DPI: TRUTH OR LIE?

[mgeduld]

For those of you who don't know, I teach graphics and multimedia classes for a living (Flash, PhotoShop, etc.) For several years now, I've been explaining to students how to optimize graphics for the web, and inevitably the subject of dpi/ppi comes up. I usually tell students to save files (or resample files) to 72dpi, because dpi stands for dots-per-inch and there are 72 dots (pixels) per inch on computer monitors. (We've discussed the PC/MAC difference here before: PCs display at 96dpi -- actually many MAC monitors do too nowadays. Xara is a PC app, so its default resolution is 96.)

Trouble is, this isn't true. Monitors DON'T display 72 pixels per inch. Nor do they display 96 pixels per inch. In fact, the concept of an "inch" doesn't make much sense when talking about monitors. For instance, if you design a 72 pixel by 72 pixel graphic at 72dpi, this should display at 1 inch x 1 inch, right? You should be able to hold a ruler up to your monitor and measure it at 1 inch x 1 inch, right? Okay, try displaying it on one of the monitors in Times Square. It will probably measure 1 yard x 1 yard.


Pixels have no set size! Monitors have no set size. So what, exactly, does the 72 (or 96) dpi setting (in Xara, PhotoShop, etc.) mean?

As best as I can understand it, 72dpi means "if you have a 13-15inch monitor, every inch of this image will contain 72 pixels. If you have a larger monitor, every UNIT will contain 72 pixels, and it's up to you to figure out what a UNIT is."

Does anyone here have a REALLY good gut understanding of dpi (a.k.a. ppi)? Say I'm designing a graphic that's going to appear during the title sequence of a major motion picture (shown in the cinema). What does 72dpi mean to me?

I've searched the web for details, but almost every graphic site says "set resolution to 72dip, because monitors display at 72dpi."

Hope this doesn't seem wildly off-topic.

Marcus Geduld
{ email me } { visit me }

[Klaus Nordby]

I know the score on this pretty well, Marcus - and I agree with you that what we hear all around is a lot of confused hogwash. You're right: "72dpi" for a movie or a web page or a Times square screen means NOTHING. It's a mere nominal, arbitrary value, since there must be SOME value in a file's header about the "resolution". This file header info - the "DPI" - is only usable information by printers, when the image is to be rendered into absolute physical dimensions.

The best simple advice I know is to forget ALL about DPI/PPI and only consider the number of horiz./vert. number of pixels in a file - e.g., 600x800, 2000x3000. Only this states the REAL amount of information in a digital image.

K

[michaelward]

Use pixels for the web and forget the DPI issue.

The only time DPI comes into play is with print, because pixels are of varying sizes on a printer.

So this is the deal: DPI means nothing to your PC monitor, it only knows pixels. Pixels mean nothing when printing, because a printers pixels will be different sizes. If you wan't a 1inch image on your printer and you are printing at 600dpi then you need a 600 pixel image.



Michael Ward
http://LeighCenturions.net

[mgeduld]

If dpi/ppi meant nothing, we could totally ignore it. Still, we all know it's better to make web images 72/96 dpi than 300 dpi. So the concept isn't meaningless. (A 1 inch by 1 inch file at 300 dpi will appear much bigger than 1 inch by 1 inch on 13-19 inch monitors set at some reasonable resolution, like 800x600).

Also, I haven't done a lot of film work, but I'm pretty sure that people making graphics for the big screen are also advised make them 72dpi. Why? (This gets even more confusing when you think that almost all films wind up on television. So the graphic is going to be displayed on a huge screen AND a small screen).

Marcus Geduld
{ email me } { visit me }

[Erik Heyninck]

Sorry Marcus, but we do not all know that its is better to use 72 or 96 Pixels per inch (not dots as those refer to screening, just like samples refer to a scanner) than 300 for the web.

This makes no difference whatsoever on a monitor. A browser cannot change the dimension of a pixel at all. Your videocard setting can, and allows you to set it at for example 800x600 or 1600x1200. In the latter setting the pixels are only half as "large" as in the former one. But when you look at the same pic with 1600x1200 it will only be one quarter of the size it has on 800x600 as heighth and width are indeed the same amount of pixels but each pixel is only half the size.
If you use 300ppi, you only need a quarter of the physical dimensions of your image to make it appear as large as one set at 75dpi. But in print, it will be a quarter of what you see.

So, no, this makes no difference at all: only the number of pixel counts and the size varies with the monitor settings.

And as far as I know, the 72 comes from the first Mac computers. I really would like to find a good site on the history of computers. I do remember vaguely a huge machine and me typing in Fortran4, cards coming out of the machine, and after putting them on a tray and they being read and interpreted by the comp, there was this unavoidable message that there were faults in the programming and it couldn't do what I asked it to do. Things have gotten easier and more powerful.

[Wolfgang]

Marcus,

Klaus and Michael are totally right!
FORGET dpi !! (when talking of screens)

Why ??

Create a 100 x 100 pixel wide graphic and save it once with 300dpi and once with 72 dpi (as GIF or JPG or PNG).

View them both in your browser and - voilá - they will have the same size.

Not anymore will they look the same, if you print them both. The 300dpi version will come out much smaller than the 72dpi version.

My clients sometimes ask me, at how much dpi I need the pictures for their webpages. I always tell them, that this does not matter and that I'd prefer this or that size in pixels. They rarely understand me ...

I guess this is, because almost nobody actually understands the dpi-concept. Well most of us graphic people here may do, but outside this place the air gets thin.

Everybody who buys a consumer printer, ink or laser, asks one thing first: "How many dpi does it have ?". Even though this number does not directly relate to the final quality, it's the only distinctive feature most people know. Or think to know.

Alas, nobody asks for dpi when they buy a monitor. The size in inches is the common known feature here.

And as long you can have the same resolution on different sized monitors, dpi will not make any sense.
I can set my 18" TFT to 640x480 which will make a 100x100 pixel graphic really big. You can set the same resolution on a 14" CRT monitor and the 100x100 will look much smaller.

What will stay the same is the relation between the size of your graphic and the size of your screen. and that's what really counts here!

Wolfgang

[Steve Newport]

I don't know exactly what DPI means, but I can tell you what I've learned from one little experience.
A Month ago I was asked to design some flyers for print advertising 2 shows. I thought, no biggy, and got them done within a week. I then sent them to the company in bitmap form (By means of Bitmap, jpeg, tiff, swf) And when they printed out, all looked well except the text. It printed out really fuzzy and too anti-anilized. The guy said the dpi needed to be higher, reall high! So I put it up to the max Xara would allow 300 (The guy wanted even more than that!) and it was still fuzzy!?!? It was really frusterating! So either Xara doesn't allow enough dpi exporting for proper printing, or it's antianilizing is too much!? Hope this help

Steve Newport

[alkolka]

I agree that once you have a bitmap, DPI is nonsense. Pixels are pixels.

But, try this:
<UL TYPE=SQUARE><LI>In Xara X set your Page Options Units to INCH.<LI>Create a 1 inch square, no outline.<LI>Include fill of your choice or leave it black.<LI>Select just the square and export it as a PNG bitmap. Set the DPI to 96 on the Bitmap Size tab.<LI>Now, export it as a PNG bitmap a second time. Choose a new filename and set its DPI to 300.[/list]
When you look at the two PNG bitmaps, you will find that they are different sizes. 96 and 300 pixels square respectively. A bitmap DPI specification has resulted in the correct translation between the inch defined vector graphic and the pixel defined bitmap graphic(s).

I suspect this also applies to many or all bitmap exports that support a DPI attribute.

I've long wondered if some bitmap formats internally carry the DPI setting used during their creation, or their real world dimensions (in, cm), for latter post processing. That would allow a bitmap to be printed at the author's intended size. If anyone can speak to that aspect, please inform us.

Happy holidays everyone.

[Klaus Nordby]

Marcus: "Still, we all know it's better to make web images 72/96 dpi than 300 dpi."

Nonsense. 300x300 pixels will look the same on your browser no matter what "dpi" you specify.

"So the concept isn't meaningless."

In the context of web and screen, it is. In the context of print it is hugely meaningful. So make up your mind whether you are dealing with web or print. If web, forget DPI. Or go on forever confusing yourself - but you have been warned! ;-)

K
K

[Erik Heyninck]

Al, that's just what it is all about. The "one inch" is of no importance when you look at a file on your monitor, except that is specifies how many pixels there will be:

1 inch with 72 pixels per inch gives 72 pixels
1 inch with 300 pixels per inch gives 300 pixels

Nothing more, nothing less.

And if your monitor is set at 800x600, your one inch with 72 pixels in it will be appr. 1/11 of your monitor's width, and your one inch with 300 pixels in it will be 3/8 (nearly half).

If you don't work against time, time often works for you.