DPI: TRUTH OR LIE?

[traceyp]

Steve

I know that Xara has a number of dpi settings in the export dialogue but you CAN save higher than that. I thought this at one time but then I thought lets try and replace the 300dpi shown with 400 dpi - and it worked. Prior to this I had to export at 300dpi and then resample in Corel PhotoPaint, a right pain in the you know what!!! Now I simply delete 300 and replace with 400 and I have no problem with fuzzy text (albeit on A4 printed material).

On a general point, I also get confused over the question of dpi/ppi and my partner gets very frustrated at my lack of understanding. I do try very hard but I do find it a difficult concept to get my head around. I'll get there though - and not before time as far as my partner is concerned, I'm sure.

Tracey

[Clements]

understanding "resolution" is an old and ongoing issue I find this thread particulary good because different people are saying the samething ... its just matter of how the they explain it and THE WAY THE READER UNDERSTANDS IT. Eric and Klaus are essentially saying the same thing; one reader may connect with what Eric has written while another connects with what Klaus has written. Bottom line, more readers MAY become better informed.

What I come away from this is:

The monitor (based on its size and setting) is a FIXED MATRIX of "screen units". One pixel is mapped to one screen unit. Its like a big piece of graph paper where ONE PIXEL of color is assigned TO ONE SQUARE on the graph paper.

If the monitor setting is increased from say 640 x 480 to 800 to 600 it would be similar to mapping a drawing on 4-square/inch graph paper and then redrawing the same image (one pixel to one square) on a 6-square/inch graph paper.

A printer, on the otherhand, has the ability to interpret the number of colored "dots" per an "inch" of paper space.

[Klaus Nordby]

Given that we're speaking of physical printing, all text - given the razor-sharp nature of letters - need to have a DPI of at least 600! If it's a big heading at 70-100pt you can get away with less, but if it's small bodycopy text at 8-9 point you'll need 800 or higher to avoid fuzzyness. Big numbers for potentially huge files!

But of course, text should NOT ever be bitmapped!!! That's why we have file formats like EPS, which allow for vector text and bitmap images in the same file, for both razor sharp text output AND a more sensible DPI for the images - around 200-300 DPI for ordinary halftone screens.

The well-known "rule" which states you should have a bitmap DPI twice that of the LPI of the halftone screen is another myth devoid of real knowledge: depending on the image, you can have far less or need far more. A totally softfocus photo can have a DPI number the same as (or even less than) the LPI number without any adverse effects, whereas an images with razor-sharp information - such as text! - needs a much higher ratio.


K
www.klausnordby.com/xara

[This message was edited by Klaus Nordby on December 08, 2001 at 13:44.]

[Klaus Nordby]

Regarding XaraX and DPI output: again, forget about DPI! XX's bitmap output is only limited to this: 32,000 x 32,000 pixels. Printed at 300 DPI, that's about a wall-sized image. Printed at 3000 dpi, that's about a page-sized image. So take your pick, depending on your output needs! :-)

K
www.klausnordby.com/xara

[Steve Newport]

Tracey, thanks a lot for the info, this will be VERY usefull in the future. My Mom was getting very frusterated at Xara because of that reason and said I need to learn new programs (Like illustrator, I hate that program!!)?? I know all those are more widely used, but people need to see that xara is sooo much better!!! It is really frusterating, the thought that before I start working in this field I'm going to have a to learn that program, or some other crappy one [img]/infopop/emoticons/icon_frown.gif[/img]

Steve Newport

[michaelward]

This my graphical attempt to explain this (though I'm not sure how well I've don [img]/infopop/emoticons/icon_smile.gif[/img] )

BTW, all figures below are theoretical.



Michael Ward
http://LeighCenturions.net

[Klaus Nordby]

Michael, it's a good graph to explain what's really very simple - except your graph is kinda wrong! Inches do not apply to screen images: they are not physical entities having fixed dimensions. To explain the 1:1 matching between an image and a particular screen "resolution" (a total misnomer and a major conceptual culprit in all this confusion) you should show how one 16x16 pixel image is still a 16x16 image on ANY screen "resolution".

K
www.klausnordby.com/xara

[michaelward]

If you read what I've put, I say that the figures are simply relative.

If you have 800x600 on one particular monitor then you will fit half as much into one inch of real space as you would with 1600x1200 (which is more or less true).



Michael Ward
http://LeighCenturions.net

[mgeduld]

I may be misunderstanding (it's late, and I'll reread all your great posts tomorrow), but it seems like many of you are saying that dpi/ppi is totally unimportant on the web. If that's true, it doesn't matter whether I make web images 72ppi, 300ppi or 3000ppi.

This flies in the face of DOZENS of books I own (some of them very well respected, like "Real World PhotoShop"), all of which insist that web images should be saved at 72ppi. Can they all be wrong?

If so, why is this myth have such a lifespan. If not -- if it IS true -- what is magic about the number 72?

I DO realize that a 3000 or 300dpi file will have a much larger file size (more pixels = more bytes) than a 72dpi file -- which means it will take longer to download. But that STILL doesn't explain why 72 (or even 96) is such a major target.

Marcus Geduld
{ email me } { visit me }

[Klaus Nordby]

Marcus: "I may be misunderstanding (it's late, and I'll reread all your great posts tomorrow), but it seems like many of you are saying that dpi/ppi is totally unimportant on the web. If that's true, it doesn't matter whether I make web images 72ppi, 300ppi or 3000ppi."

Right!

Marcus: "This flies in the face of DOZENS of books I own (some of them very well respected, like "Real World PhotoShop"), all of which insist that web images should be saved at 72ppi. Can they all be wrong?"

Yes! It's a stupid, Mac-biased myth.

Marcus: "If so, why is this myth have such a lifespan. If not -- if it IS true -- what is magic about the number 72?"

It's the default "screen resolution" of the Mac. And virtually everybody who use a Mac - including tons of graphics people - are brain-dead, because the Mac does not encourage the use of one's brain.

Also, the number "72" is "magical" (hah!) because the PostScript standard for typographical points is 72 points in an inch (which is not *exactly* the traditional measure). This is - most likely - the historical origin of that figure.

K
www.klausnordby.com/xara