Colour and luminance as components of an image

I'm trying to replicate the colour-only images that Tom Axford posted last year on dpreview in this thread:

www.dpreview.com/forums/post/66129684

using Affinity Photo, but not having much success. I have tried converting the colour image to LAB, then making only the L channel editable and visible, then filling the whole area with solid mid grey (RGB 128, 128, 128). When I switch the a and b channels to visible I see something a bit like Tom's results but the image is less distinct and much less vibrant.

Does anyone know how to do this properly?

BTW: I tried posting a reply to the thread at dpreview, but for some reason it is not showing up - perhaps the thread was considered too old.

You can take a look at the last two pages of this thread:
dprevived.com/t/what-are-your-biggest-inspirationsmotivation-boosts/4292/

@TomAxford was nice enough to explain some of the techniques there once again, because I wasn‘t able to replicate some aspects. Perhaps your question is answered there as well!

Thanks. It seems very easy in Affinity. Add an HSL adjustment layer, and in its properties box tick HSV (top left) and drag the Luminosity slider to 100%. :)

Yes, that sounds right. In HSV, the V channel is defined as V = max(R, G, B), so making V = 100% means that at least one of R, G and B is 100%, which is the maximum luminosity possible for that colour without clipping one of the colour channels.

The problem with using HSL or LAB is that raising luminosity to 100% can lead to clipping of one or more colour channels, which inadvertently reduces the colour saturation and possibly changes the hue also.

I don't use Affinity Photo, but in Ps, you just convert to Lab and turn the layers on and off.

The GIMP has 'decompose' and 'recompose' options and any space can be selected. Here's a trip to HSV and back with S set to 50% and V turned down a bit in Curves:

Adjusting the Saturation and Value affected the whites big-time!

If your only interest is in the hues, there's an app called Show Image which will show all them all in living color at 100% saturation:

kronometric.org/phot/color/ShowImage.exe

Still runs on my Win 7 computer, don't know about later - or Mac for that matter.

Yes, that is bound to be the case. Pure whites and neutral greys have 0% saturation, so when the saturation is set to 50%, the only way to do that is to turn it into some arbitrary colour. If the colour is just off-white, setting the saturation to 50% exaggerates the colour hugely.

In JPEG images, most "whites" will actually be very slightly off-white because of JPEG compression errors and these errors will be hugely exaggerated when the saturation is increased to 50%.

Errors of this sort can be substantially reduced by using 16-bit TIFF images instead of JPEGs.

Thanks, that‘s very interesting! Is it possible to reduce the amount of one of those layers/channels (L,a,b) or create a copy of how the image looks when L is deactivated for example?

No doubt.

Currently, I am using 32-bit floating point Linear in the GIMP. To get a pure Hue Map, I decompose to HSV; fill S and V with some shade of gray; then recompose.

... the result is not for the faint-hearted!

Sure. Edit one or more of the channels.

With L* set to 50:

Thanks - I've no idea how to do that though... I can't change opacity/visibility of channels.

Select the L* channel.

Select All:

Fill with 50% gray:

Turn on all channels:

Go to layers to check your work:

Thanks a lot - I‘ll try that!

I'm thinking that extracting the hues-only in the GIMP and then partially merging the result (adjust opacity) with the original could be interesting.

Original:

Hues:

Observe the shadow colors ...

Flower is herbertia lahue

Yes, there are many interesting possibilities.

Here's another one I quite like:

(done by creating a duplicate layer of mode "HSV Value", then raising the black point in that layer and using Unsharp Mask a couple of times to bring out the detail)