It seems to me that there is no reasonable efficient way to isolate the lead bars other than tracing them.
As a first step you could remove the black bars from the new image. To do this, select them and use Content Aware Fill to fill the gaps. This may need some hand-retouching after.
Then drag this image into the church image.
First step, flip the image horizontally to reverse it.
Then use Free Transform in Perspective and Distort mode to get the image into position over the window.
I would suggest using a combination of blending modes and layer masking to combine the new image with the one below. You could also create a displacement map from a copy of the church window. Adjust the levels of the displacement map before applying it.
Alternatively, you could use the Vanishing Point tool to clone or paste the window into position.
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I am not sure that we have enough information about what you need to achieve. I mean if you just want to create a version of the window as seen from outside, then it would look something like this
If you really just want the leading without any of the coloured glass, then I can't see and easy way of achieving that other tracing as Christoph suggested. You could maybe ease the tracing process a wee bit by looking at the best channel for a given area, and it is helpful that the image you are using is of good quality and high resolution.
Google tells me that the window is from Grouville church on the isle of Jersy. Do you live there? I'm wondering if you could search for a better image taken from the outside, or even find a local to take the photograph for you?
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Most of what you see on the outside is reflected light from the sky, contrasted with no reflection from the surroundings.
The only common element is the outline of the lead bars.
You'll have to rebuild this, and then add reflection to each pane. They will probably be at slight angles that reflect different parts of the sky and/or the surroundings. In addition the glass will have a very irregular surface, further complicating the reflections.
In short, this is a very complicated task. It can be made to look convincing, but it will be extremely time-consuming. This is strictly manual labor.
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I don't think there is any alternative than to trace the leading.
Once done you can make a material for the lead and one for the glass. I assume you are using this texture for 3D - if so then you can set the reflections to balance the reflection of the sky but just see a hint of colour in the glass.
I used these steps to quickly make a corner of the window in Photoshop 3D below. I did not have time to trace the whole window but hopefully you get the idea. The stonework is a 3D extrusion from a paving slab.
Thank you to everybody who replied. It appears that this is a bit of a head scratcher but I have a few ideas now on how to tackle this challenge.
I'll apply your suggestions and see what happens.