 1200x1800 would print at 4x3", rather than 4x6", and as a result be very distorted...

[ricktas]

same as your screen is not square! and probably has a resolution of 800x600 or some other ratio. The PPI quoted is often based on a standard print shape 6x4 etc

[peterb666]

Generally speaking, you don't need to worry too much about printer resolution. The key is to maintain maximum quality, the printer resolution should be at least twice the output in dpi.

Early laser printers were all 300x300 dpi and generally early injet printers around 360x360 but the dots overlap anyway - they have to otherwise you couldn't get black as there would be white bits between the dots as there is no such thing as a square dot!

The reason that printers have a greater horizontal resolution than vertical is that it is easier to do mechanically. Same thing with scanners. So a printer with a resolution of say 600x1200dpi is just fine. Note that I have been using dpi (dots per inch). ppi is pixels per inch which is fine for describing image quaility and the other term is lpi or lines per inch.

OK, as long as your images is going to wind up at least 150 ppi, it is going to look OK and should be printed by a printer with at least 300 dpi. No problem doing more and most people use about double those figures these days as a minimum.

So what does that mean?

If you have an image of 3600 x 2400 pixels (8.6 megapixel) and decide that your minimum printing resolution should be 600dpi, you will get the best quality image when printing at up to 300 pixels per inch. That means an image of 12" x 8" is the go (approximately A4 paper size).

When viewing the printed image with your eyes from just about any viewing distance including stupidly close, you are not going to see any advantage over one that is printed at 150 ppi, then you can have a nice image up to 24" x 16". Of course the larger the image, the further way you can view it so with posters you can print at even lower ppi and get away with it.

[peterb666]

same as your screen is not square! and probably has a resolution of 800x600 or some other ratio. The PPI quoted is often based on a standard print shape 6x4 etc

ppi is pixels per inch. I have yet to see a 1 inch by 1 inch object that is not square.

[reaction]

in my case I was looking at the machines used by photo kiosks for prints. the spec said 300x600 I can't remember dpi or ppi.

Now ppi doesn't make sense - one direction would look more dtailed, and dpi sounds quite low, as you need many dots to make a pixel.

[arthurking83]

I remember reading on this topic about scanners.
Printers are very similar technology to scanners, with the obvious difference of producing an output vs an input job.

The way a scanner works is that it has a fixed optical resolution(say 300dpi) horizontally, and then it has a stepper motor to move along the vertical axis travel.
This stepper motor can be more finely tuned at the design and production stage to vary in quality.
The higher dpi figure is not an optical resolution value, simply the step value of the motor system.

There is only one optical resolution and that is always the lower value of the two.
So, say when you scan, by default for a high quality scan, it may do so at the 300x300 dpi resolution figure, as I had my old scanner set up years back.
But there was a setting where I could use 300x600dpi, and that change from x300 to x600 simply meant that the stepper motor's vertical step setting was halved(from 1/300" per movement to 1/600" per movement).
Of course x600 scans too longer to produce as each step which took the same amount of time as the x300 scan, but of course there were double the number of them.
This meant a bigger file size(more than double) but I saw no appreciable increase in detail in any of the scans I'd made.
Still have that scanner, but is incompatible with Win7(no driver).

(I'm assuming) Printers work the same way.
The print head has one maximum resolution possible, and I know with most printers you can lower that if need be, and then you can do stepping .. but I don't think I've seen printers rated in the same way as a scanner has been.
(then again they may well be, I've just never noticed it)
Print head in a printer has a fixed maximum resolution, and then you can step the motor drive with various degrees of refinement.
I'm assuming this is what is referred too by the OP's question.

If the dpi values for the print alter the image's aspect ratio, then I'm sure that the software will allow the user to crop the image as a priority, and if the user chooses not too, it may interpolate the print resolution to stretch the image.
I remember the Canon software did this on my old ip4000, but I barely used this printer in this manner and it died early on it it's life time.

This 'specification' marketing speak, may also be tied to particular technologies too.
When I went looking for a replacement printer, I wanted laser(I do mainly office printing), and from what I gather laser printers usually specify the specifications at the more acceptable XXX x XXX resolutions. (eg, my HP colour laser's specs are 600x600, and with software enhancements can print up to 2400x2400)
Lasers don't print the same way as inkjets do, there are no nozzles and drops as such.
The tech is different.

[Lucas]

I've just done some additional reading, as this still didn't make sense to me, and thanks to your question, I've finally learnt that dpi is NOT the same thing as ppi.
Pete mentioned that the dpi should be twice that of the ppi. This suggests that that print kiosk would only produce acceptable quality of photos you want printed at 150ppi?

Even at 150dpi, each dot is still less than 0.2mm. This would result in 1 ink dot making up each pixel for a 150ppi image. If that image is printed at 300dpi, then each dot is now less than 0.1mm, and each pixel is made up of 4 ink dots. I'm guessing this is where Pete's 'dpi should be atleast twice that of the ppi', and that 4 ink dots is considered the minimum necessary so that tonality is not lost. To say that you need many dots to make a pixel could be incorrect, from Pete's guidelines it sounds like practical experience suggests you need only 4.

Maybe that is just saying stuff that everyone else already knows - but it was helpful for me to type it out!

In regards to the uneven axis resolution... I found this in an HP document called 'understanding dpi'

'In the figure at left, nozzles spaced 300 per inch on the printhead give 300 dpi in the vertical (“V”) direction. The ability to place dots on 600 dpi centers along the scan axis (“H”) gives a printing resolution is 300 (V) X 600 (H) dpi. As seen in the figure at left, dots are sized to print in a 300 X 300 dpi grid but they are placed with 1/600-inch precision along the scan axis. With finer control over where large dots are printed, a printer can achieve both high throughput and high quality. Throughput is based on high area coverage per unit time, and this requires high ink flux: the volume of ink delivered each second to the print medium. Image quality is improved with increasing precision of dot placement at high-contrast edges in an image, such as for text, lines, and graphical elements.'

I'm kinda making an assumption here, but it seems to suggest that a 300x600dpi resolution is going to have the same number of dots as a 300x300dpi print, so isn't REALLY a higher resolution, but they get better accuracy so it does improve print quality.

Also, found this in regard to how many dots are needed to make a pixel:
Fine Art - Min Res: 1200dpi
Small format prints, text, technical graphics - 600dpi
Point-of-purchas, Event graphics - 300dpi
Billboards - 150dpi.

No mention of ppi... However fine art images at 300ppi, printed at 1200dpi would have 16 dots per pixel, I believe.

[peterb666]

One point to note is the dots overlap even with the same colour and each colour can be printed on top of one another. Thinking of the dots as being discrete, individual things in little rows is off-track.

The reality is you probably don't need as many dots as you think and there are a variety of factors that limit/control image print sizes including processing artifacts.

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