Question about resizing

sofasurfer · **Joined:** Nov 04, 2010 **Posts:** 67

GIMP Version: 2.6.11
Operating System: Linux
GIMP Experience: Beginner Level

I have an image of about 4000x5000 (19mg) and 490dpi. I resized it (using Nautilus resizer) to 603x480 (283kb) and 490dpi. I then opened it in GIMP and changed the resolution to 72dpi. The size is still 183kb.
Why?
And when I zoom in, the pixelation for the 603x480/490dpi is the same as for the 603x480/72dpi.
Why?

I am including both images...640x480/490dpi and 640x480/72dpi.

PhotoComix · **Joined:** Apr 12, 2010 **Posts:** 5870

because only printers consider dpi , for monitor are not relevant
And about Gimp that is just a default value you may change as you wish.

You may found very long explication and discussion about, but a big point is that the dpi IS NOT the resolution but a info you give to the printer about the resolution, you may even save a jpg with a crazy high compression but at 1200dpi (result will be crap of course).

Anyway you get the best resolution not from jpg but from png ( and tiff but have much bigger file size) and then you may also set dpi to a reasonable high value (900 as example )

And again will be no change in the file size , or on the screen even zooming in:you are simply saying to the printer that the image is at high resolution, if is not will not help,but if it is the printer will try to do its best

PhotoComix · **Joined:** Apr 12, 2010 **Posts:** 5870

PS it is not what you asked but if you resized to print, that was a mistake!!

Never scale down images that should be printed, set the dimension in the printer dialog or you will get crap:
a image of 640x480 cannot be printed much bigger then a post stamp (not at a decent quality )

sofasurfer · **Joined:** Nov 04, 2010 **Posts:** 67

Here is what I am doing...
I have images that I save in high res, scaled to 11"x14" and 300 dpi. These I make prints from. I resize them into a separate folder, at about 1000x600 to be used on ebay or my website, http://www.early1900photos.com.
I used to always resize them and then change the dpi to 72. Now I have learned that I do not need to change the dpi for on line display, Correct?
But I was always told that you need to lower the dpi so people can not make quality prints from your displayed images. Is this true of not?
Thanks for your time.

PhotoComix · **Joined:** Apr 12, 2010 **Posts:** 5870

yes and not...as you see is simple change the dpi, would be better use not scaled png for quality print (again for best quality resizing must be done in the printer dialog...) and for ebay post jpg of just the sufficent size and quality to display well on the web ,but no more

for sure by a compressed jpg of that size would be no possible print a decent quality (still you may lower the dpi, just to be more safe

paynekj · **Posted:** Sun Oct 02, 2011 2:30 am **(#6)**

Quote:

And when I zoom in, the pixelation for the 603x480/490dpi is the same as for the 603x480/72dpi.
Why?

By changing the resolution from 490dpi to 72dpi all you are changing is one value stored in the image file, which tells a user how big to print the image at it's "natural" scale. The image data doesn't change at all - hence the file remains the same size.

That value could be used by the printer, but mostly it's ignored as people choose what size to print rather than accepting the size the image says it is. So, yes, you are wasting your time changing the dpi in the resized image. Reducing the number of pixels is the only thing that will make a difference.

Note that for an image 1000 pixels long, I could print an image 6" long at 166dpi (1000/6 = 166), which is going to be a fairly good quality. Granted it's not going to be good enough to be a fine art print, but who are you worried about making their own prints? Even at 72dpi, print quality isn't that bad, so your 1000 pixel image would look OK at 1000/72 = 13.9 inches long.

The only sure way to prevent people stealing your images is not to put them online.

You can make it much more difficult to steal by watermarking, but obviously this makes the images unpleasant to look at.

Kevin