Question about GIMP

*eadaoin

Hey guys,

I'm just getting used to the GIMP and have a question.

After RAW processing in with my Canon software I'm converting images to JPEGs and then opening them in GIMP for cropping, resizing, layers etc.

The thing is GIMP automatically open images at 72dpi but I'm working on them with the intention of printing so I'll want to save them at around 350dpi. I thought it was not good to up the resolution to 350dpi manually as this can affect the quality of the print? At least this is what I think I've figured out so far, I'm still learning about all this stuff so I may have gotten it wrong!

GIMP isn't changing the actual size in pixels so the images are still fairly large and would probably make a decent print, but I want to make sure I'm creating the best possible file for the best possible print so I'd like be sure I'm doing everything correctly.

My question is: is it possible to open images at their original dpi and then only change it to 72dpi if i want to put them on the web?

PopeBuckfastXVI

Doesn't it just depend on how many pixels there are and what size you are printing?

Isn't dpi effectively meaningless until you actually go to print something?

*eadaoin

Well my point is that I am actually going to print something, like I said I'm working on these images to print them so I want to make sure I get this stuff right.

magicbastarder

if you change the DPI in GIMP and leave the actual pixel dimensions of the shot the same, it won't actually affect the image in any way in GIMP in terms of quality or size - the DPI is just an instruction to the printer, and will not have any effect on processing.

*eadaoin

Cool thanks, that's great to know

It's hard to get your head around this stuff sometimes, especially when you're learning from books and the internet rather than classes!

AnCatDubh

There was some comment around here recently to the effect that ppi/dpi/etc has little meaning in the context of an image which has been produced from a camera sensor which has a defined height and width in pixels.

Where it is relevant, is if you are actually working in inches - for example, a publisher says to you that they want the image in 10x8 format at 360dpi. They are then telling you that they want an image at 3600x2880 (they obviously may accept larger numbers in the same crop ratio). Thus your image now has a defined height and width that you are working with.

For ease of use purposes if you change your measurements to inches rather than pixels then the setting of dpi or ppi (one print the other screen but effectively the same measurement) comes into play - altering that setting will have zero effect on the number of pixels (height and width) which makes up your image but will tell you if printing at a certain dots/pixels per inch how large (in inches) your print will be.

Many printers will get away with much less than 360dpi and may allow you 200dpi.

So if you go from 100 to 300 pixels per inch, your image will appear to shrink by 3 times if measured in inches but the pixels remain identical.

All you are actually doing is saying to the Gimp, that instead of telling you in 72dpi what the print measurements will be, calculate it assuming a larger value such as 360.

So think of it as a form of measurement rather than a for of image manipulation.

If on the other hand you were scaling your image (resize) and scaled it in inches, then it would have an effect on the number of pixels increasing or decreasing as was required.

Also, if memory serves me, you can tell the Gimp to default to 360 or any other dpi value. I'm guessing the reason it defaults to 72 is for screen resolution purposes - so to display your image out of camera on screen, it would be based on a calculation width/72 x height/72 = w x h in screen inches (pixels).

I think that is the gist of what was posted previously on the subject.

Hope i've made sense!

*eadaoin

Ok I'll admit I'm slightly confused by all of that, but I'll give it some time to sink in at home this evening and I'll understand it in no time I'm sure.

Once upon a time the relationship between ISO, aperture and shutter speed seemed impossible to understand but if I can figure out that stuff then surely I can figure out this stuff too!

AnCatDubh

I guess i've probably put too many words on it Assuming that you are working in exact measurements then the main thing to know is how many inches and how many dots per inch = multiply one by the other and it gives you the desired number of pixels for your image.

hbr

*eadaoin wrote: »

Hey guys,

I'm just getting used to the GIMP and have a question.

After RAW processing in with my Canon software I'm converting images to JPEGs and then opening them in GIMP for cropping, resizing, layers etc.

This introduces an unnecessary loss of quality in the RAW to JPEG
conversion. It would be better to convert from RAW to a non-lossy
format like TIFF. When you are finished working on the image, you
can save it to another TIFF or in Gimp's own native .xcf format.

The thing is GIMP automatically open images at 72dpi but I'm working on them with the intention of printing so I'll want to save them at around 350dpi.

The default 72dpi figure in the Gimp print settings is only a way of
calculating the final image size. This has nothing to do with the size
or quality of the stored image.

GIMP isn't changing the actual size in pixels so the images are still fairly large and would probably make a decent print,

That is the key point. The image is unchanged regardless of the print dpi settings.
The printed image will be a different size to the image on the screen unless your
printer happens to have exactly as many dpi as your screen.

but I want to make sure I'm creating the best possible file for the best possible print so I'd like be sure I'm doing everything correctly.

For printing, it is best to use a non-lossy format instead of JPEG.

My question is: is it possible to open images at their original dpi and then only change it to 72dpi if i want to put them on the web?

The requirements for the web and e-mail are different. File size is a consideration
so you should re-scale your images to a suitable size and save them as JPEG.
Choose a compression setting that gives a reasonable compromise between
quality and file size. This is usually in the 60-90 range on the Gimp slider
thingy.

magicbastarder

from a print perspective, print size, pixel size and dpi are interrelated - print size is pixel size divided by dpi; so you can't change one and leave the other two untouched.

e.g. a 2000x2000 pixel image will print to 20" x 20" if the dpi is set to 100dpi.
if you want to change the dpi to 200dpi, (keeping pixel size constant) the 2000x2000 pixel image will drop to 10"x10" print size OR (keeping print size constant) to 4000x4000 pixels at 20" x 20" - i.e. it differs depending on whether you want to hold pixel size or print size as you change dpi.

in PP, it's usual to change the dpi but keeping pixel size constant, as this is the only one which really matters on screen. your computer will generally ignore print size and dpi while displaying the picture.

*eadaoin

Thanks guys, you've all been very helpful. I'll read through this stuff a couple of times and see if it will start to piece together in my noggin

hbr I didn't know that about TIFF files, I guess I should work with those from now on if I want the best quality print.