Thanks Ian and Glenn
I agree that your photos don't show this effect, which might be because of your workflow. I am seeing it in some other photos on the BLP site for example Diana Womersley's most recent Moderators Pick (Jacky Winter) shows distinct halo effects around the bill and legs, which might be due to sharpening during raw processing but might also be due to downsizing (or both) (apologies Diana for picking on you, its just a convenient recent example). In my own photos it occurs in some situations when downsizing, noticably around very abrupt transitions like bills, and (as I said) can be reduced in various ways. I just have to keep an eye on it.
I have found a number of sites which refer to an 'overshoot' or 'ringing' problem (which I understand are not quite the same thing) from resampling algorithms. For example, this from Wikipedia's article on acutance: "Low-pass filtering and resampling often cause overshoot, which increases acutance, but can also reduce absolute gradient, which reduces acutance. Filtering and resampling can also cause clipping and ringing artifacts." A Cambridge in Colour article on image resampling states: "All non-adaptive interpolators
i]SP - this includes bicubic[/i attempt to find an optimal balance between three undesirable artifacts: edge halos, blurring and aliasing. Even the most advanced non-adaptive interpolators always have to increase or decrease one of the above artifacts at the expense of the other two — therefore at least one will be visible." Cambridge in Colour includes a separate tutorial on reducing moire when downsizing for the web. I found some other articles a while ago but can't bring them up at the moment.
Perhaps your technique involves a less sharp starting point, and results in more blurring during the transition; this would reduce halos and moire, but places more emphasis on post-resampling sharpening?
Glenn, in answer to your question I have used two techniques and the effect can be seen in both. First, I use DxO Photolab, which includes a bicubic or bicubic - sharper option. I am tending to use this now that we have a square 1800px maximum allowance; DxO makes it awkward to downsize to a maximum pixel size specified in both the horizontal and vertical dimensions, as it only allows you to specify a dimension for the longest edge. In terms of sharpening during raw processing ie before downsizing, DxO's main tool is their proprietary lens/camera measurements/corrections which automatically correct for lens issues. DxO has an unsharp mask tool which is off by default and I never use it.
Second (when we specified maximum dimensions in both X and Y axes) I found it easier to use Lightroom for downsizing. This involves exporting a full size 8 bit TIFF from DxO to Lightroom (this happens directly via a plugin) and then downsizing and exporting a JPEG from Lightroom. Although this seems clumsy, once set up it literally takes just a moment. Adjustments to sharpening can be made to the TIFF in LR if one wants. You are correct in that Lightroom (at least the version I have) does not give you much specific control over sharpening in the downsizing process, although it has a few generic settings.
Simon