72/96dpi PC/MAC

[ruben]

Is it correct that PC uses 96dpi and Mac uses 72dpi?

And if it is, could someone please explain why.

[LiquidReflex]

You mean for screen resolution? I'm pretty sure 72dpi is the standard resolution no matter what platform you are running. It basically alters the size displayed on screen. For example, a 2"x2" image at 72dpi (144x144pixels) will show up the same size on screen as a 1.5"x1.5" image at 96dpi (also 144x144pixels). Maybe someone with more knowledge on the subject can chime in though to verify.

[ruben]

What I know is this: If you make an image that fits perfectly in the frame of a PowerPoint presentation in Mac (can't remember the exact numbers), it won't fit in a presentation on a PC unless it's 96 dpi.

[ChrisRoss]

Its all 72dpi for all platforms, I'm not sure what your saying about Power Point, Power Point could careless about the DPI of an image. This is one of those Mac vs. PC rumors that have been around for a while.....

[Eagle]

::
*cough*

http://www.google.co.uk/search?num=1...e+Search&meta=

[mmj]

It's not so much that the two platforms have different resolutions for fonts, it's just that they have different default settings for the number of pixels per inch. This setting should ideally depend on the monitor, but the default settings are:

Mac: 72 pixels per inch
Windows: 96 pixels per inch (small fonts) or 120 pixels per inch (large fonts).

It's up to the graphics driver, or the user, to specify the exact measurement.

The "points" measurement for fonts is 72 points per inch. This is the standard, and it has nothing to do with the platform used (a point is a standard unit of measurement equal to 1/72 inches). You may notice that this 72 points per inch matches the 72 pixels per inch used on Mac OS systems by default. What this means is that on a Mac, one pixel is exactly the same as one point by default, whereas on a PC, pixels and points are always different.

This is not due to any bugs in Mac OS or Windows, and is the correct behaviour.

In my opinion, you should not specify font sizes in points for on-screen use. You should only use points for printed media. On screen, you should use relative/keyword font sizes, or pixels.

You do not need to worry about points measurements in graphics for the web, because all browsers use pixels, not points, to determine the size of images. You do need to worry about points only if you're specifying lengths (such as font sizes) using the "pt" measurement in CSS.

[RockyShark]

If I change my resolution from 1024x768 to 800x600, haven't I changed the dpi I'm displaying? Similarly, if I plug a 21 inch monitor in instead of a 17 inch monitor, without changing any settings, haven't I also changed the dpi?

I never really understtod how those figures were arrived at either, as they are subject to so many variables.

[mmj]

Originally Posted by RockyShark

If I change my resolution from 1024x768 to 800x600, haven't I changed the dpi I'm displaying? Similarly, if I plug a 21 inch monitor in instead of a 17 inch monitor, without changing any settings, haven't I also changed the dpi?

You have. However, your graphics driver is not able to detect the size of your monitor, so your graphics driver has to make a guess. That guess is 72dpi on Mac, and 96 or 120dpi on Windows, though you can change this to match your monitor size more exactly.

For example, in Windows, go to the Display Properties dialog, the Settings tab, Advanced button, General tab, and see under "DPI setting". Note, however, that some applications are optimised for best performance under the default settings (96 or 120).

[greg.harvey]

It's 96 dpi (smaller fonts) on a Mac and 72 dpi on a PC traditionally. However, this isn't an issue very much any more. For a long time Mac browsers have compensated for the difference in screen res. Only NN4.x for Mac is still about and rendering fonts smaller because it doesn't compensate for the difference. But it's 10 years old. So who cares.

Off Topic:

No one should be using pixel sizes for fonts anyway.

[RockyShark]

Originally Posted by mmj

For example, in Windows, go to the Display Properties dialog, the Settings tab, Advanced button, General tab, and see under "DPI setting". Note, however, that some applications are optimised for best performance under the default settings (96 or 120).

Ahhhh... Never really noticed that before. Cheers!

[mmj]

Originally Posted by greg.harvey

It's 96 dpi (smaller fonts) on a Mac and 72 dpi on a PC traditionally.

Nope, other way around.

Off Topic:

No one should be using pixel sizes for fonts anyway.

There's nothing wrong with using pixel sizes for fonts. In my opinion, pixel sizes are suitable for use on screen media and should be avoided for print media, whereas absolute sizes such as cm, pt, etc are suitable for use on printed media and should be avoided for screen media. However, you're quite welcome to use either on either - they're quite valid.

[greg.harvey]

Originally Posted by mmj

Nope, other way around.

Oh yeh. Sorry.

Originally Posted by mmj

There's nothing wrong with using pixel sizes for fonts. In my opinion, pixel sizes are suitable for use on screen media and should be avoided for print media, whereas absolute sizes such as cm, pt, etc are suitable for use on printed media and should be avoided for screen media. However, you're quite welcome to use either on either - they're quite valid.

I completely disagree with you on this. So do the W3C:

http://www.w3.org/2003/07/30-font-size

G

[mmj]

Originally Posted by greg.harvey

Oh yeh. Sorry.



I completely disagree with you on this. So do the W3C:

http://www.w3.org/2003/07/30-font-size

G

There is nothing in that page that disagrees with what I said. In fact, they too recommend against using absolute length units such as pt (or cm, etc) for on-screen use.

My point was that using px for font sizes is quite valid, and the W3C confirms this:

http://www.w3.org/TR/REC-CSS2/fonts....pdef-font-size
http://www.w3.org/TR/REC-CSS2/syndat...lue-def-length

In fact, "px" is part of the CSS standard (all versions), and can be used anywhere that you can specify any length.

[greg.harvey]

Originally Posted by mmj

There's nothing wrong with using pixel sizes for fonts.

Originally Posted by W3C

Do not specify the font-size in pt, or other absolute length units. They render inconsistently across platforms and can't be resized by the User Agent (e.g browser).

Not quite sure how you figure that? That's an absolute and unequivocal "disagreement" with what you said. The documents you posted appear to state that PX will resize relatively according to the display device. Well great, but that doesn't help you if you want to make the text on a web page more readable there and then, does it...?

G

[greg.harvey]

Ok. I'm not quite getting something here. I've checked your first link and it barely mentions PX as a measurement. I certainly don't understand how it supports your argument. Please could you elaborate? I'm not being facetious... I want to know.

The second link says this about it (so far as I can tell, or have I picked out the wrong paragraph?):

Originally Posted by W3C

Pixel units are relative to the resolution of the viewing device, i.e., most often a computer display. If the pixel density of the output device is very different from that of a typical computer display, the user agent should rescale pixel values. It is recommended that the reference pixel be the visual angle of one pixel on a device with a pixel density of 90dpi and a distance from the reader of an arm's length. For a nominal arm's length of 28 inches, the visual angle is therefore about 0.0227 degrees.

But I still don't understand how that helps with this fundamental problem:

Whether the user agent should be able to resize pixel sizes is open for debate. I'm not sure what the W3C line is on this, but as far as I'm aware (and I stand ready to be corrected by a demo piece of CSS -- I'm not prepared to trawl through pages of documentation) Internet Explorer does not resize the PX form of dimensioning font sizes. This being the case and IE being the dominant browser by a long way, like it or not, PX is inaccessible.

No?

Like I say, I'm quite happy to be proven wrong on this as it would make my life a hell of a lot easier, but I'm not convinced I will be...

G

[mmj]

What more can I say, other than that you need to become more familiar with the CSS spec? There are many different types of length units available. px is one of them. Any length unit may be used for font-size. px is one of them. px is not an absolute length unit. Examples of absolute length units are pt and cm. Although they're valid, in my opinion (and that of the W3C) you should avoid using absolute length units for displaying on screen media. Nothing wrong with px though.

If you need more information, it can all be found in the spec.

[carrotflowers]

On the font size issue, I have to say that like greg.harvey, I've eschewed using px for font sizes. I used this wiki http://css-discuss.incutio.com/?page=FontSize
to review font size options. I rely mostly on em. Besides the resize issue with px, I've always just feared that at higher resolutions, the px sized font would be too small.


Back to On Topic things: neat! Never knew that about 72/96 dpi...

[greg.harvey]

Originally Posted by mmj

If you need more information, it can all be found in the spec.

I've read the spec you posted and it doesn't address the accessibility issues. You haven't said whether my interpretation is correct or not. I believe it is. In which case, in actual effect pixels are absolute from a user perspective, regardless of the technical definitions. In which case they fail on accessibility. I'm yet to see anything to suggest otherwise. I know I'm at risk of repeating myself, but you don't seem to be getting my point. Your apparent unwillingness to offer any kind of a solution in terms of code implies to me that this whole thing is a waste of time... I was kinda hoping you'd have a solution. Now I'm just regretting getting this thread off topic. So to draw a line under it and leave it as agreeing to disagree:

Off Topic:

For anyone who is interested, please also see information on why you really shouldn't use PX values here:

http://www.wilk4.com/webdevres/fontcss3.htm

And more typography tips from Joe Gillespe, the eminent screen font typographer, here:

http://www.wpdfd.com/advanced/ch3_8.htm

There's much more info if you use the arrows in the bottom right corner to navigate.

And even Joe himself has said before that PX are a no no:

http://216.239.59.104/search?q=cache...hl=en&ie=UTF-8 (Forums currently offline, but I'm sure they'll be back soon.)

Cheers,

G

[RockyShark]

Allow me to make one observation. You said:

Originally Posted by greg.harvey

So do the W3C:

http://www.w3.org/2003/07/30-font-size

G

But the link you provided doesn't say the W3C dissapproves of using px. In fact, on that page is a link to http://www.w3.org/TR/REC-CSS2/syndata.html#length-units where they group px with em as a relative value.

Now - I agree that px is not a good method for all the text in your site, but there are times when it's useful, and valid. The W3C does not dissaprove of using px based upon the links provided thus far.

[mmj]

If you like, I can split these posts about accessibility into a separate thread. Let me know.

[greg.harvey]

Originally Posted by RockyShark

Now - I agree that px is not a good method for all the text in your site, but there are times when it's useful, and valid. The W3C does not dissaprove of using px based upon the links provided thus far.

Depends on how you read it and what the facts are. It still isn't clear. If PX are an absolute measurement (as Joe Gillespe, rightly or wrongly, states) then the W3C do dissaprove as in my original quote. If PX aren't considered to be absolute then the W3C don't dissaprove.

[mmj]

Originally Posted by greg.harvey

Depends on how you read it and what the facts are. It still isn't clear. If PX are an absolute measurement (as Joe Gillespe, rightly or wrongly, states) then the W3C do dissaprove as in my original quote. If PX aren't considered to be absolute then the W3C don't dissaprove.

I think the source of your confusion may be due to your believing that px is an absolute unit of measurement. In CSS, an 'absolute' unit of measurement is one which is a real life measurement, rather than a metric that is relative only to something which can vary, such as the computer screen or the font size, or so on.

The CSS spec defines it thus:

Relative units are:

* em: the 'font-size' of the relevant font
* ex: the 'x-height' of the relevant font
* px: pixels, relative to the viewing device

It specifies absolute units thus:

Absolute length units are only useful when the physical properties of the output medium are known. The absolute units are:

* in: inches -- 1 inch is equal to 2.54 centimeters.
* cm: centimeters
* mm: millimeters
* pt: points -- the points used by CSS2 are equal to 1/72th of an inch.
* pc: picas -- 1 pica is equal to 12 points.

The 'px' unit is specifically included in the spec for use on screen devices (including a computer monitor) so that designers can specify lengths and sizes based on a number of pixels. There is no more suitable place for the px unit of measurement than when designing for on-screen, as this is its specific purpose.

So you have 3 options for all lengths in CSS. 1) choose an absolute unit. 2) choose a relative unit. 3) Use percentages (%). Some properties (including font-size) also allow a 4th option: a keyword (ie, 'large', 'small')

Now, for displaying a font on screen, your options narrow. If you are avoiding absolute units for display on screen, which you probably should, then that leaves you with relative, percentages or keywords. Percentages are only applicable when the parent item also has a font size, so somewhere or other, you have to have a parent element with a font size that isn't a percentage. This leaves you with keywords or pixels (or, of course, you could also choose not to set font size at all and go with the browser's default).

From there onwards, it's personal choice. If you require the font a certain size in pixels to fit your design, use pixels. If the font size can be more flexible, use keywords. Both are quite valid, and both have benefits and drawbacks. For example, the drawback of using keywords is that they are inconsistent across browsers and if the user has set his default font size too large or small, it may break your design. The drawback of using pixels is that the user's default font size settings will not influence the text. It would be wise, then, to use keywords for body text etc which can scale, and pixels for text in controls, menus etc if they are of fixed size. However, it is free for you to decide.