"Erase White Paper" to a duplicate layer causes overall darkening

[Ken Hjulstrom]

Hi,

I was using Affinity Photo, experimenting with filters to try to improve the appearance of a rather overexposed image, and quite by accident, I discovered that by duplicating the original photo layer, and applying the "Erase White Paper" filter to the duplicate layer, the result is a generally darker appearance, which is exactly what I was looking for.  Since the documentation for "Erase White Paper" states that all it does is "apply transparency to white areas of an image", I wasn't expecting any visible change to happen at all, since I would expect that the transparency that was just created in the top layer would just allow the white in the underlying area to show through.  Both layers are set to the "Normal" blend mode, and both have an Opacity value of 100%.  Could someone fill me in on what's actually happening here?

 

In the attached file, there are two layers, "Copy of Background" (which had the "Erase White Paper" filter applied), and "Background", which is the unaltered original image.  If you toggle the visibility of the "Copy of Background" layer, you can see that the original image is significantly brighter than when the visibility of both layers has been turned on.

Thanks,

Ken

 

 

EraseWhitePaperTest.afphoto

[PaulEC]

Quite simply "Erase White Paper" removes all the white from an image, not just areas of solid white. (It's really intended to remove the white background from a purely back and white image.) So solid white becomes completely transparent, black stays completely opaque, and all other colours/tones in-between will become semi-transparent in varying degrees.

In your example you are seeing the darker areas on top of (effectively added to) the lower layer. If you hide the lower layer and replace it with a flat coloured layer you can see the varying transparency more easily. A more controllable way to darken an image, in a similar way, is to duplicate the layer, give it a multiply blend mode, then adjust the opacity of the top layer.

[thomaso]

2 hours ago, PaulEC said:

If you hide the lower layer and replace it with a flat coloured layer you can see the varying transparency more easily. A more controllable way to darken an image,

If I hide the lower layer and replace it with a white layer I see the initial look (= the look of the hidden layer). – What makes transparency placed on top of non-transparency darken the latter? Why does the same (semi-)transparency not darken a white layer in the same way it darkens the original image?

[PaulEC]

37 minutes ago, thomaso said:

If I hide the lower layer and replace it with a white layer I see the initial look (= the look of the hidden layer). – What makes transparency placed on top of non-transparency darken the latter? Why does the same (semi-)transparency not darken a white layer in the same way it darkens the original image?

If you add a white layer all you are doing is replacing the white that was removed. In the OP's example the upper layer (which has had the white element removed) is now mostly semi-transparent, hence you are "looking through" a semi-transparent layer at the original layer, which means that the darker areas now appear darker because you are looking at the original layer plus the semi-transparent layer above. (Areas that were already quite dark are hardly affected.)

"Why does the same (semi-)transparency not darken a white layer in the same way it darkens the original image?" – Because it is affecting a white layer (so it looks the same). If it is on top of the original image the semi-transparent areas are being added to the ones below, effectively darkening already darker areas, more than the lighter areas.

If you look at at the uncropped image, with a red, rather than white, background, the varying transparency is more obvious.

RedBG.afphoto

[thomaso]

16 minutes ago, PaulEC said:

which means that the darker areas now appear darker because you are looking at the original layer plus the semi-transparent layer above.

Do you mean the brighter areas get darkened? In the OP's file the initially dark areas are not affected but the bright areas in the lower layer get dimmed if the layer above shows its transparency at these spots.

Ah, now I understand: Only the semi-transparent areas darken the image below – while pure white areas aren't affected because the upper image is 100% transparent at these spots. Or: where the original image does not contain pure white areas the "Erase White Paper" causes semi-transparency, which then darkens the image below.

 

 

[PaulEC]

2 hours ago, thomaso said:

the darker areas now appear darker

Sorry, not very clear, I meant that the dark areas will become darker, (I suppose really I should have said the midtones will become darker!)

[PaulEC]

2 hours ago, thomaso said:

Ah, now I understand: Only the semi-transparent areas darken the image below – while pure white areas aren't affected because the upper image is 100% transparent at these spots. Or: where the original image does not contain pure white areas the "Erase White Paper" causes semi-transparency, which then darkens the image below.

👍

[Lisbon]

12 hours ago, Ken Hjulstrom said:

Since the documentation for "Erase White Paper" states that all it does is "apply transparency to white areas of an image"

If "Erase White Paper" only affected transparency there shouldnt be any change in the RGB values.
This is clearly not what happens.

GIMP has a function called "Color to Alpha" that lets you make a given color transparent.
What this function does is answer a simple question. What color, with a certain transparency, do I have to mix with white to get the original color?
To answer this question, you get a color with different transparency and RGB values compared to the original. Therefore, slight changes in color are to be expected.

I suspect that "Erase White Paper" works very similarly to GIMPs "Color to alpha".
("Color to alpha" works with any color. Not just white)