Geoffrey wrote:
> So here's a question which will demonstrate my ignorance.  I've got some
> digital photos I took that, when opened with GIMP are identified as
> being 75dpi x72dpi.  I need images that are 300 dpi, so is it possible
> to convert the 75dpi image to 300 dpi??  I selected to scale the image
> and happened to notice that I can make this change at this point.  Is
> this doing what I'm expecting?

That is a question that only a mind reader can answer.  (Sorry :-))

Here's the thing:  a resolution in dpi is not a property of the image
per se, it's a property of the way the image is displayed, on the
screen or on paper.  When you see a resolution of 72x72 dpi for an
image, what those numbers represent is somebody's judgement that the
image will look good when displayed with 72 pixels per inch; and 
usually this means "look good when displayed on a monitor", because
nothing looks very good when it is printed at 72 dots per inch.

Now suppose you have a 300 x 300 pixel image, with a nominal resolution
of 72 dpi, and suppose you want to convert it to 300 dpi for printing.
There are two approaches you could take.  (Actually more, but let's keep
it simple for the moment.)

(1) You caould say, okay, 300 pixels is about 4 inches at 72 dpi.  I
    want my print to have the same size.  So, I will scale the image
    to 1200 x 1200 pixels, and set the resolution to 300 dpi.

(2) You could say, okay, I don't want to make the image look blurry
    by scaling up the number of pixels, so I will simply set the resolution
    to 300 dpi without changing the pixel dimensions, thereby getting a
    print about 1 inch across.

Both of these are legitimate choices, and so are many others.  The best
way to do it depends on your image and your printer.  What makes it
complicated is the fact that printer dots have much poorer color resolution
than monitor dots.  On most modern systems, a monitor dot encodes 24 bits
of color information.  On a typical high-quality printer, a single dot encodes
about 6 bits of color information.  Thus, simply changing the resolution so
that a single monitor dot becomes a single printer dot is usually not the
best thing to do.

  -- Bill

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