Masking using blend mode

[David Edge]

I'd like to mask / select a foreground object by blending a photo of the scene without the object and one with the object present. It seemed intuitively clear that some sort of blend mode like difference would do this, but difference doesn't give priority to the foreground object so you get a cocktail of the foreground object and what's behind it.

Any bright ideas chaps?

 

d.

[GarryP]

Would it be possible for you to attach the images involved?
Finding the best method for doing something is usually dependent on the specifics and the original images would be very useful.
If you don’t want to share the images themselves, is there an example of what you want the final result to look like?
Also, can you be a little bit more specific about what you mean by “blend” as that’s a fairly loose description which can be easily misinterpreted.
The more clear and specific information you give the better we can help.

[David Edge]

Apols if it wasn't clear. See the attached example. I want to cut out the walking couple by subtracting the buildings by some method. There will be hundreds to do so I'm looking for a straightforward method although some manual retouching will be needed.

[carl123]

2 hours ago, David Edge said:

I'd like to mask / select a foreground object by blending a photo of the scene without the object and one with the object present.

Still unclear exactly what you want to do, but you may be looking for

File > New Stack

You generally need more than 2 images though.

[David Edge]

Well the version with nobody in it was produced by median stacking lots of images. So I now wish to subtract the background and just have the couple. Lots of alpha, two little people.

(FWIW the building background will be replaced by a cyanotype and the people superimposed on clear acetate.)

[firstdefence]

To select the people spend 30 mins drawing around the people with the pen tool to get a nice selection,  make sure you fill the curve you create and use the selection from layer option from the Select menu, you can refine the selection after that, maybe a small amount of feather but if you make a neat job that shouldn't be necessary.

Did this in 6 mins flat, (Yes I timed myself lol!) this is a rough rip and I'd normally spend more time zoomed in to get some sweet, sweet lines but it shows what can be done using the pen tool, the beauty is you can go back to the curve and keep tweaking it until you are happy.

 

Closeup and personal.

[firstdefence]

I'm feeling merciful tonight 😉

Street shot cropped people.afphoto Saved with history.

I also added a very small amount of gaussian blur to the edge of the people by cmd clicking on the people layer to get a selection, shrinking the selection by .5px and then adding .5px gaussian blur. using the live filter.

[David Edge]

The Gaussian blur's a good tip thanks. Vector outlines may not be the answer in the context of the project which involves a kilometre of high street and multiple remixes of each building tho'. 

Is anybody aware of a computational approach? Even using non-Serif software, shudder.

 

d.

[David Edge]

OK, I found this in a forum related to "the other lot"

The term you are looking for is a "difference mask".

Basically you align the two layers, set one to difference, and use the result to create a mask (you will have to tweak contrast, add some blur, etc.).

However, it is a very bad method for masking. You will get quite crap/inaccurate results from doing it I'm afraid (image noise, shadows/etc. from the object, some pixels on the object being too similar to pixels in the original background, etc). Could be sufficient for just a garbage matte or just locating differences.

This type of masking you typically only see in webcam applications where it tries to remove the background in realtime (quality not important, and works with any background as long as it's different enough from the object).

I was thinking "difference" and missing the "mask" bit. So that would do what I was imagining, but in the view of the poster, not very well. And of course it will work in Affinity too.

[firstdefence]

Computationally it would have to be the reverse wouldn't it, i.e. the building moving but the people static.
 

The vector outline is only to get a snug fit selection that selections tools might find hard to get, having said that it would be tedious considering the amount of images that need to be edited because I'm assuming the people are in a different position in each "frame" in which case video editing software like AfterEffects or BlackMagics Fusion might be a better option for motion matting if the image sequence is more like a flip book. 

[carl123]

Try www.remove.bg

It's super fast and does a reasonable job

You can use it for free (limitations) or sign up for a free trial to test one of your full sized images

 

 

[David Edge]

Yes, I agree that vector plus blur is quicker than messing about with selections when there's little contrast between object and background. You can come and see the final result from 6 March at The Small Print Company in Derby.

Re Carl's suggestion, I'll give it a go although I note in the sample image the young lady is standing in front of a brick wall, my backgrounds are a bit more challenging.

Thanks both

 

d.

[David Edge]

Quote

 

Ah sorry Carl - I hadn't seen that you had attached a sample, that might very well be the answer for bulk editing and the book version, with the vector crop for the awkward ones or work on the wall. Bingo!

d.

[firstdefence]

4 hours ago, David Edge said:

Yes, I agree that vector plus blur is quicker than messing about with selections when there's little contrast between object and background. You can come and see the final result from 6 March at The Small Print Company in Derby.

Re Carl's suggestion, I'll give it a go although I note in the sample image the young lady is standing in front of a brick wall, my backgrounds are a bit more challenging.

Thanks both

 

d.

Awesome that's just down the road from me, cool.