Transferring brightness to transparency, Affinity Photo

[Starbase1]

OK, here's an odd one.

I don't know Affinity Photo in much depth.

And I'm hoping to use it for an image sequence that's going into a video.

The image as rendered is a lens flare. I can produce it either with, or without transparency - but as it's an optical effect, if I turn on transparency, the whole thing is 100% transparent.

What I would like is to start with an image with a bright flare on a black background.

And then make the darker areas transparent, keeping the bright areas opaque, (with a soft border), in such a way that I can run it over a folder full of images. 

Is it possible to take the luminance/brightness channel, invert it, and apply it as transparency? Or some other trick?

Clues for the clueless very much appreciated!

Nick

[Starbase1]

Sorry, I don't understand what am I blending with, and how does putting it over another layer change transparency?

 

I hope I'm no being thick here...

[h_d]

You could try:

Layer - Invert - this makes all the white pixels black, and vice versa

Filter - Colours - Erase White Paper - this removes the (now white) background.

Cmd-click (macOS, presumably ctrl-click for Windows) on the layer in the Layers panel to select all the pixels. The 'marching ants' won't appear to select everything, but don't worry: some pixels will be 'half selected' and should get picked up when you...

Select - Save Selection > To File... keep the .afselection file somewhere handy.

Close your original file without saving.

To apply the starburst:

Open another image, then

Select - Load Selection from File - navigate to your .afselection file and plonk it in.

Set your foreground colour to white, then

Layer - New Fill Layer.

Right-click on the Fill Layer and choose Rasterise and Trim to create a Pixel layer containing the starburst above the background layer of the image. Deselect (cmd-D on macOS). Select the Move tool, adjust the size and position of the starburst and play with the layer's opacity and with blend modes to modify the effect.

One possible downside is that the starburst will be the same size (in pixels) as the original. So if you place it larger images, it will be proportionally smaller and enlarging it drastically will tend to pixellate the edges.

You might also speed things up by saving the starburst layer as an Asset.

[Starbase1]

H-D, thanks for that, lots to think about! 

I can control the lens flare to any required size, so that is not a problem.

Nick

[Starbase1]

Thanks Hens, a video is a lot clearer!