- Edwin - wrote:
Hi!
On Thu, 16 Mar 2006 14:31:38 -0500
John Hinton <webmaster@xxxxxxxx> wrote:
Simon M. Campden-Main wrote:
Well, there's the meat of it, isn't it? I wonder how Paint
Shop Pro comes up with PPI (Image - Image information).
I have several thousand scanned images with resolution
ranging from 72 PPI up to 1200 PPI (As reported by Paint
Shop Pro) and want to discard, or more likely tag as
rejected, any that suffer a resolution of less than 150
PPI. As you might imagine, I find the thought of
doing it manually with Paint Shop Pro repugnant.
Okay, if it was scanned at 72ppi then you can't really print
it out again even with a near-similar quality. However, having
an image with a 72ppi does NOT necessarily mean that you
have an image unsuitable for printing.
PPI or pixels per inch is a printing term.
Last time I checked, "dpi" is the printing term. (^_^)
dpi (dots per inch) -> printer
ppi (pixels per inch) -> monitor/screen
Still missing it.. Yes, dpi is a printing term.. but has absolutely
nothing to with with images. dpi or "Dots per Inch" is purely a term
that describes the limits of your printer. If for instance, we are
talking about an ink jet printer with a 600 x 600 dpi capability, with
black and three colors, that printer can print any of the four color or
not print in a space 1/600th of an inch as it moves along. And there is
no blending within that one dot... it's simply one of the colors or left
the white or the paper color.
One might think that an image should be scanned at 600 dpi to provide
and equal quality, but really you can scan at a much reduced resolution
and get the same results with a basic inkjet color printer, because the
scan will most often be done at 16.7 million colors.. the printer only
has 4(+paper color) so it really takes a 4x4 dot area or so to start to
get close to matching a single pixel. Now, lets don't even go there on
printers. The above is greatly understated and was how old inkjets
worked.. much has changed and I really haven't followed exactly what
they are doing now, but obviously it seems that there is a blending over
top of other colors in today's printers..... Just trying to provide
simple theory, quick to type, easy to grasp.
So, yet still ppi is being misrepresented.. Please stop doing this.
If you don't believe me try this. Open a quality paint program. In fact,
Macromedia's Fireworks in the image sizing dialog box separates pixel
dimensions from ppi putting ALL ppi function under the heading of "Print
Size". The example.....
Open or create an image 600 pixels x 300 pixels.
Make sure you stay at 100% zoom factor.
Set the printing Pixels/Inch to 150, but don't let the program change
the Pixel dimensions. You will be given a print size of 4" x 2". The
image will take up 600 x 300 pixels of your monitor screen space.
Now with that same image, change only the resolution to 300. Leave the
pixel dimensions the same (one has to be careful with the locking of
proportions and samplings to be sure the program doesn't change the
pixel dimensions when changing the Pixel/Inch). You will now see that
the print size is 2" x 1", but yet the image size on the screen has not
changed sizes.
A pixel is a pixel to a monitor. A pixel is sent thru an algorithm on
the way to a printer and by and large, print quality has to be much
greater than monitor quality to 'look' as good. Basically, you can't get
a 4" wide image on the screen to look as good when printed at 4" wide.
A general guideline is images for print should be no less than 150 ppi,
newspaper quality, and 300 to 600 is recommended for color brochures and
"near photo quality". So, using our example above and assuming the
printing company has requested 300 ppi images, that image at 600 x 300
which fills a major portion of one's monitor screen, can only be printed
under this constraint at a maximum of 2 inches x 1 inch. If you're going
to print, check with the printer before beginning as print quality is
generally rising at all levels.
The question of throwing away all images with 72dpi is irrelevant.
The questions are:
What resolution, pixels per inch, do you want to print at? Lets assume 300.
What size are the images in pixels? Lets assume 1200 x 900. (I know, odd
size but easy math to follow)
Are you happy with printing these images at 4 inches x 3 inches? ((1200
pixels / 300 pixels per inch = 4 inches) x (900 pixels / 300 pixels per
inch = 3 inches))
If yes, keep the image.
If you started with an 8 x 10, scanning at 72dpi and are happy with
150ppi, you can get a good print out at 4 x 5. If you want 300ppi, then
2 x 2.5 inches.. and so on.....
Also, if working in .jpg filetypes, there is normally a quality setting
in the paint program. If you're going to print, set it to the highest.
If to the web, medium to medium high is generally not a noticeable
difference. My testing showed that these settings affect the number of
colors in the image. Yes, we have 16.7 million to work with, but if you
do a 'count colors' you'll normally find the image has 50 to 80 thousand
colors or less. Setting the quality lower reduces this number which
somehow reduces the filesize (I have no idea how the data is stored in a
jpg file). And the algorithms used by the various paint program vary
widely. This is where quality programs shine and cheaper ones show a
lesser quality... generally.
I was told by a Canon rep, that a 5megapixel image is the size needed to
get the best print from their best consumer grade photo printer while
printing an 8 x 11. That mathematically divides out to just under 300
ppi... so it makes sense.
So, an image which is 600 pixels x 300 pixels is going to display on
screen as 600 pixels by 300 pixels regardless of printer dpi or ppi.
Sorry as this is really off topic, but it seems to be a basic missed by
so many who are ultimately creating web pages.
And yes, do look at the powers within ImageMagik, which is installed on
most 'nix systems. There are hundreds of simple command line functions
available for use within PHP. ImageMagik also does a good job of
optimizing images for web use. Their website provides a list of most if
not all of these functions.
http://www.imagemagick.org/script/command-line-tools.php
Best,
John Hinton
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