Glen Adomovicz wrote in message <6i78t7$a8m$1@oz.aussie.org>...
>
>He is an example, let's say I want a final image
>with 304.8 pixels per inch and the size to be 4x5 inches. You will make
this
>file in Photoshop and then change the resolution to 72 pixels per inch and
>the size then goes up to 16.931x21.167 or 1219 x 1524 pixels.

The question is, why use Photoshop for the simple calculation, that 304.8
pixels * 4 equals 1219 pixels and 304.8 pixels * 5 equals 1524 pixels? I
would say that a simple calculator does the job ;-)

No really, this question (or a similar one) pops up on this or other groups
frequently, but:
DPI is an output-device unit. Images (that is, pixel-rasters) don't have any
DPI size themselfs, they are just a bunch of colored squares arranged in a
checkerboard like fashion!

And even formats (like TIFF) that "support" DPI don't *really* support DPI:
A TIFF stores the "intended" or "preferred" DPI size of the image as a
convenience, so that when you load the image in some other program (for
example in Word, where you paste it into your text), you don't need to
specify how large you want the image to be.
So if you scan an image with a certain number of DPIs and save it as a TIFF,
the number of DPIs is stored with the image, _to your convenience_!. When
you then paste the image into a Word-doc (without resizing it) and print the
Word-doc, the image will have the same physical size as it had on the
original.
Very neat, but that doesn't give the bunch of pixels that actually compose
the image any DPI-releated property. The DPI related property was simply
stored at some other place in the file, where Word knew how to read it. Sort
of like a TGA (that does not store any DPI infos) saved to a floppy with a
Post-It sticker attached where you write down: "I scanned this image with
X.. DPI".

Even the often used 72 or 75 DPIs for monitors are actually rubbish: A 21''
monitor showing 640x400 pixels VGA would have (very roughly with pythagoras)
38 DPI, while a 15'' monitor showing 1024x768 pixels would have about 85.3
DPI.

It is quite simple:
If you have an image of 600x600 pixels and print it with 600dpi, then it
will be 1x1 inch large. If you print the same image with 300dpi you get a
2x2 inches image etc.

So you only need to know the final print-size (in DPI) and how large the
image is to be on the paper (in inches). Then you simply render an image
that has enough pixels to fullfill that: If your image is printed with
300dpi and is to cover an area of 5x4 inches on the paper, than you simply
render an image with (5x300) x (4x300) = 1500 x 1200 pixels.

Johannes.

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