Monday, April 4, 2011

Understanding ppi and dpi

We are once more left scratching our heads about posts in the Kindle Forum [and in general] about "saving as 300 etc ppi/dpi".

The ref to dpi [dots per inch] can be easily be dealt with, dpi is purely a function of how many dots an inkjet printer might lay down and is NOT necessarily same as the ppi, BUT Kindle has nothing to do with print so let's forget that totally.

The simple path then says forget all about ppi as well as far as Kindle is concerned, but THAT will take a bit more explaining.

Here is a cover image we were about to feature as The Chainmakers. This blog layout will resize/scale images to 400 pixels wide max, so in order to save upload time we are taking our standard 900 x 900 image as supplied to customers and resizing to 400 x 400.

So to show the absurdity of "saving at 300 ppi" etc we saved the image twice, one "at" 96 ppi and one "at" 300 ppi, and guess what, they are both exactly the same and here is a screenie from Windows Explorer.

So chain96.jpg and chain300.jpg are BOTH 175KB and BOTH 400 x 400

To explain, it is simply a matter of showing the other 2 images in that list, per:

And of course, because these show text, the "rules" say save as a gif and not a jpg

These screenies are from Jasc Paint Shop Pro and because it is designed to be used by professional image editors it does not say "Print Information - FOR INFORMATION ONLY" because users KNOW it is for info only.

So for the benefit of those thinking they are actually saving "at" 300ppi etc we have added the GREEN arrows to show that the ppi and the inch size of a PRINTED image are contained in an INFORMATION BOX, but in both cases the saving is simply the SAME 400 x 400

So here is the actual image

Now part of the confusion might be caused by info in the Kindle Guide saying save images at 300 ppi, but what that means is SCAN at 300 ppi.

For example you have a 4 inch by 4 inch photo you want to appear as same size on the screen, so assuming your PC is set up at 96 ppi then if you scan at 96 ppi the image will be same size on the screen, BUT it won't look too good.

That is because a scanner is a transducer and all transducers have less than 100% perfect transduction. SO, if we scan at 300 ppi we have 9.76 pixels to be resized down to ONE pixel meaning the software can do all manner of "dithering" to grab the best of the 9.76 pixels in coming up with one near perfect pixel.

Simple as that.

here is a gif to show graphically [or maybe gifically]

on LHS is a "blowup" of a scan at 100 ppi and 300 ppi on RHS. Look at letter "F" to see it has only 3 pixels on LHS to do its thing, so it is blurred, but has 9 pixels on RHS so is able to render clear lines

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