Premultiply alpha against black color? How?

[Alex_M]

Is there a way in Affinity Photo to premultiply the alpha against black color? I'm doing some renderings and I'm getting black halos around my objects. In Photoshop there's "Remove Black Matte" and in After Effects there's "Premultiplied - Matted With Color:" What about Affinity Photo?

[Dan C]

Hi Alex_M

Unfortunately there's no 'remove black matte' option in Affinity currently, although it has been previously requested here.

I can see that you have also previously made a post of a similar nature here, where my colleague provided an answer for Defringing, if you could provide an example then we may be able to help further!

[Alex_M]

Ok, please see the attached image. You will notice the black outline around the objects on the faucet to the left. It should look like the one to the right - no black outline. Do you have any ideas on how to get rid of the outline?

[Dan C]

I see, thanks for that.

Any chance you could provide the source image, so I can try playing with it to see if I can help remove the matte?  

[Alex_M]

Sure, please see attached.

EDIT: I Think the problem stems from the way the alpha is saved. The PNG format always saves alpha premultiplied so rendering against black background produces halos around edges of objects. In Photoshop the halo can be removed with Matting > Remove Black Matte and in After Effects you just tell it weather the alpha is premultiplied or straight. How do I do that with Photo? TGA format allows choosing to save the alpha as unpremultiplied (straight). Then I don't get the black halos in neither Photoshop nor Photo.

faucet.zip

[Dan C]

Thanks for that Alex

I've played around with the files and the closest I can get is changing the Gamma settings under the blend options for the image, see below.

As you can see, this makes a slight difference to the black haloing around the object.

Another option, if possible to your workflow, could be to use the EXR format as a straight alpha option is available in the export options of Affinity for this format. 

[lepr]

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[Alex_M]

Thanks for these examples! The Apply Image filter actually seems to work best. I had to change the Opacity parameter at the bottom to 50% to make it look good otherwise I get a white outline.

[lepr]

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