Super imposing two images

[PaulMellia]

Hi,

I have a solid black Map image of Scotland (below) on a white background and I want to super impose another image but only to appear in the black area of the image and not the white area, any help on how to do this would be appreciated? Thanks!

[Ofnuts]

One way:

• Open your other image
  
• File>Open as layers and open your Scotland image
  
• Move/Scale Scotland to match
  
• Colors>Invert to make it a white on black image
  
• Hide your other image layer
  
• Open the Channels list and drag one of the RGB channels to the general list to copy it (that should be your Scotland image). Make sure that all chanles remaine selected (dragging the channel xwill have deselected it)
  
• Remove/hide the Scotland layer
  
• Select the other layer and make it visible again
  
• Layer>Transparency>Add alpha channel (if greyed it is already done)
  
• Layer>Mask>Add layer mask and initialize to the copied channel
  

[rich2005]

There are various ways to get that result. Another way, still using a layer mask, but first you need to have a look at image sizes.

The map is quite small 490 x 512 pixels, You probably need to scale that up to suit the background image or maybe the other way, scale the background down to suit. Up to you. Assuming the map has to be scaled up

Open the background image
File -> Open as Layers the map, so it is on top of the background.
Use the scale tool to size the map, you can temporarily reduce opacity to see where you are.
Once scaled, tidy up with a Layer -> Layer to image size

The next bit works because the map is black and white.
With Map layer active Layer -> Mask -> Add layer mask and select Greyscale copy of layer.

and that is about it: Just get all that in a one minute example : https://i.imgur.com/960fElt.mp4

[Krikor]

• Having two layers, the one on the map below and the other on the top. Zoom in/out to taste (use Scale Tool - Shift + S)
  

• With the Select by Color Tool (Shift + C) click on a black part of the map.
  

• With the selection still active, select the layer with the other image and click the right mouse button:
  

• Add Alpha Channel then click again - Add Layer Mask - check the Selection option.
  

[PaulMellia]

Thanks all!

[tmanni]

Another way without using layer mask nor selection :

• create a layer group and move the map layer inside it
  
• apply Colors > Color to Alpha... on the map to send the white color to alpha channel (decrease the Opacity threshold to keep the dark area full opaque)
  
• File > Open as Layers to import your second image and move it inside the group, on top of the map layer
  
• double-click on the second layer to open the Layer Attributes dialog
  
• change the Composite mode from Auto to Clip to backdrop
  
• finally, create a new white layer for the background at the bottom of the layer stack