Commons:Graphics village pump
Artefacts at certain resolutions of scanned imagesEdit
I frequently find when scanning images from books etc., that while the scan looks fine at full resolution, at lower resolutions the image appears with crosshatching or line patterns that can at times badly distort it. As an example, I downloaded a screenshot of this image. In the screenshot, at full resolution one can see a pattern of white dots superimposed over the image (which may be alternatively seen as a dark grid).
Having read the help page on scanning, I'm assuming they are some sort of moire effect? However, it's not clear to me what if anything can be done to prevent it. Can anyone help? Thanks, Gatoclass (talk) 11:45, 17 April 2021 (UTC)
- Hi @Gatoclass: It's because printed material uses halftone screening comprising repeated dots of different sizes to simulate greyscale. As you pointed out, a moiré effect arises when the display resolution doesn't match the dot spacing, analogous to beats in music.
- The best solution is to source higher-quality material, such as original photographs before halftoning, but that might be impossible. A workaround is to slightly blur the image using an optical low-pass filter but that degrades the image.
- A better solution takes the Fourier transform of the image and discards frequencies corresponding to the pattern. Here's the procedure in The Gimp: http://en.wikibooks.org/wiki/GIMP/Remove_Coherent_Noise and a more in-depth analysis.
- P.S. I used your screenshot to illustrate w:aliasing.
@Cmglee:, first of all, thank you so much for the assistance! I got the plug-in working, and have had excellent results with it so far!
One minor issue though is that the tutorial states that the "Decompose" function is found at "Image > Mode > Decompose", actually it's found at "Colors > Components > Decompose" - at least on my version, which is 2.10.14. I see there's a new version out so maybe the tutorial is correct.
Other than that, I am very pleased to have found a way to eliminate this annoying issue from my uploads! On the way, I also learned how to scale images in GIMP - which is really very simple, I don't know how I overlooked it before, so that's another important new string to my image manipulation bow! Additionally, I learned the advantages of scanning and uploading images in .png, which is another handy thing to know. So this has been a very worthwhile exchange for me. Thank you once again, your assistance is much appreciated! Gatoclass (talk) 10:07, 23 April 2021 (UTC)
A lot of images taken with Category:Taken with Olympus XA have dark corners and a overligthed middle. It is not en lens defect as pinhole cameras wirhout lenses have the same problem. Its physics: There is less ligth coming trougth a diafram hole from if it comes at an angle. Good lenses correct this problem. Is there a way or tool to correct the images digitaly?