Yes, the rigging in the resized but not sharpened image looks bad, so any sharpening will make it look worse. You might post a link to the full size version.
To my eyes, the water and foliage look good in the final version, so you might use an adaptive sharpening that leaves the boat alone. But it’s better to sort out the resizing first.
I wonder why you used cubic. When you downscale, you can avoid aliasing problems by pre-blurring the picture by the same amount as your scaling (Gaussian 3px blur if your downscale 3x, for instance). This removes high-frequency components that could cause aliasing and wouldn’t be visible in the result anyway (since they would be smaller than a pixel).
Everyone - many thanks for this. This is just what I needed!
Right, so, I chose cubic just because it was the default in gimp 2.10. I used to use sinc lanczos3, now it’s not there and just went with cubic…
So I downscaled again and it seems that lohalo is the best. If I zoom to say 400%, I’m not seeing the artefacts.
Would you say that this is a good starting point before I start trying to sharpen???
Resized using lohalo:
Here’s a link to the full-sized image:
(link removed)
Cheers,
Jules
I’m seeing sharpening artifact on that image. I don’t think there’s a way to fix that easily.
Where are you seeing sharpening artefacts? In the ropes to the left of the front mast? Those do not look quite right to me, but I’m not sure if it’s from the sharpening or not.
It is most apparent in the ropes of the mast, yes. That’s where I’m noticing some white lines.
Maybe this is what @Reptorian is referring to?
Those seem like they shouldn’t be lighter there, but I wouldn’t know if it were due to sharpening or not personally.
Okay - great - now I’m getting it. I can explain the “lighter” bits. They were caused by me inadvertently double sharpening in Canon’s DPP software. I used the lens optimizer and Canon says to turn off the sharpening or it will cause artefacts - I hadn’t done this. So, I have now turned off the sharpening and the “lighter” bits seem to be gone to me.
Here’s the modified original:
(link removed)
Now, it looks legitimate or better to work with. There is constrained sharpening on G’MIC, that might be your answer.
@jules Definitely need to be wary of DPP because it could sharpen without you knowing about it. In camera, I normally try to make everything as unmodified as possible; e.g., setting picture profile to Neutral. You can do that in DPP as well. That way, you have more control when you use GIMP, etc.
You should also straighten it. Masts in a sailboat are at best vertical, or slightly leaning aft (“rake”).
Just curious: shot in Scotland?
I really appreciate this - many thanks!
@Reptorian Thanks, I’ve now applied Constrained Sharpening.
@Ofnuts Yes, it’s Scotland, and still needs straightening. 
@afre Thanks for the DPP tip.
So, with everyone’s input, I took a fresh TIF, scaled it down to 650x490 and applied Constrained Sharpening. In terms of downscaling and sharpening a downscaled image, is this now all right? (I know other things could and should be done to this image but for this exercise I wanted to focus on downscaling and sharpening.)
Thanks again for your input, advice and opinions!
I cannot usually get the result I want just by applying simply one sharpening to the whole image.
Here is my common workflow regarding sharpening when dealing with photos publish in the web. I am shooting mostly portraits.
First consider local contrast (LC): if the pic asks for some larger scale pop, GMIC has some nice tools like Local Contrast Enhance and Texture Enhance. Choose the area in preview image which needs strongest LC settings, mess around with settings to get a good result and click ok. Remember that you can always pull down opacity if the effect is too strong. Usually you have to apply a layer mask and
a) paint black the areas where you don’t want LC (usually: bokeh, OoF areas, face, skin)
b) paint white the areas where the used LC setting is ok
c) use shades of grey in the areas where you want less LC
! If you use strong LC settings, be especially careful with black-white transition zone’s in masks. Use medium-soft brush. It is a good idea to use the free select tool to roughly set the area of effect, pour in a can of paint and finish the borders with brush.
! Sometimes you don’t need LC and dodge & burn is enough.
Sharpening is the final step before watermarking (if use one) and exporting. Nowadays I prefer GMIC’s RL-deconvolution for sharpening and it is done the same way LC is applied: pick up the area which needs most sharpening, get the settings, apply layer mask and paint:
a) black: skin, bokeh, OoF, etc. you don’t want sharpened
b) white: eyes, eyelashes, clothes, props, and so on
c) grey: lips, hair, …
! Sometimes you have to create two sharpening layers and use masks to get separate them
=> Export and be happy <3
In your case I would use mask and ease sharpening on ropes and other areas where you get jagged lines with full setting.
And yes, I am aware that some things like applying LC to bokeh is kind of matter of preference / taste, but I am here just sharing my opinions. Feel free to disagree.
As an example, below is one portrait edited this way and a screenshot from Gimp showing layers and masks. Didn’t get rid of all jagged lines in her sunglasses, but otherwise sharpening is ok to my taste.
https://www.flickr.com/photos/jannebokeh/30799549278/in/dateposted-public/
Unfortunately there’s no room for straightening if don’t resort to cloning the sky.
Compare these images by opening them in tabs and switching back-and-forth.
GIMP-2.10.4 NoHalo:
RawTherapee-5.5 Lanczos:
@Janne - Many thanks for sharing the workflow tips- very much appreciated!
@Morgan_Hardwood - Yes, I see that. What a difference! I did the same test - resized in Gimp (nohalo) and in RT. And resizing in RT is far, far better… Kind of annoying to have to swap to RT to downscale but proof’s int pudding, as they say!
Aggressive methods tend to have more artifacts whereas gentler ones tend to be much softer. NoHalo is supposed to be softer than the other methods.
I’ll not bring my two cents about sharpening, but:
@Francois_C - Many thanks for the input. The reason I posted a reduced image was that I was trying to figure out how much sharpening needed to be applied to a downscaled image. And what I discovered was that my original image had been over-sharpened by DPP. Once I turned that off, those artefacts went away and I then was off to the races, as they say.
Again many thanks to everyone for the input.
Cheers,
Jules