jcrous wrote:Joe, I had some idle time at work today, so again I went into it again.
Amazon.com flagged my book cover images in the past that the dpi was lower than 300 dpi. I resized it with PSP X8 too 300, but it was 300 pixels per cm.
Despite what the Amazon flag said, your image wasn't lower than 300dpi. It was lower than 300ppi (perhaps I didn't make that as clear as I could have in my previous post about the difference?). And you did not resize it to 300dpi but to 300ppi as the box for entering that change is also labelled.
jcrous wrote: If I change it to ppI it is 762. Then Amazon did not flag it as lower than 300 dpi.
Of course it is 762ppi and didn't get flagged as lower than 300ppi (even though Amazon incorrectly uses dpi when communicating, which is their error). Let's look at the terminology again.
PPI stand for Pixels Per Inch. PPCM stands for Pixels Per Centimeter. So you resized it using the PPCM option which provided a print size of 300 Pixels Per Centimeter. There are 2.54 centimeters in an inch. So 300 pixels per centimeter equals 762 pixels per inch (300ppcm x 2.54 = 762ppi). So that's way over 300ppi and much more print resolution than needed but the Amazon people can resize it to a lower print resolution and come out with the correct print size they need. They can't do that with an image that has a print resolution that is too much below 300ppi because they would have to upsize the image, which causes noticeable image degradation, much more so than downsizing.
jcrous wrote:Photoshop CS6 and PSP X6 & 2018 also use ppi or ppcm.
Of course Photoshop and PSP use ppi or ppcm. They are professionals who actually know the difference between ppi and therefore use the correct terminology.
jcrous wrote:HOWEVER:
Affinity Photo is making use of dpi.
Then that should tell you something about the people at Affinity, Amazon as well as others who use incorrect terminology. If you actually understand the difference between ppi and dpi then wouldn't you prefer to use the correct terminology so that knowledgeable people actually know what you're talking about?
jcrous wrote:So I conclude that PS and PSP is simulating dpi with ppi or ppcm and it is close enough that the automatic servers of Amazon is not throwing it out.
Then you would be making an incorrect assumption. PS and PSP are using the correct terminology for the process you are doing, which is providing information to embed in the image that tells the printer how may pixels of the image to print to one inch of paper. Amazon's servers wouldn't throw it out because it contained AT LEAST the minimum 300ppi they require. They can work with that better than with an image containing less than 300ppi.
If you are saying that you think that ppi and dpi are close enough that is completely incorrect because they are two different things, even though some people still use the terms interchangeably because, for the most part, they don't understand the difference and have also been led by others who should know better to think they are the same thing. And people who think that are lacking the knowledge about how their printers work also.
A printer is generally rated as being able to produce output at x number of DPI (generally being the best that the printer can achieve). Let's say it's 600dpi just for argument. If you print a photo and select the highest quality print setting it will print at the maximum 600dpi (dots of ink per inch of print to put it very simply) and you'll get a sharp photo. But if you print the same image using the Draft setting the printer will conserve ink and spit out - for example - only 200dpi or dots of ink per printed inch, resulting in an image that isn't nearly as sharp or colorful as the former. (I'm sure you've seen the difference between draft, standard and high settings in your printer software). BUT, regardless of the printer setting (High, Standard or Draft, which changes the DPI output), that same image will always still print out at the same size! That's because the printed size of the image has nothing to do with DPI (which is only a printer specification function) and is ONLY a factor of the PPI setting embedded in the image by the graphic software.
If you instead are saying that ppi and ppcm are close enough that is a matter of interpretation. As you've seen above, 300ppcm is equal to 762ppi, which isn't even close to 300ppi (or, to use Amazon's incorrect terminology, 300dpi). Image editors give you the option to work in imperial measure (inches) or metric (centimetes) per inch to accommodate people in countries in which one or the other is the measurement standard used. I suspect most Europeans use the ppcm option because they are familiar with metric. Americans aren't, so they tend to use ppi.[/quote]
So I repeat what I said in my previous post. Professional software like Photoshop and even PSP do not have a setting for changing dpi because it is impossible for graphic software to do so. They only have a setting for PPI. And because you use PSP I wonder why, when you see the dialogue that allows you to change the PPI, you call it DPI when you are at least now aware that they are different animals and your software uses the correct terminology?
If you wonder why I stress this topic, it's because over the years I have seen dozens and dozens of posts on different forums where people express confusion between the terms PPI and DPI. And the reason they are confused is because so many people insist on using the terms interchangeably even when they are aware that they are not the same thing at all. It's a confusion that seems to have started many years ago when printer manufacturers began manufacturing scanners and, because their people were so used to the term dpi when it came to printers, they used it in their scanner software when, in fact, the scanner software is really asking at what ppi do you wish to scan the image. Apple was the worst offender in adopting this incorrect terminology in its manuals, etc. So you shouldn't be surprised that Affinity uses the incorrect terminology also given that it was originally released as Mac software.
If you are still not quite sure of how PPI works, you might want to read the thread at the link below. The original poster was one of those persons confused by PPI. My post (the fourth one down in the thread) in reply used some examples to try to assist him and seemed to be received favorably. If you were to read it and still have questions then feel free to ask.
http://forum.corel.com/EN/viewtopic.php?f=56&t=61213
