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Selection contours - Printable Version

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Selection contours - gm40 - 07-08-2019

I would like to manipulate a picture of a group of people in which one person is missing. I also have a picture of that missing person, scaled down to match the size .My problem now is to introduce the individual's head into the picture of the group such that the manipulation cannot be seen, i.e. I need to copy the contours of the individuals head into the group picture, cut it out and then copy the head as a new layer  into the picture of the group. Is that possible?
I hope I've described my problem clearly enough. Of course, I appreciate any suggestions to achieve my goal on another way. Thanks so much!

RE: Selection contours - Ofnuts - 07-08-2019

Yes but not in that order...

  1. You need to have a clean cutout of the person (in other words, that person on a transparent background). This is typically hard to do if the photo wasn't taken with this in mind (white background or "green screen"). Some tools may help (Foreground selector, for instance), but this is rarely perfect (loose hairs...).
  2. Open the group's picture
  3. Duplicate the groups layer
  4. Add the person's picture as a third layer (File>Open as layers or drag it from the image opened in Gimp)
  5. Scale it to match. Some color adjustment could be necessary. This is also when you realize that the lighting directions don't match and that you have to find another picture(*) Smile
  6. Move the person's layer between the two group layers,
  7. With the eraser (or better, a Layer mask), cut a hole(**) in the first layer through which the person can be seen. This is the other hard part, the people in front of the person have to be cutout rather cleanly and the automated tools cannot be used.
(*) The other usual catch is that many of these projects are made starting from a head shot. But in a group picture, you can often see more of the persons in the back rows

(**) Possibly several, around several body parts, see previous note

RE: Selection contours - gm40 - 07-08-2019

Thanks so much for your quick response! Up to item 6 all is working well. However, at 7 I'm running into trouble. I added a layer mask and selected a hole in the top layer with the lassoo. When I tried to 'cut' this hole nothing happened. I would have expected to see part of the new head through it as it is the next layer down. Where is my thinking ar action wrong? Also I wonder whether it wouldn't be possible to take the individual's head and cut around , thus creating a pattern that one could introduce into the group's picture for cutting there.

RE: Selection contours - Ofnuts - 07-08-2019

If you have two identical layers one above another, when you cut a hold in the top one you see the very same thing from the bottom one, so the hold isn't visible. Of course if you are cutting where you inserted the persons layer you should see it. Two possible explanations:

  1. Since 2.10 you have to "commit" the selection before using it ([Enter] or double-click), so maybe there was no selection when you hit [delete].
  2. Maybe you cut the wrong layer (was the top layer the selected one in the layers list?)
The holes you have to cut in the top/front layer depend on the content of that top/front layer, not on the content of your inserted layer. The inserted layers on the other hand automatically masks off the layer back/bottom layer where needed.

RE: Selection contours - gm40 - 07-08-2019

I'm using version 2.8. The hole which I cut is white, i.e. 100% opacity. So how do I get the opacity just for that hole down to 0%?
I found the source of the problem but still don't understand it: The picture of the group is a jpeg formatted. When I cut a hole in it and clear it I get a white spot. However, I copied the whole picture into the clipboard and exported it as a png formatted, read it in again. And now cutting and clearing the hole is giving me the correct result, namely an open hole that is making the layer underneath visible. What happened by transforming the picture?

RE: Selection contours - Blighty - 07-08-2019

(07-08-2019, 12:31 PM)gm40 Wrote: I found the source of the problem but still don't understand it: The picture of the group is a jpeg formatted. When I cut a hole in it and clear it I get a white spot.

To cut a hole the layer must have an alpha channel (ie supports transparency). jpeg format does not support transparency, png does. If the layer name is shown in bold it does not have an alpha channel. Right click on the layer and select Add Alpha Channel.

If the layer does not have an alpha channel: When you cut a hole the hole is filled with background colour, in your case white.

RE: Selection contours - gm40 - 07-08-2019

Thanks a lot for all your help and the information!

RE: Selection contours - Ofnuts - 07-08-2019

(07-08-2019, 12:31 PM)gm40 Wrote: The hole which I cut is white, i.e. 100% opacity. S

Not exactly the "When I tried to 'cut' this hole nothing happened" Smile