DPI vs. PPI - What's the difference?

[Midgardsormr]

Not at all. I just wanted to be sure I wouldn't lose too much crispness in the labels at the lower resolution, and I have little experience with designing for print.

Thanks!

[Redrobes]

Gosh, what a thread ! So much information and so much misunderstanding.

The other day on some board I saw that some chap ask for a high res board game and the map was produced here by somebody. He then said that this map was 7200 x 4800 pixels and was for a board 24x18 inches and at 600dpi. Now my math is not stellar but even I can see something is amiss there.

(Edit -- Ahh found it... http://ritz21.deviantart.com/art/Myt...d-Map-70277509 got the numbers a little wrong but comment still applies ! Well I guess you can print it at 600dpi but it wont be 600dpi)

One thing that has been overlooked is that most image formats embed the DPI into the file spec. A png file has its DPI written into the format as does BMP. I am fairly sure TIFF does but I dont know about GIF or JPG. Most paint packages allow you to scale by DPI as well as image size. You can scale in PSP by a different DPI and keep the image the same res with its Actual Size box.

A post earlier was suggesting that it does not make any difference with a VTT what the DPI is. I believe that *in general* this is incorrect as some of them scale the map based on its pixel res. I.e if you want something 5feet across then it had better be exactly 200 pixels if you want all the icons on top to be in scale which makes large maps very difficult in those VTTs.

Since I write ViewingDale I can add a little more info too about VTTs. For some of them - namely mine - the res does not fundamentally matter as it is a zoom browser where you can enter any image for any scale tho whether its of high enough res to make it usable is another matter. So for me 200 pixels per 5ft scale may not be enough in certain circumstances - for example if you wanted to print that map for miniatures at 1:60 real scale on a commercial print job. So I would welcome the large gigapixel images for poster sized maps - bring it on !

ViewingDale maps can be export at any DPI up to about 7200. The scale of that image is independent of the DPI but the real size, DPI and map scale are interrelated so that you can ask for an image which is 200 pixels for 5ft if you so wished.

Finally, I would suggest that if you do want to use a map in a VTT then its better not to have a grid on it as all VTTs that I know of can put one on at the selected scale. DM's also like for it not to have fixed items like monsters and stuff like that where icons will be used on top and moved dynamically. Now how dynamic something has to be before people like it is very variable.

As far as I can see, an image is just a bunch of pixels. If that image is supposed to exist in the real world at a certain size then it has a PPI / DPI value for that real world size. That value might be very different on screen or when scaled to print via a printer driver.

[RobA]

Most paint packages allow you to scale by DPI as well as image size. You can scale in PSP by a different DPI and keep the image the same res with its Actual Size box.

Wow! I am amazed at how often this topic keeps coming up

Just a clarification on your point. Changing the DPI of a file SHOULD not change any of the pixels of the image. So changing the DPI should not be called "Image Scaling", and IMHO any program that puts the "change DPI" option under image scaling is doing users a huge disfavor by continuing to spread confusion. The proper place for "change DPI" should either be in the print settings, as this is the only place it has meaning....when printing.

A post earlier was suggesting that it does not make any difference with a VTT what the DPI is. I believe that *in general* this is incorrect as some of them scale the map based on its pixel res. I.e if you want something 5feet across then it had better be exactly 200 pixels if you want all the icons on top to be in scale which makes large maps very difficult in those VTTs.

The 200px/5ft scale was stated (AFAIK) by Dundjinni and has become standard for photo-realistic/artistic battlemaps (as they are intended to be printed and 200DPI gives a nice print result). But again, DPI has nothing to do with the px/ft representation of an image. Some VTT programs may use this as a default setting when using consistently sized art, but hopefully they aren't tied to it. Personally, if I was doing VTT work I'd stick with a DPI (or more correctly PPI, when referring to screens) of 75-100 which is closer to actual screen resolutions.

As far as I can see, an image is just a bunch of pixels. If that image is supposed to exist in the real world at a certain size then it has a PPI / DPI value for that real world size. That value might be very different on screen or when scaled to print via a printer driver.

DING-DING-DING We have a winner in the "most succinct explanation of why DPI/PPI is irrelevant when discussing an image (file, that is) category"!

-Rob A>

[Midgardsormr]

Wow! I am amazed at how often this topic keeps coming up

My fault. I bumped it with my question to GP.

I thought it might be helpful to see the Resample dialogue from CorelPaint and look at how changes in DPI settings affect both the print size and the resolution of an image when working in that package.

The left column of the first image is how the dialogue looks when examining the image in inches. The right column is how it looks when looking at the pixels. I started with my current WIP, which is ~26 X 20 inches at 300 dpi. As you can see, that makes the image 7629 X 5894 px.

When I changed the dpi to 75 but kept the size in inches did not change. When printed, this map will still be 26 X 20. However, the total number of pixels in the image has changed to 1907 X 1474, which will make it far smaller on the computer screen--only 25% of its original size in each dimension.

If you check "Maintain original size" in CorelPaint then change the DPI, the pixels remain constant, but the print size changes. I attached a second image to show the effects of checking that box. Note the new print size is 100 X 78 inches, but the size in pixels is still 7600 X 5800 (and is greyed out--it cannot be changed in this mode).

In Corel, the DPI is set as part of defining the image--You can change any two: DPI, size in pixels, and print size. It's a bit like an electrical calculation with voltage, current, and resistance. You can change two, and those two combined determine the third. If you're designing for the screen, only pixels matter, though, so you can ignore the other two. If you think there's a chance that someone might one day print the map, though, it's probably worth at least glancing at them.

[RobA]

And I still think that CorelPaint (any many other programs) do a great disservice to their users by doing it that way. All it does is lead to confusion...

-Rob A>

[Carnifex]

The proper place for "change DPI" should either be in the print settings, as this is the only place it has meaning....when printing.

Not entirely correct. Every professional graphic designer sets the ppi and printing size in Photoshop so that the image shows up correctly in Indesign/Quark.

[Carnifex]

72 dpi works as long as the map file created was made at your correct dimension and scaled at 72 dpi when you export the file.

If you convert a higher or lower resolution file to 72 dpi or rescale your map to 20 x 30 when it wasn't that in the first place - then 72 dpi is not what you want - it will look crappy.

Using high resolution like 300 dpi is required if your map file has to be rescaled to fit the dimensions and resolution you ask.

In other words, if you don't have to rescale 72 dpi is fine, if you do have to rescale start with a higher resolution...

I hope that wasn't too complicated?

I really recommend at least 150 dpi for printing and 300 if you want the best quality. It all depends on the quality of the print. In offset printing you should know the resolution of the raster (lpi (lines per inch)) - and you should have lpi x 2 in dpi.

Also scaling down is usually not a problem (say from 600 to 150 dpi).



Here's a little mini-guide to scaling things in Photoshop:
If you want to change only the dpi (to 200 dpi) but keep the pixel size:
Image > Image Size (or alt+ctrl+I)
Make sure "Scale styles" and "constrain proportions" are checked. "Resample image" should NOT be checked.
Type "200" in the resolution box and click OK.

If you want to scale the image and increase both pixel size and resolution (to 200 dpi):
Image > Image Size (or alt+ctrl+I)
Make sure "Scale styles", "constrain proportions" AND "Resample image" are checked.
Type "200" in the resolution box and click OK.

Does it make sense?

Be careful when you increase the number of pixels of a picture - you'll always loose quality.

[alucard339]

Thanks for the mini guide, I always find it confusing to work it out.

Like my original wold map: its in 150 DPI, but at a size so huge (12000 by 8000 for 70mb) that its not fun to work with for small change.

ciao,
Alu.

[RobA]

Not entirely correct. Every professional graphic designer sets the ppi and printing size in Photoshop so that the image shows up correctly in Indesign/Quark.

We can agree to disagree then. You are accepting that what is done by a particular software package or combination of packages is "correct". While this may be what is necessary to get it to work properly, it does not make it "right".

(And the fact that you even need to provide a mini-tutorial just reinforces my point that the applications have confused the issue to the point of being incomprehensible to most users. )

-Rob A>

[Carnifex]

We can agree to disagree then. You are accepting that what is done by a particular software package or combination of packages is "correct". While this may be what is necessary to get it to work properly, it does not make it "right".

(And the fact that you even need to provide a mini-tutorial just reinforces my point that the applications have confused the issue to the point of being incomprehensible to most users. )

-Rob A>

Not quite

Well, the way I see it, the way PS does it is the only way to do it when you work professionally with layout. This is the way it have worked since the dawn of DTP. If you have any other suggestion to make it work you're welcome.

To import the image to InDesign/Quark/Pagemaker in sceen resolution would be a pain (this is possible nowadays if you want) and then you'd have to check the DPI of every image in the layout program every time you want to resize it. That would be impossible. Some programs automatically resize the images to a certain dpi - but that is not optimal imo when you want maximum control and that would complicate certain other things as well (b/w images for example where you want 1200 ppi for printing).

(When you make a PDF for offset print you can also set the DPI to limit the size in case you used too large images.)

(PS is not just any application - it's industry standard. But I agree that they could make the image size much more clear.)

I also agree with you that dpi/ppi is completely useless until you want to print something - the only thing that matters until then is the number of pixels.