How to solve colour banding in resized images?

[joe_c5]

<p>The first one was just done in gamma 2.2 space. That one was with ImageMagick, but GIMP didn’t look too diferent. If it looks better to you than the other two, then your monitor may have a gamma different from 2.2.</p>

<p>The sinc downsampled version should have flat frequency response out fairly close to Nyquist, so I would not expect it to need sharpening. Any sharpening is likely to add high frequency artifacts, which given the high frequency repeating pattern on the stairs could cause Moiré patterns even without true aliasing.</p>

<p>And yes, do everything else in your usual workflow, using GIMP only for the final resize for troublesome images like this.</p>

[syd]

<p>Joe,</p>

<p>You've gone over my head a bit with some of the tech stuff above ... I'm getting there slowly with this Digital Darkroom learning curve however so bear with me. One thing I should state outright is that my monitor is not profiled and it is just an office junker I have been using at home ... any minute now I'll be buying the NEC and will profile that correctly ...I will also try and profile my scanner as well. So how this Moire is looking on this screen is kind of a nice baseline for the absolute worst case scenario of people viewing the image ... in a way this is good.</p>

<p>Questions ... what is Nyquist?</p>

[joe_c5]

<p>The Nyquist frequency is half of the sampling rate. In the case of an image displayed at 1:1 on a monitor like that, it would be one line pair (a dark line next to a light line) per two pixels. You can see that some sections of the downsampled images are like that, with alternating light and dark lines on each pixel.</p>

<p>Any spatial frequencies higher than Nyquist cause aliasing, where they look like a lower frequency that is not supposed to be there.</p>

<p>I am not sure whether the patterns that show up when sharpening some of the downsampled images are aliasing or not, but they definitely do not look right.</p>

[tom_mann1]

<p>BTW, Simon, color profiling has nothing whatsoever to do with Moire patterns. You can get Moire, halos, and such even on B&W images.</p>

<p>More later,</p>

<p>Tom M</p>

[joe_c5]

<blockquote>

<p>color profiling has nothing whatsoever to do with Moire patterns.</p>

</blockquote>

<p>That is correct, however what I mistook for Moiré patterns in the first image I posted were not. Instead they were caused by a mismatch between the gamma of the monitor and the image when it was resized. Profiling a monitor should help to get the correct gamma curve, improving the last image I posted.</p>

[jacopo_brembati]

<blockquote>

<p>So ... how do I resize the image down for web viewing and avoid this geometric banding to preserve the normal view?</p>

</blockquote>

<p>Simon, the only way to avoid aliasing artifacts is to filter the high frequency using an optimal low-pass filter.<br>

Try PhotoResampling :<br>

http://www.photoresampling.com/index_eng.php</p>