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Batch convert ppi?

May 23, 2012, 10:05
Post: #1
Batch convert ppi?
I have done LOTS of detailed research into ppi, dpi etc.

I believe I do understand the difference. Whenever I open a picture in gimp it opens at 72ppi.
I wish to scale all my photos to 300 ppi so i can use a template I made of the exact same size paper as my printer uses without having to worry about the printer rescaling my photos from 72 ppi to whatever dpi it needs.
I have converted lots manually and am assured that this works for my purposes (not professional just scrapbook and photo album prints)

I have two questions though;
1. Is there a freeware program, or a way through gimp, that I can batch convert multiple files up to 300ppi.
2. Why when I change the ppi of a photo does the filesize become larger? Surely the ppi is just a reference number to know how many pixels to display and to convert for printing?

Thanks in advance for any help.
Pepperisit
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May 23, 2012, 11:40
Post: #2
RE: Batch convert ppi?
Gimp assumes 72PPI because there is no PPI encoded in your picture. The JPEG format has support for that information but it is only meaningful on scanner output where you can relate a pixel to a physical dimension of the scanned document. With photographs out of a camera, a pixel can represent 0.001 inch (macrophoto) to several hundred yards (landscape). So this information is only meaningful when you wan to print.

So, be careful... if your picture is assumed to be 72PPI and you want unconditionally scale it for 300PPI, you are increasing its dimensions by a bit more than 4x... Which means a big blurry photo, since for each pixel the scaling software will have to invent 15 more (each pixel becomes a 4x4 pixel square)....

If you want a very specific final dimension and not lose detail, you have to keep all the pixels, but set the print definition to whatever is needed to obtain the given size using the pixels you have (ie print definition= available pixels/expected print size).

See this: http://gimpforums.com/thread-basic-scaling-question

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May 23, 2012, 12:31 (This post was last modified: May 23, 2012 12:32 by rich2005.)
Post: #3
RE: Batch convert ppi?
@Pepperisit
As well as all the above and to answer your first question, there is a plugin that will batch change the ppi settings in images.
http://registry.gimp.org/node/26259

A screenshot http://i.imgur.com/JM5ON.jpg

The ppi value does not affect the pixel size of the image only how it is interpreted/displayed by application/printer. Turn off 'dot-for-dot' in gimp view menu and there is the equivalent representation.

example http://i.imgur.com/W3ebY.jpg

Gimp is not the only application that will batch change ppi,
If you need a GUI - XnViewMP for example
http://newsgroup.xnview.com/viewtopic.php?f=60&t=25455
or if you are not afraid of command line, ImageMagick, from memory 'convert' with the '-density' switch.

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May 23, 2012, 14:13
Post: #4
RE: Batch convert ppi?
Thanks to both of you very helpful answers and good information.

In regards to you ofnuts;
I made a template of the size of the printer paper at 72ppi, imported the photos and resized them as I required (using scale to mm) and printed them out. They were exactly the correct size but because they had been scaled down (physically) but the ppi was still 72, the image lost a lot of pixels before printing and the small photos (as I said for scrapbooking) and they were blurry.
Hence why I am pushing the images UP to 300 ppi. THEN reducing the sizes down to the size I require onto my new template that is the correct paper size at 300 ppi.

to rich2005 I will have a look at those plugins and hope I can understand one of them


Any ideas about why the mB size of the photos goes up when only the dpi has been changed? Or is it because its been opened by gimp and resaved, because gimp has done other things as well as change the ppi?
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May 23, 2012, 14:46
Post: #5
RE: Batch convert ppi?
(May 23, 2012 14:13)Pepperisit Wrote:  In regards to you ofnuts;
I made a template of the size of the printer paper at 72ppi, imported the photos and resized them as I required (using scale to mm) and printed them out. They were exactly the correct size but because they had been scaled down (physically) but the ppi was still 72, the image lost a lot of pixels before printing and the small photos (as I said for scrapbooking) and they were blurry.
Hence why I am pushing the images UP to 300 ppi. THEN reducing the sizes down to the size I require onto my new template that is the correct paper size at 300 ppi.
Why don't you use "Image/Print size" and set the print size in mm/inches and let Gimp adjust the intended print PPI accordingly... This is what this setting is meant for.

If you image hasn't got enough pixels the output will be at something less than 300PPI, but the looks will be as good as if you upscaled the picture in Gimp to increase its pixel size and then printed it at 300 PPI.

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May 23, 2012, 14:53
Post: #6
RE: Batch convert ppi?
Very valid point. Quite simply because GIMP and my printer somehow don't seem to be able to communicate! When I use the gimp print function and set the size of paper as 5x7 it prints off centre even though the printer is set up correctly.

So my workaround is quite simple. I use the template of the print size at 300ppi then import the photos to that, resize each layer then save it and use the software that came with the printer to print that image file.
This also means I am able to print multiple photos on one piece of paper to save on waste

I am sure there are always better ways but the limted use I get out of gimp and the printer means I only use it once every few weeks
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