Hi Sean P and MEB,
Now got my act together and here is the info you asked for. And a resume of what I've done. A bit of long post I'm afraid.
In Raw Develop Persona:
I use a basic routine to prepare Raw files to give a good starting point for tonal adjustments. (See Affinity video on Maximising Raw Latitude.)
1. De-fringe problem arises mainly when I am trying to target colours other than the default mid blue.
2. As MEB notes, the grey blocks appear when Edge Brightness Threshold is reduced to catch colours like the magenta edges on the chrome/metal-work. (Affinity tutorial video shows taking Edge Brightness Threshold right down to catch hard-to-reach fringing.)
In Develop Persona:
Screen Movie attached:
3. Basic Raw Latitude Maximisation routine.
4. Chromatic Aberration.
5. De-fringe in Raw Develop Persona:
To remove orange fringing: Hue 30º, Edge Threshold 56%, Radius 10px, + Complimentary Hue.
Desaturated areas appear. Varies with view zoom.
6. Develop image - artefacts mostly disappear but some desaturated ‘halos’ remain - not subject to zoom view factor.
In Photo Persona:
7. Basic Raw develop routine (No de-fringe at this stage.)
8. De-fringe as live filter layer. Similar settings as under Develop Persona, But: Edge Threshold 70%, Radius 20px.
Desaturated artefacts appear, and vary with zoom.
Flatten file: artefacts remain.
9. Back to beginning, apply de-fringe filter to a duplicate layer directly. Same result, BUT artefacts almost disappear when I hit “apply”. Some still remain in small areas as seen in video.
In Photo Persona using Frequency Separation technique.
10. Back to beginning. Add Frequency Separation filter.
11. Set high frequency quite low - about 1.7px.
12. Add HSL filter and drag into High Frequency layer.
13. Drag Saturation down almost all the way. (take it too far and all colour blacks out.) Most of the magenta/cyan hot pixels neutralised.
14. Use sponge tool to knock back these areas directly.
15. Flatten image then use Vibrance to restore any colour lost in this process - only a little needed.
I guess I am not understanding the implications of the De-fringe slider settings. Following the excellent Affinity videos helps most of the time, but recently I’m getting these grey blocks. Is this to do with app updates?
Screen Shot 2018-06-05 at 22.46.01.tiff
Screen Shot 2018-06-05 at 22.46.20.tiff
Screen Shot 2018-06-07 at 18.02.13.tiff