Inverting "L" channel in LAB Colour space

[Adil]

I saw a recent YouTube video of someone using a Lightbox to take macro images. They looked great but looked better when they inverted the white background to become black. The method they used to do this in photoshop was relatively straight forward. They changed the colour gamut to LAB profile for the image they were editing in PS. The "L" channel holds the black and white details and there is an option in PS to invert this channel. I am trying to find a way to replicate this process with Affinity Photo and cannot see to find a way to invert a channel.

Anyone have any ideas?

Thanks

[R C-R]

If I understand what you want to do correctly, in AP change the color format to LAB/16 via the Document > Convert Format/ICC Profile menu option, then in the Channels Studio panel, make sure the pixel layer (probably named "Background by default) is selected, right click on Lightness (probably named "Background Lightness") & choose "Invert" from the menu that pops up.

If there are foreground objects you do not want inverted, first select them with the 'marching ants' selection tools, refine that selection as needed, & invert the selection (via the Select > Invert Pixel Selection menu item) before doing the above. (You can do it the other way around by selecting all the background, excluding the foreground, but it is often easier to select the foreground & invert that.)

Then do a copy & paste to add the foreground as a separate layer (so it won't be inverted), reselect the original background layer & then invert that layer via the Channels panel.

[Adil _P]

Hi thanks for this however was not able to do what you said.

 

From the screenshot below the image is loaded in LAB colour profile. 

The channels available are:

• Composite Lightness
• Composite AOpponent
• Composite BOpponent
• Composite Alpha

If I right click on the Composite Lightness channel whilst back ground pixel is selected the choices I get are as follows:

• Toggle Visible
• Toggle Editable
• Load to PIxel Selection
• Add to PIxel Selection
• Subtract from Pixel Selection
• Intersect with Pixel Selection
• Create Spare Channel

There is no option to invert channel.

An example of what a finished product looks like before and after inversion is shown below (Not my work). As you can see from the example given by inverting white to black effectively effects all of the original colours - this is the effect I am trying to achieve with the first image below.

"Invert Channel" does not appear when searching for help?

Any ideas would be appreciated.

 

 

 

 

[R C-R]

1 hour ago, Adil _P said:

The channels available are:

• Composite Lightness
• Composite AOpponent
• Composite BOpponent
• Composite Alpha

With the "Background (Pixel)" layer selected, try scrolling down in the Channels panel. Below the  composite channels, you should see 5 more items, including the "Background Lightness" item I mentioned.

BTW, I find it useful to set the "Show scroll bars" option in System Preferences > General to "Always" on my Macs so there is never any confusion over whether or not I am seeing everything there is in a window.

[Adil _P]

Thanks got it!

Here is an example of what I was looking to achieve.

 

Thanks for the help and advice!

 

[lepr]

An alternative solution: open the image file, add a Curves Adjustment, change the adjustment's mode to LAB, change its target channel from Master to Lightness, and then invert the curve (drag bottom left node to top left and drag top right node to bottom right).

 

[R C-R]

45 minutes ago, anon2 said:

An alternative solution: open the image file, add a Curves Adjustment, change the adjustment's mode to LAB, change its target channel from Master to Lightness, and then invert the curve (drag bottom left node to top left and drag top right node to bottom right).

Not just an alternative but a better one, since 1) there is no need to change the document format to LAB & 2) this curve can be added to the AP Adjustments panel as a preset.