Create mask from Image

[James Parker]

I am trying to create a mask (to use on an adjustment layer) from an image.  I have imported the image, converted it to a pixel layer using rasterise, but whatever I do (following the various guides on this forum and elsewhere) I cant get it to convert properly to a mask which can then be applied to an adjustment layer.  The obvious 'rasterise to mask' just results in a completely black mask as does 'apply as mask to below'.

 

Attached is the selected mask layer (its a 'star mask' - ie it masks the areas identified as 'stars' in a stacking program').  The end goal is to 'bring up' the stars in a sky scene of comet Neowise (second attachment) without over-exposing the sky or the comet.

 

[MEB]

Hi James Parker,
Welcome to Affinity Forums
Are you sure Rasterise to Mask results in a completely black mask? Have you alt-clicked the mask layer in the Layers panel (to see the mask a a grayscale image) and checked it? Your first screenshot only shows a pixel layer selected not a mask.

- select the layer you want to use as a mask. Go to menu Layer > Rasterise To Mask (you should see "Mask" rather then "Pixel" between parenthesis after the Layer's name)
- with the mask layer selected go to the Channels panel - right-click the Mask Alpha channel there and select Create Spare Channel (it will appear in the bottom of the Channels panel)
- create the adjustment layer you want as usual (let's say Levels for example)
-  go to the Channels panel, right-click the Spare Channel you created earlier and select Load to Levels Adjustment Alpha (should be the last option)
The Levels adjustment should now have the mask loaded.

[James Parker]

Hi

 

Thanks for the response.  I followed the instructions and they do work thanks.  So I can remember in future what is the bit with creating the spare channel doing and also why is the mask layer not visible in the normal way as a child to the (brightness/contrast) adjustment layer?

 

 

 

[MEB]

Hi James Parker,
The Spare Channel bit is just to store the mask as a new channel so you can load it to whatever adjustment/filter/mask you want from the Channels panel. Ideally we should be able to simply copy/paste from a layer mask to another mask or filter/adjustments built-in mask, however this is not currently possible.

Regarding your second question - why is the mask layer not visible in the normal way as a child to the (brightness/contrast) adjustment layer? That's because all adjustments and live filters already have a mask built-in. So for example, let's say you have a colour image of a flower in a garden and want to turn everything black and white except the main subject (the flower) - you can add a black and white adjustment to the layer, (turning all the image in black and white using the sliders the adjustments offers), then with the adjustment layer still selected, select the Paint Brush Tool, set it's colour to Black (when working with masks black conceals, white reveals) then paint over the flower on canvas to remove (or hide) the black and white adjustment effect over that area. When you paint with black with he adjustment (or live filter) layer selected you are actually painting in the adjustment/filter's built-in mask (if you disable the adjustment/filter unticking it in the Layers panel you will see the image layer is still full colour).

Despite this you can still attach a regular mask to an adjustment or live filter. Select the adjustment or live filter layer in the Layers panel, then click the Mask Layer icon on the bottom of the Layers panel. Depending on the Assistant setting Add mask layer to selection (menu View > Assistant Manager in Photo Persona) the mask will be added either as a child layer or as a new layer. For this to work the mask layer must be a child of the adjustment or live filter. If it wasn't added as a chid, simply drag the mask layer over the thumbnail of the adjustment or live filter.