hi,

i found any downsampling ( scale image to a smaller size ) will definitely blur the image ( scaned from film ) to an undesired degree. is there any suggestion around that? using a unsharp-mask after every downsampling ?

--
woody

On 4/12/06, Leeuw van der, Tim wrote:

Hiya,

What kind of interpolation do you use when rescaling the image to a smaller size? (And what version of GIMP do you use?)

i was using Linera interpolation. GIMP version seems 2.2.x on Linux

GIMP 2.3.x added a new kind of interpolation, Lanczos I believe it's called, which is said to work better; both for upscaling and downscaling. Binaries of GIMP 2.3.7 are available for windows, but if you don't use windows, then I don't know how you would best get binaries for your platform.

i think i can download source tarball and do a compile/install, right?

Cheers,

--Tim

-----Original Message----- From: gimp-user-bounces@lists.XCF.Berkeley.EDU [mailto:gimp-user-bounces@lists.XCF.Berkeley.EDU] On Behalf Of steven woody
Sent: woensdag 12 april 2006 10:00
To: gimp-user@lists.xcf.berkeley.edu Subject: [Gimp-user] How to prevent downsampling from bluring my image

hi,

i found any downsampling ( scale image to a smaller size ) will definitely blur the image ( scaned from film ) to an undesired degree. is there any suggestion around that? using a unsharp-mask after every downsampling ?

--
woody

On 4/12/06, Leeuw van der, Tim wrote:

Hi Steven,

Sorry that my reply is getting a bit messy but outlook on windows isn't very well in quoting text...

never mind and i thank you!

To get the latest version depends on your distribution. You can get the source tarball and compile, fetching any updated dependancies yourself. But a distribution like Debian is likely to have packages already built and available as part of 'unstable'.

ok, i know how to do you.

Btw, since your images are scanned from film, perhaps they are already somewhat noisy?

yes, i found scanned images are usually noisy than digitial camera's. it is actually a pain for me. what's the best way of handling the case?

Did you try to reduce noise, or perhaps increase sharpness / reduce blur / etc, before scaling the image? Just a thought - but it seems to me that (down)scaling a noisy image will give worse results than downscaling a cleaner image.

i do the noise reducing using despeckle filter. but it was adviced in many other articles that tools like despeckle will also introduce blurring ( i found it's really true ) and should be down in the very final step, especially after downscaling or upscaling. so, i think i have some problem to get your point here. did you really suggest i do the despeckle or some unsharp mask things before downscaling ?

Another option would be to try scaling the images with imagemagick. It might already have lanczos scaling and other scaling options built-in with the version installed (or installable as pre-built package) with your distribution. You can even write a small shell-script to scale all images in some directory, if that's your thing.

supposing my Gimp has already equiped a Lanczos interpolation, when comparing to imagemagic in scaling, which one performs better in term of quality?

thanks.

--
woody

ok. thank you for your information and i will try it.

On 4/12/06, Leeuw van der, Tim wrote:

Hi Steven,

I'm by no means an expert on image manipulation. If experts recommend to do despeckle as last step, then perhaps that might be best. My logic was that a noisy image would be harder to scale right because the noise could introduce more artifacts. But I could be wrong -- I don't have that much experience, and don't have time now to do the experiments. (Would first need to search for a suitable photo...). However, I thought that if you already have a number of photos at hand to experiment with, then it might be a quick experiment for you to first reduce noise in the image, then perhaps sharpen it, and then scale it.

About quality of ImageMagik vs. GIMP, I have no idea. I never made a detailed comparison of the two in quality of scaled image. I did make a comparison of scaling of ImageMagik and PIL (Python Imaging Library) and found them to be comparable for the photos I needed to rescale at that point; after which I wrote my script in Python using PIL to scale down (without any further enhancements) a full copy of all images from my digital camera to fit my screen. But, as I said, they're images from a digital camera and not from a scanner. And I didn't care if a single image would come out with poor quality; it was a massive batch operation.
At that point I had already given up on writing a GIMP script to perform this operation in batch so I didn't compare the quality of GIMP scaling with the others.

Cheers,

--Tim

--
woody