Gimp re-sizing photos

Iristxo

Hi guys

I have used gimp to touch-up a few photos (a good few) and I have just by complete chance realized that it has re sized them as well. Some of them are like slightly bigger or whatever but some of them are smaller. Does this not mean that they loose quality? Like if I'm going to print out the photo in a big size it'll be all pixelated and all that. In a few instances a photo that was originally over 2Mg is now less than one! how is this possible and why does it do it? And I have deleted the originals as well, big mistake, I know that now...

AnCatDubh

The Gimp won't resize an image unless you tell it to - IMAGE->SCALE IMAGE. You then enter parameters and hit scale so I don't think it has resized them on you.

I'm thinking the compression which you've used might be responsible for the shrink in file size, however the images size as defined in width x height terms should be unchanged.

What format are you saving in ? JPEG's / TIFF's

If jpeg, then compression will be applied by the gimp or any other image processing software. Multiple saves of Jpeg will cause image degradation.

So I don't think you've an actual resize issue - rather a compression issue.

Iristxo

AnCatDubh wrote: »

The Gimp won't resize an image unless you tell it to - IMAGE->SCALE IMAGE. You then enter parameters and hit scale so I don't think it has resized them on you.

I'm thinking the compression which you've used might be responsible for the shrink in file size, however the images size as defined in width x height terms should be unchanged.

What format are you saving in ? JPEG's / TIFF's

If jpeg, then compression will be applied by the gimp or any other image processing software. Multiple saves of Jpeg will cause image degradation.

So I don't think you've an actual resize issue - rather a compression issue.

Really? So are yo telling me that if I touch-up the image with the windows photo gallery editor and save, then the same image with the gimp and save again, I am degrading the image? And this translates into what, a loss of quality when I print them or whatever? They are JPEG images. Thanks so much for your help.

Iristxo

One last question, the windows editor allows me to "restore" the photo to the original image. Is this the original image with the original quality and all? Thanks again

5uspect

Not sure about the windows editor but yes every time you save a jpeg you apply compression.

As a comparative example this is what happens when you upload a video to youtube 1000 times. Videos are compressed like jpegs and when you upload to youtube they add some lossey compression to save themselves some bandwidth.

A lot f people here shoot in RAW format from which they compile a jpeg. The RAW file is an uneditable digital negative. If your camera can shoot in RAW I would recommend it.

If you must use jpeg always retain the original file and never save over it.

Heebie

I totally agree.. if your camera shoots JPG format.. as soon as you offload the images from the camera.. back them up somewhere, like to a CD or DVD.

Then, once you know you have copies of the original, unedited files, then you can work with them.

If you have to save bits of an image before you're "done" editing, use a format that does not have loss, such as a TIFF (a.k.a. TIF) as that will not have any compression. (If you're using Gimp, it has it's own native format, which I think is called an XCF, and in Photoshop you can use a .PSD or a .PSB) These files you can open again and re-save without any quality loss.

When you're DONE editing the picture, you can then save as a JPG. When saving as a JPG, you can save the image at the highest quality setting available to cause LESS compression, so less distortion of the image.

Some programs use a lossless JPEG compression (also called a "100% quality" JPG.. or a quality level of "12" in Photoshop) These still compress the image quite a bit, but don't degrade it. Most program that handle jpeg files do NOT have the lossless algorithm available.. if you save at 100%.. it's still using a lossy compression.. so it's always a good idea to assume the software you're using does not understand the lossless JPG format. (assume it's going to compress and degrade the image if you save as a JPG in other words.)

AnCatDubh

Iristxo wrote: »

Really? So are yo telling me that if I touch-up the image with the windows photo gallery editor and save, then the same image with the gimp and save again, I am degrading the image?

Yes. Each time you save compression is applied. Actually, good points made by subsequent contributors above - the Gimp has a 100% quality setting when you save as JPG. There are other options in advanced settings which I haven't toyed with but by the looks of them they will also effect the JPG image quality - one in particular which looks like a smallest filesize v best quality -- this is usually the tradeoff.

And this translates into what, a loss of quality when I print them or whatever? They are JPEG images. Thanks so much for your help.

Yes, unfortunately the more saves you do, the more likely that the quality of your print will suffer in that you are altering the source from which you will be printing. Depends on how harsh the compressions have been to the image - often you can get away with murder when it comes to print so long as you aren't printing something the size of a house.

If working in the gimp, I recommend saving in the native xcf format -- this is the gimps own format and is lossless so you are safe. Your filesizes won't reduce but your quality will remain in tact. The gimp likes working with it When you first open your JPG in the gimp just immediately do a Save As and call it <<filename>>.xcf -- Always then as a last stage of your workflow, output the image to a JPG if that's your final desired result. This way you can alter the image as often as you like -- that is working with it saved as xcf -- and finally output as JPG as often as you like. Because your source is the xcf filetype, your final JPG's no matter how often you revise them will have full quality. Remember after you Save As JPG, and if you are continuing to work on the image, to reopen the .xcf; then continue to edit at will.

TIFF is also a good lossless format to be using.