Enhance scanned photographs: DPI ??

"Nicolas Hoch" wrote in message

Hi,

in my photo class, we developed black&white prints on multigrade photopaper (no digital camera so far). Unfortunately, most of them were not developed long enough, so they are too bright. I would like to scan the photographs (they are 10×15 inch), adjust brightness, contrast, gamma, etc. in Photoshop PS and then burn them on a CD and have them developed in one of those 1-hour-photo-shops.
My question is: What DPI should I use for this purpose? The default value is 200, but I think this is too low. For Web-purposes I usally choose 72dpi and for other things 300. But if the pictures shall be developed in a photo-shop on professional photo-paper resolution should be as high as it makes sense.

What do you suggest?

Also, do you think it is worth the effort to scan as TIFF instead of JPEG (alter options, more space needed)?
Are black&white prints from JPEGs from a CD more expensive than color?
Thank you very much!!
Nico

If you are going to do any form of editing (including playing with the brightness etc) you should always save as TIFF. Jpg should be used for emailing a final version, or posting to the web only.

Prints hold about 300dpi of information, so scanning at a greater resolution than that doesn’t achieve a lot, though if you were going to do significant editing it can make detailed cloning and healing etc easier if you scan at a higher resolution than that.

Hi,

in my photo class, we developed black&white prints on multigrade photopaper (no digital camera so far). Unfortunately, most of them were not developed long enough, so they are too bright. I would like to scan the photographs (they are 10×15 inch), adjust brightness, contrast, gamma, etc. in Photoshop PS and then burn them on a CD and have them developed in one of those 1-hour-photo-shops.
My question is: What DPI should I use for this purpose? The default value is 200, but I think this is too low. For Web-purposes I usally choose 72dpi and for other things 300. But if the pictures shall be developed in a photo-shop on professional photo-paper resolution should be as high as it makes sense.

What do you suggest?

Also, do you think it is worth the effort to scan as TIFF instead of JPEG (alter options, more space needed)?
Are black&white prints from JPEGs from a CD more expensive than color?

Thank you very much!!
Nico

"Nicolas Hoch" wrote in message

Hi,

in my photo class, we developed black&white prints on multigrade photopaper (no digital camera so far). Unfortunately, most of them were not developed long enough, so they are too bright. I would like to scan the photographs (they are 10×15 inch), adjust brightness, contrast, gamma, etc. in Photoshop PS and then burn them on a CD and have them developed in one of those 1-hour-photo-shops.
My question is: What DPI should I use for this purpose? The default value is 200, but I think this is too low. For Web-purposes I usally choose 72dpi and for other things 300. But if the pictures shall be developed in a photo-shop on professional photo-paper resolution should be as high as it makes sense.

What do you suggest?

Also, do you think it is worth the effort to scan as TIFF instead of JPEG (alter options, more space needed)?
Are black&white prints from JPEGs from a CD more expensive than color?
Thank you very much!!
Nico

pardon my not answering what you asked but
why scan already poor quality prints? the scan will have one step poorer image than the originals.
A scan of a bad print is not going to get you other than what you see now, except darker or lighter. If the highlights are burnt up you would have to draw whatever in there, no amount of PS can bring back what isn’t there. Get a scan of the original negatives and work from those.or just bring the negatives, print film is quite forgiving and photo stores have machines that read a variety of quality of negatives.
If you are you trying to pass off commercial prints as yours for your photo class you won’t fool anyone.

"Nicolas Hoch" wrote in message

Hi,

in my photo class, we developed black&white prints on multigrade photopaper (no digital camera so far). Unfortunately, most of them were not developed long enough, so they are too bright. I would like to scan the photographs (they are 10×15 inch), adjust brightness, contrast, gamma, etc. in Photoshop PS and then burn them on a CD and have them developed in one of those 1-hour-photo-shops.
My question is: What DPI should I use for this purpose? The default value is 200, but I think this is too low. For Web-purposes I usally choose 72dpi and for other things 300. But if the pictures shall be developed in a photo-shop on professional photo-paper resolution should be as high as it makes sense.

What do you suggest?

Also, do you think it is worth the effort to scan as TIFF instead of JPEG (alter options, more space needed)?
Are black&white prints from JPEGs from a CD more expensive than color?
Thank you very much!!
Nico

If the prints are under developed, or under exposed, then they are fit only for the bin. Do not even think about scanning them, the detail is just not there.

If they had been over developed or exposed, then they maybe could be rescued, but the sensible thing is to get back into the darkroom and do them again and do them right this time.

Do not be tempted to pull them out of the developer before they have had their full time. That is the most common mistake made by beginners.

Roy G

In article ,
"Nicolas Hoch" wrote:

in my photo class, we developed black&white prints on multigrade photopaper (no digital camera so far). Unfortunately, most of them were not developed long enough, so they are too bright. I would like to scan the photographs (they are 10×15 inch),…

Already you are walking down the wrong path.

If the prints are underdeveloped, they have no detail in the hilights. There’s no image information there, so no amount of playing in Photoshop will create image information there. Go back into the darkroom and print them again.

adjust brightness, contrast, gamma, etc. in Photoshop
PS and then burn them on a CD and have them developed in one of those 1-hour-photo-shops.
My question is: What DPI should I use for this purpose? The default value is 200, but I think this is too low. For Web-purposes I usally choose 72dpi and for other things 300. But if the pictures shall be developed in a photo-shop on professional photo-paper resolution should be as high as it makes sense.
What do you suggest?

300 pixels per inch is fine; there’s little gain in going above that, and no gain at all in going above 350-400 pixels per inch.

Also, do you think it is worth the effort to scan as TIFF instead of JPEG (alter options, more space needed)?
Are black&white prints from JPEGs from a CD more expensive than color?

JPEG is "lossy." In English, what that means is that JPEG is a format that deliberately degrades image quality in order to make a file smaller.

JPEG was invented for situations where file size is critically important and image quality is not important. It should never, ever be used unless you have a clear, specific, and good reason why you have to use it and no other format will do, such as for the Web. You should never make a JPEG your only copy of an image. You should never use JPEG for general-purpose use. Never use JPEG unless you absolutely have to, and if you do, keep a copy of the image in a lossless format such as PSD or TIFF.

—
Art, photography, shareware, polyamory, literature, kink: all at http://www.xeromag.com/franklin.html

"Nicolas Hoch" wrote:

in my photo class, we developed black&white prints on multigrade photopaper
(no digital camera so far). Unfortunately, most of them were not developed
long enough, so they are too bright. I would like to scan the photographs (they are 10×15 inch),…

If you still have the negs, scan them in and you’ll be much happier.

If you only have the prints, use 300 pixels per inch. You may be able to improve things a little, but tacit and others are correct that this is definitely second best.
—

Mike Russell
www.curvemeister.com

hi,
most printers today can print in the resolution of 600 x 1200 dpi. epson inkjets has a resolution in the 2400 range.
why not scan them at that resolution?
300 dpi was suggested because several years back 300 dpi was the standard resolution for most inkjet printers. since they have improved shouldn’t scanning follow this standard?

most printers today can print in the resolution of 600 x 1200 dpi. epson inkjets has a resolution in the 2400 range.
why not scan them at that resolution?

Because dots are not pixels in that context.

Andrew

Nicolas Hoch wrote:

most printers today can print in the resolution of 600 x 1200 dpi. epson inkjets has a resolution in the 2400 range.
300 dpi was suggested because several years back

Can somebody please explain that? It occured to me before as well, but I think printing resolution is something other than scanning resolution. But somehow it has to be interconnected/related?
Why wouldn’t it make sense to scan an image at 600 or even 1200 dpi, if you can print at 1200?

Thank you very much!
Nico

A pixel is a picture element. A dot is a dot. If you look at an inkjet print through a magnifying glass, you will see that each pixel is made up of lots of tiny dots.

A scanner actually scans pixels, not dots, but unfortunately with consumer scanners the resolution is referred to as dpi when they should say ppi. An inkjet printer prints pixels, each of which is made up of many dots. The pixel resolution of an Epson printer is 360ppi (some people reckon 720ppi).

Andrew

most printers today can print in the resolution of 600 x 1200 dpi. epson inkjets has a resolution in the 2400 range.
300 dpi was suggested because several years back

Can somebody please explain that? It occured to me before as well, but I think printing resolution is something other than scanning resolution. But somehow it has to be interconnected/related?

Why wouldn’t it make sense to scan an image at 600 or even 1200 dpi, if you can print at 1200?

Thank you very much!
Nico

I would scan them in to Photoshop. Make a duplicate of the images layer (so you have two) and then change the top ones layer blending mode to Multiply. You can make other copies of the image on top of this and set them to multiply as well.

"Nicolas Hoch" wrote in message

Hi,

in my photo class, we developed black&white prints on multigrade photopaper (no digital camera so far). Unfortunately, most of them were not developed long enough, so they are too bright. I would like to scan the photographs (they are 10×15 inch), adjust brightness, contrast, gamma, etc. in Photoshop PS and then burn them on a CD and have them developed in one of those 1-hour-photo-shops.
My question is: What DPI should I use for this purpose? The default value is 200, but I think this is too low. For Web-purposes I usally choose 72dpi and for other things 300. But if the pictures shall be developed in a photo-shop on professional photo-paper resolution should be as high as it makes sense.

What do you suggest?

Also, do you think it is worth the effort to scan as TIFF instead of JPEG (alter options, more space needed)?
Are black&white prints from JPEGs from a CD more expensive than color?
Thank you very much!!
Nico

In article ,
wrote:

hi,
most printers today can print in the resolution of 600 x 1200 dpi. epson inkjets has a resolution in the 2400 range.
why not scan them at that resolution?

Because "dpi" is not the same thing as "pixels per inch," though this is a mistake many people often make.

It is true that an Epson inkjet printer prints at a very high resolution. However, one dot made by the printer is not the same as one pixel. It takes many, many printer dots to make one pixel.

Inkjet manufacturers do not advertise this, but the effective resolution of an inkjet printer, measured in pixels, is typically between 280 and 340 pixels per inch. Anything above this *does not* improve image quality.

—
Art, photography, shareware, polyamory, literature, kink: all at http://www.xeromag.com/franklin.html

"tacit" wrote in message

In article ,
wrote:

hi,
most printers today can print in the resolution of 600 x 1200 dpi. epson inkjets has a resolution in the 2400 range.
why not scan them at that resolution?

Because "dpi" is not the same thing as "pixels per inch," though this is a mistake many people often make.

It is true that an Epson inkjet printer prints at a very high resolution. However, one dot made by the printer is not the same as one pixel. It takes many, many printer dots to make one pixel.
Inkjet manufacturers do not advertise this, but the effective resolution of an inkjet printer, measured in pixels, is typically between 280 and 340 pixels per inch. Anything above this *does not* improve image quality.

—
Art, photography, shareware, polyamory, literature, kink: all at http://www.xeromag.com/franklin.html

hi,
most printers today can print in the resolution of 600 x 1200 dpi. epson inkjets has a resolution in the 2400 range.
why not scan them at that resolution?

—–

Scan resolution is how many digital pixels will be recorded per inch of film or print that is being scanned. This results in larger or smaller output files.

Lets say you have a slide that is 1.00 inches wide, (1.00 for easy math)and you scan it at 3200ppi (ppi same as dpi but the preferred terminology for scanner heads), You end up with a file that is 3200 pixels wide.

Now, as others have said, you should print this file at 300dpi, (aprox.. Many images print great at 150dpi or less) for best quality. greater than 300dpi is meaningless as your eye cannot tell the difference.) Printing it at 300dpi will yield a print 10.6inches wide, (3200/300)

Now your 2400dpi printer will put 8 dot’s in every one of those pixels,(2400/300). More printed dots per pixel more better…

Now, there is more to it but that is the basics. Some printer/scanner heads may chime in with the reasons why this is not EXACTLY correct but this will make it easier to understand, hopefully.

Scan large = print large

BIG mega pixel DSLR = Big print capability..

Later

http://imagequest.netfirms.com

wrote in message

300 dpi was suggested because several years back 300 dpi was the standard resolution for most inkjet printers. since they have improved shouldn’t scanning follow this standard?

1440dpi 4-color ink-jet printer:
1440/4=360 optimal dpi number

2880dpi 7-color ink-jet printer:
2880/7 = ~400 dpi

Oversimplified, but illustrative.

—
.. .::xcat
::::::::::::::

"Nicolas Hoch" wrote in message

For Web-purposes I usally choose 72dpi and

dpi has absolutely no meaning when web purpose is in question.

Dpi means dots per inch ON PAPER, or whatever else print medium, not computer screen.

Computer screen has density of pixels (not dots) and therefore you have, for example 96 ppi (pixels per inch) on my 19 inch monitor with resolution of 1280*960. If I change resolution to 1024*768 that would result in 74,5 ppi, or 57,7 ppi for 800*600. 21" monitor have different ppi’s for same resolutions…

GIMP has utility witch allows you to easily measure vertical and horizontal ppi density of your computer screen.

—
.. .::xcat
::::::::::::::

.. .:xcat wrote:

1440dpi 4-color ink-jet printer:
1440/4=360 optimal dpi number

2880dpi 7-color ink-jet printer:
2880/7 = ~400 dpi

Oversimplified, but illustrative.

No. The number of ink colours available does not relate to the dots per pixel.

Andrew

In article ,
"DBLEXPOSURE" wrote:

Now your 2400dpi printer will put 8 dot’s in every one of those pixels,(2400/300). More printed dots per pixel more better…

Actually, it will put 64 dots in each pixel. To get the dots printed per pixel, you use (printer resolution/pixel resolution)^2.

—
Art, photography, shareware, polyamory, literature, kink: all at http://www.xeromag.com/franklin.html

In article <din41o$80l$>,
". .:xcat" wrote:

1440dpi 4-color ink-jet printer:
1440/4=360 optimal dpi number

2880dpi 7-color ink-jet printer:
2880/7 = ~400 dpi

Oversimplified, but illustrative.

You do not divide the printer resolution by the number of colors; the number of colors of ink is absolutely, positively irrelevant to the effective pixel resolution of the printer.

—
Art, photography, shareware, polyamory, literature, kink: all at http://www.xeromag.com/franklin.html

wrote in message

hi,
most printers today can print in the resolution of 600 x 1200 dpi. epson inkjets has a resolution in the 2400 range.
why not scan them at that resolution?

Because of the difference between dpi and ppi.

Inkjet printers have a mechanical dot placement accuracy. Some (variable) picolitre droplet or multiple droplets can be placed with that spatial accuracy. This accuracy can typically be 1/1200th or 1/2400th of an inch for many HP printers, upto 1/9600th of an inch in one direction for Canon printers. Epson printers typically have a droplet placement accuracy of 1/1440th upto 1/2880th of an inch.

In order to create more colors than the couple of ink cartridges (and the paper color in the absence of ink) provide, the intermediate colors are produced by mixing (multiple colors/droplet sizes per placement position) or dithering (in a carefully designed pseudo-random pattern) of adjacent multiple colors. The size of the dithered color rectangle/square is typically 1/600th or 1/720th of an inch. Because the size is fixed, it is easier to optimize the pattern within and thus allows to avoid repetitive patterns.

It is therefore valid to describe the native resolution of such printers as 600 or 720 Pixels per inch (PPI). Images that have more or fewer pixels than the native printer resolution, will (!) be resampled by the printer driver to 600 or 720 ppi. Some printer drivers do a decent job, while others use very simple resampling algorithms.

300 dpi was suggested because several years back 300 dpi was the standard resolution for most inkjet printers. since they have improved shouldn’t scanning follow this standard?

To reliably(!) scan a feature of 1/600th or 1/720th of an inch, twice that regular spatial sampling density should be used (so 1200ppi or 1440ppi). It is also referred to as the "Nyquist frequency". Scanning/sampling at a higher spatial frequency will not get more image detail out of the image, while scanning/sampling at a lower frequency will produce more or less visible aliasing artifacts (moiré). This may produce large scan files so it may be necessary to down-sample that file, depending on its final use or on storage limitations.

Digital photochemical prints typically have resolutions like 300ppi or 400ppi, because each pixel is continious tone (no dithering), and therefore can be scanned at lower (minimally 600 or 800 ppi) resolutions. If that produces too few pixels for a certain output size, one can choose to scan at a higher resolution (which will require more storage space but won’t have more image detail), or resample (which also won’t add resolution, but will allow to add some type of sharpening that mimics added resolution).

Bart

"Andrew Morton" wrote in message

No. The number of ink colours available does not relate to the dots per pixel.

I stand corrected.

—
.. .::xcat
::::::::::::::

why scan already poor quality prints? the scan will have one step poorer image than the originals.
A scan of a bad print is not going to get you other than what you see now, except darker or lighter. If the highlights are burnt up you would have to draw whatever in there, no amount of PS can bring back what isn’t there. Get a scan of the original negatives and work from those.or just bring the negatives, print film is quite forgiving and photo stores have machines that read a variety of quality of negatives.

Hi Kat Woman,

I have just begun scanning about 200 old family photos, however, I have found it difficult to obtain good results from negatives, and have been scanning the prints instead. I guess that once again, I’m doing something wrong in the scanning process judging by your answer above. Have you any ideas?
I am scanning at 400 or 600 dpi, as black & white negs. The auto adjust setting is on. I have a Umax astra 3450.
Any help appreciated.
Thanks, Chrissssssss…………..

PS
Re my previous post about not being able to see more than one image, I had no idea that the full page Browser window should even be closed. I only use the Open option now!

"Chrisssssss………" wrote in message

why scan already poor quality prints? the scan will have one step poorer image than the originals.
A scan of a bad print is not going to get you other than what you see

now,

except darker or lighter. If the highlights are burnt up you would have

to

draw whatever in there, no amount of PS can bring back what isn’t there. Get a scan of the original negatives and work from those.or just bring

the

negatives, print film is quite forgiving and photo stores have machines that read a variety of quality of negatives.

Hi Kat Woman,

I have just begun scanning about 200 old family photos, however, I have found it difficult to obtain good results from negatives, and have been scanning the prints instead. I guess that once again, I’m doing something wrong in the scanning process judging by your answer above. Have you any ideas?
I am scanning at 400 or 600 dpi, as black & white negs. The auto adjust setting is on. I have a Umax astra 3450.
Any help appreciated.
Thanks, Chrissssssss…………..

You don’t say what size the negs are but 400 or 600 doesn’t sound like enough. Consider this: at 600 pixels per inch a 35 mm image comes out to about one half megapixel. Have them scanned in a commercial shop with high res scanner.

In message <dj3gpi$89f$>, Chrisssssss………
writes

I have just begun scanning about 200 old family photos, however, I have found it difficult to obtain good results from negatives, and have been scanning the prints instead. I guess that once again, I’m doing something wrong in the scanning process judging by your answer above. Have you any ideas?

The best results would come from a film/slide scanner to scan the negatives – the photos are derived from the negative. There may be an adapter available to improve your scanner’s ability at scanning negatives. Or you could take them to a photo lab and they will bung them on a CD for you.

—
Timothy

In article <dj3gpi$89f$>,
"Chrisssssss………" wrote:

I am scanning at 400 or 600 dpi, as black & white negs. The auto adjust setting is on. I have a Umax astra 3450.
Any help appreciated.

In order to scan negatives or slides, you should scan them at a resolution so that the final image is 300 pixels per inch at 8×10. That means scanning the negative itself at a much, much higher resolution.

If your scanner is a flatbed scanner with a film-scanning attachment, expect to get poor-quality scans. You need a film or slide scanner, not an attachment to a flatbed scanner, for good results.

—
Art, photography, shareware, polyamory, literature, kink: all at http://www.xeromag.com/franklin.html

"Chrisssssss………" wrote in message

why scan already poor quality prints? the scan will have one step poorer image than the originals.
A scan of a bad print is not going to get you other than what you see now, except darker or lighter. If the highlights are burnt up you would have to draw whatever in there, no amount of PS can bring back what isn’t there.
Get a scan of the original negatives and work from those.or just bring the negatives, print film is quite forgiving and photo stores have machines that read a variety of quality of negatives.

Hi Kat Woman,

I have just begun scanning about 200 old family photos, however, I have found it difficult to obtain good results from negatives, and have been scanning the prints instead. I guess that once again, I’m doing something wrong in the scanning process judging by your answer above. Have you any ideas?
I am scanning at 400 or 600 dpi, as black & white negs. The auto adjust setting is on. I have a Umax astra 3450.
Any help appreciated.
Thanks, Chrissssssss…………..

PS
Re my previous post about not being able to see more than one image, I had no idea that the full page Browser window should even be closed. I only use the Open option now!

sorry I have never used a flat bed scanner
I use a canoscan 4000
it is designed for negatives and slides
my scanner gives a res of 4000dpi but small size like one inch by 1.5 inches the actual size of the slide. If
I don’t resample it, it comes out to about 9×13 at 300 dpi for printing.

Nicolas Hoch wrote:

most printers today can print in the resolution of 600 x 1200 dpi. epson inkjets has a resolution in the 2400 range.
300 dpi was suggested because several years back

Can somebody please explain that? It occured to me before as well, but I think printing resolution is something other than scanning resolution. But somehow it has to be interconnected/related?

Why wouldn’t it make sense to scan an image at 600 or even 1200 dpi, if you can print at 1200?

Thank you very much!
Nico

Hi Nico,

Tacit and the others have explained this topic very well. Just to comment on this issue in a slightly different way, maybe look at it this way: The resolution of the actual digital image, expressed in ppi, tells you how much detail that digital image holds. Generally speaking, a digital photo needs to be at a resolution of 300ppi to give a photo quality print. (eg. an image to be printed at 6 x 4 would measure 1800 x 1200 pixels ie. 6 x 300, 4 x 300).
The dpi rating of the printer is telling you how fine the printer can print, meaning the higher the dpi, the more dots will be printed to represent each pixel of the photo.
So summing it up, ppi is amount of detail in the photo of a given size, dpi basically allows one to calculate (using the formula Tacit provided) of how many dots the printer will lay down per pixel of the image.

Regards,
Brian.

most printers today can print in the resolution of 600 x 1200 dpi. epson inkjets has a resolution in the 2400 range.

Of course, that is incorrect.

The dpi rating of the printer is telling you how fine the printer can print, meaning the higher the dpi, the more dots will be printed to represent each pixel of the photo.

If only the manufacturers would be honest with the "dpi" rating. For common inkjet printers, it remains 360 at best.

"Lorem Ipsum" wrote in message
SNIP

For common inkjet printers, it remains 360 at best.

Not really, try printing the target from
<http://www.ddisoftware.com/qimage/quality/> and follow the instructions enclosed with the file.

You’ll probably find there’s additional quality to be gotten from printing at 600/720 ppi.

Bart

Bart van der Wolf wrote:

"Lorem Ipsum" wrote in message
news: SNIP

For common inkjet printers, it remains 360 at best.

Not really, try printing the target from
<http://www.ddisoftware.com/qimage/quality/> and follow the instructions enclosed with the file.

You’ll probably find there’s additional quality to be gotten from printing at 600/720 ppi.

Bart

This page looks quite scientific an competent – but what they present is a black and white image.

For a black and white image ppi and dpi are identical as there is no need for shading.

Replace all the black in the sample image with a color that requires mixing (like rgb 45/76/93) and look for the difference 🙂

For full color printing there is a need for shading and shading means reduction of resolution: Assume ink dots printed at 720dpi being circles, the center of two circles is 1/720th of an inch apart and The size of each circle is such that it fully covers a 1/720th" square. Two adjacent circles overlap a little so a full colored area with no white can be printed.

Now printing each other black dot will create a 75% grey, printing one of three dots will create a 50% grey. And if you want to print just 30 shades of grey you need an 8×8 dot pattern. 8 dots linear in each direction for one pixel reduce those 720dpi to 90ppi. 5760dpi are reduced to 720ppi for printing 30 shades of grey.

Then think of colors: You need to print each ink in a separate place to avoid color mixing. So when you print with six inks you need additional possible dot spaces and you always need some unused space for white.

Usually you will need about 40% unused white, and you will want to print at least 30 shades of every ink for a full color image. Lets calculate: 30 shades per ink means 30 dots per ink, with six inks that’s 180 dots. That’s 60 percent as 40% of the dots will be unused. 100% are 300 dots. The next smaller square from an integer is 289 (17×17) and assuming that some shades of cyan and magenta are replaced by the light cyan and light magenta will reduce the space required for one pixel to 256 or 16×16 dots.

5760 dpi divided by 16 is 360ppi.

Higher resolutions can be achieved when printing "business graphics" with only a few shades or text using a single ink but for true color images there is a visibly lower limit.

Michael

tacit wrote:

In article <din41o$80l$>,
". .:xcat" wrote:

1440dpi 4-color ink-jet printer:
1440/4=360 optimal dpi number

2880dpi 7-color ink-jet printer:
2880/7 = ~400 dpi

Oversimplified, but illustrative.

You do not divide the printer resolution by the number of colors; the number of colors of ink is absolutely, positively irrelevant to the effective pixel resolution of the printer.

This is not entirely correct 🙂

If you have for inks (C,M,Y,K) and need a 50% Magenta you would need 3 dot positions (one Magenta, two White).
If you have 6 inks (C, lC, M, lM, Y, K) you can create that 50% Magenta using one dot of light Magenta – so for the lighter shades of Magenta and Cyan you need less white dots. This need of fewer white dots reduces the total space needed to do the shading for a desired color and thus increases the resolution in ppi for the same dpi printer.

But still, 360ppi from an inkjet printer is an exceptional good value.

Michael