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Re: ppi vs dpi - or saving images in Genuine Fractals



Kennedy - 

Your logic seems prety clear to me. I guess I was getting confused about 
the relationship of ppi (pixels) to dpi (printer droplets). 

In general - does it make sense to keep the Genuine Fractals image at the 
6 x 6 cm size, 1200 dpi, or to resize it to either 7 1/2" or 11" square, 
at a lower ppi?

Perhaps there really isn't any difference in the data, if you maintain 
the same information. If that is the case, then using your numbers, it 
would look like this:

These size/ppi combinations should contain the same amount of data:       
   

6 cm at 1200 ppi
19 cm  at 380 ppi
28 cm at 260 ppi

Then the file sizes should (in theory) be the same, and in fact, it would 
not matter what image size/ppi you chose from the this list, would it?

So heres the rub:  if I save the file in GF at 380 ppi for 19 cm 
image(for example) and then later want to print at just that size, the 
file opens up significnatly quicker than if it were saved at some other 
size and then re-sampled to a different size/ppi combination.  So  this 
suggests that one should pick a preferred print size/ppi combination and 
save the image at that size, yes?

Thanks again for your insights. 
Geoff


>Just so I understand where you are coming from Geoffrey, at 1200ppi on a
>6x6cm negative, you end up with an image of about 2800x2800 pixels.
>
>If you print this at 7.5" square, (ie. 19cm square) you are enlarging
>the scan by x3.17, resulting in about 380ppi on the page.  Why do you
>want to resample this to 300ppi?  It seems that you would end up with
>less information than you started with.
>
>Printing at 11" square (ie. 28cm square) you enlarge the scanned
>original by x4.65, resulting in about 260ppi on the page.  Whilst there
>may be an argument for GFing this to 360ppi or some higher figure, I
>doubt that it would make much difference in the end print.
>
>My advice would be just to print at the scanner's output without any
>rescaling at all.
>
>Remember that the 720dpi refers to the ink droplet placement.  It may
>help to discriminate in your own mind between pixels per inch (ppi) and
>dots per inch (dpi).  Many ink dots can be placed for each pixel in the
>image.  Indeed there are sound reasons for not getting too close to one
>droplet per image pixel in most printers.  That is why you are getting
>good results from the 360ppi prints.
>
>What matters is what you can actually see on the page - and at anything
>above 200-250ppi, you certainly won't see individual pixels or the jaggy
>edge effects that this produces.  
>- -- 

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