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resample with constrained proportions
I have an image that is the correct size (length and width) but it is only 98 dpi.  Before printing, I need to resample the image to increase it to 150 dpi without changing the size.   Photoshop has this capability ("constrain proportions"), but I can't figure out how to do it in GIMP.  

Is this possible in GIMP?
Gimp is going to be a bit more hands-on.

98 ppi ?? strange value however.

Use Image -> Scale Image and let Gimp do the arithmetic for you.
1) Multiply the width by 150 then divide by 98   computer type notation *150/98
2) Change the resolution to 150 ppi 
3) Click the scale button
Width and Height / X and Y resolution are linked change one and the other adjusts automatically.


Then check the print size as required Image -> Print Size

..and a 40 second animation of that

There is a page on printing resolutions here:
Thanks!  I'll give it a try

Regarding the 98 ppi, I have an image that is 2500x3750 pixels (1.5:1 aspect ratio).  When I scale it to my desired size 25.5" x 38.25" the adjusted ppi is ~98.
Still not sure what a dedicated PS 'constrain proportions' but Gimp has links between height and width in various tools to maintain an aspect ratio.

With the pixel size known and a target of 150 ppi it becomes scaling 25.52" x 150 and 38.25" x 150 = 3825 x 5738 pix.

After scaling, use Image -> Print size and change the ppi to 150. This is just information contained in the image file which 'tells' the printer the size.

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