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Reducing pixels per inch

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Reducing pixels per inch John Culleton 29 Nov 16:21
  Reducing pixels per inch GSR - FR 30 Nov 16:39
John Culleton
2004-11-29 16:21:34 UTC (about 20 years ago)

Reducing pixels per inch

I have a large photograph that I cropped and then reduced in size. But when I do that the pixels per inch goes up. How do I reduce an image in size yet keep the pixels per inch at e.g., 90? I scanned at 50 PPI (the lowest Xsane will do.) But when I reduce it to roughly half size by resizing the image

image->scale image

the PPI shoots up to 164. If I reset the resolution to 90 then the dimensions go up.

GSR - FR
2004-11-30 16:39:44 UTC (about 20 years ago)

Reducing pixels per inch

john@wexfordpress.com (2004-11-29 at 1021.34 -0500):

I have a large photograph that I cropped and then reduced in size. But when I do that the pixels per inch goes up. How do I reduce an image in size yet keep the pixels per inch at e.g., 90? I scanned at 50 PPI (the lowest Xsane will do.) But when I reduce it to roughly half size by resizing the image

image->scale image

the PPI shoots up to 164. If I reset the resolution to 90 then the dimensions go up.

I started with a 10*10 inch image and 90 DPI (so it will use 900 pixels in each dimension). It is "big" and 90 DPI, and want it to be half size, 5 inches but still 90 DPI.

Then in that dialog:

1. Set the units in the top most drop down menu to in(ches), so two top most labels will show 10.000", and the two entry boxes below it will show 10.000. This step is optional, but shows how things change.

2. Input 0.5 in the Ratio X, and as the link button is on, Y will change to 0.5000 as soon as you hit Tab. New Width and Height above it will change to 5.000 automatically. The boxes in Print Size & Display Unit frame will too. But Resolution boxes stay at 90.

3. Accept that settings and wait for Gimp to do the maths.

Finally check in View/Image Info that each axis is 5 inches at 90 DPI, and logically using 450 for that.

Playing with Print Size and Display Units makes one go up when you lower the other, and viceversa, they are interrelated and they are merely the way to access the fields some file formats have to record suggestions for printing. To really modify the pixel data, you have to use the top frame controls, Pixel Dimensions.

Instead of Ratio, you can also use New Width and Height in Pixel Dimensions ones. You _have_ to do step 1 so Gimp know you are going to input inches, and then you can type 5 in one of the top boxes, Ratio will be the ones that change to 0.500.

As it has been said already, all this interrelations can sound complex, but are useful *, provided you know "what will move when you pull from a thread". This could get a video, or some graphical representation of the relations at least:

Easy: Top frame sizes Top frame ratios

Bottom frame sizes Bottom frame resolutions

Complex: Top frame ----> Bottom frame sizes

Bottom frame size boxes ----> Top frame sizes if set to physical units

* Image from a 3D app or a screen capture or whatever, the point is that nothing matches, it says 1000 pixels, 72DPI and 13.889 inches, but you need 2 inches and 300 DPI. So you set 300 in DPI box (PS&DU's W and H will say 3.333), then change top drop down menu to inches and type 2 in PD's Width (PS&DU's W and H will go to 2, and PD's Scale will go to 0.600).

GSR