Re: PNG format and DPI


"Jon" <jk@xxxxxxxx> wrote in message
news:46c0783e$0$1632$ed2619ec@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Owen Ransen wrote:
On Sat, 11 Aug 2007 00:23:29 +0100, Jon <jk@xxxxxxxx> wrote:

If I scan an image at 600 DPI and save it as a Tiff the resolution (as
displayed in Bridge) is, as I would expect it to be: 600 DPI.
However, if I save it as a PNG the resolution drops to 72 DPI!
Is there a way to change this?

The titleof your post

PNG format and DPI

may explain it all. PNG does not have, in itself
a DPI setting. PNG does not know the size of the
final image when printed. It knows how many pixels
there are, but that is all.

See also:
http://www.ransen.com/Articles/DPI/Default.htm


Easy to use graphics effects:
http://www.ransen.com/

Thanks for the link. It is not the first time I have read about DPI/PPI
and to be honest I'm still confused.

When I scan a 7" x 5" image into Photoshop I set my scanner software to:
Resolution: 600 DPI
When the image appears in Photoshop I check 'Image Size' and that tells me
the following:

Pixel Dimensions:
Width: 4200
Height: 3000

Document Size:
Width: 7"
Height: 5"
Resolution: 600 Pixels/inch

I then save the image as a PNG and Photoshop Image Size states exactly as
above.

I then view the PNG image in Bridge.
This tells me the following:
4200 x 3000 @ 72DPI

OK, so if DPI is purely to do with the Printer regardless of whether I
print the image or not then why is there a discrepancy between PS and
Bridge?
And what if the printer I send the images to hasn't the capability to
print at 600DPI?
Well, I can't print the image directly from Bridge anyhow so presumably
the 72DPI IT shows is meaningless and possibly a PS/PNG bug.

However, what throws the whole DPI/PPI/Resolution thing into a cocked hat
is that the Resolution/DPI I set in the scanner software determines the
Pixel dimensions of the resultant image that shows in Photoshop ie.
setting the Scanner software to Resolution 300DPI for a 5" x 7" photo
gives me an Image Size/Pixel Dimension in Photoshop of Width 2100 - Height
1500.
And this determines the Print size I can achieve when I send the image for
printing to an online photo printing outfit.

So if there's anyone out there who can explain what all this is about, I
would be most grateful.

Furthermore, while they're at it, could they also explain the purpose of
Photoshop's 'Image size' Auto button which confuses the issue further by
introducing Lines/inch. I notice that within this 'Auto' option, if I
select 'Best' then it reduces the Pixel Dimensions of the image to:
Width: 1862
Height: 1330
Resolution: 266 Pixels/inch

Arghhhh!!!

First let me say I do not understand the reason why many of the 'Auto'
options were included.

Dots per inch (dpi) is a size measurement for input or output devices.
The actual size of the digital image is read as pixels.

Dpi does not really relate to the onscreen image because the onscreen image
can be viewed at different sizes by zooming, and it can be otherwise
manipulated by software.
However, dpi is important when scanning and printing. When you scan at
600dpi you are telling the scanner to look 600 times for every inch it
travels in width, and height. When you scan at 72dpi you are telling it to
look only 72 times for every inch, resulting in a lower resolution scan.

The dpi of the printed image is usually determined by the software from
which it is printed. Some software determines/adjusts the dpi by the
requested size of the print, say 4x6, or "to fit within margins". Some
software imports images at a set resolution, Outlook Express and IE, for
instance, only display and print at 72 dpi. Most printers/graphics layout
software imports at a set resolution like 300ppi, but allows you to resize
it in the layout. Most graphic apps, like PS, will print at the saved
resolution, (i.e. 600dpi).

72dpi is the closest resolution to the 100% size onscreen, so it is the
default resolution in many web authoring apps. PNG as there name implies,
(portable network graphics), is a format closely ascociated with the
internet so many apps will use 72dpi as a default value, for that file
format. GIFs are treated similarly. I am not familiar with Bridge, but try
printing the image from there to see how it is printed.

4200pixels at 600dpi is 7"
3000pixels at 600dpi is 5"
Image will be sharp.

4200pixels at 72dpi is 58.333"
3000pixels at 72dpi is 41.667"
Image will be slightly pixelated.

Hope this has helped.
Scott


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