There are times the image registration doesn't do what you want, the perspective transformation doesn't allow you to see through the target image. I decided to write my own script to help. I know I should have bring the tripod, get a wind shield, but I am only an ordinary photographer with a fixed lens, weighted 269g, camera. How this work?
1. create a 4 points path for the target image

2. create a 4 points path for the active image which match the same reference points in the target image

3. run the script under filters/distorts/Align to path

This is an ad hoc version and not much error checks, let me know if you have problems.

Due to 3 files limit, the script and sample file posted here.
installation: place the .py file into your user/.gimp/plugins directory
test: run animation play

okyl168, I'm unsure of your goal; the Align to Path plug-in seems to shift a limited set of pixels rather than matching all available pixels (within a clip or crop area)?

No it doesn't, it only uses the first 4 points on each path to create an affine transformation matrix and apply to the whole image. It is similar to tools/image registration except only work on 4 points. There are occasion that you want to fix certain objects in the animation but applying image registration won't guarantee such fixing.

You can also do things like setting 2 paths, one for the object and one for the desired shape and apply the script. Example on Issa's monasteries.

okyl168,

Please excuse my butting in on this thread, but could you explain what you have done on Issabellas monastery picture (above).
I tried to achieve the same result by a different method and failed miserably!

david.

Thank you for the reply and script. I'm familiar with a pixel matrix from GNU Octave and affine transformation. So it makes sense now, I'll give it another go later.

1. create the target path (click on 4 corners of the rectangle), see the red lines in the image and name it ref
2. create the active path (click on 4 corners of the monasteries), see the blue lines in the image
3. run the align to path script, select the active path on top, select the ref path for next, then click ok
4. The changes will apply on the selected layer, you can run fit canvas to layer to show the new image.

@Okyl168
This is a very clever plug-in, Oberon ..I think it could be very useful. It certainly reduces the amount of guess-work compared with using Gimp's standard Perspective Tool ..at least for situations where you know the shape that you want to transform part of the image to.

Just some minor points of nomenclature:
- in Gimp the term "align" just means moving an item left/right and/or up/down to align with something else ..what your plug-in is doing is perspective transform and I think it should be named and described as such ..perhaps you could call it something like "python_fu_ob_perspective_transform" with blurb "Perspective transform from path to path";
- the terms "active", "target" and "reference" for the paths can be a little confusing ..the term "target" is often used to refer to the image/layer to be worked upon (rather than the result) and "active" could mean either ..it may be better to use simple terms like "From path" and "To path" or "Initial shape path" and "Final shape path".

@David + Odin
Just to clarify what Okyl said above:
- the "active" path describes an area in the input layer;
- the "reference" or "target" path describes the shape you want that area transformed to;
- the necessary transformation is applied to the whole layer.

So to transform Issabella's monastery pic:
- select Path Tool, then click on each corner of the portico in turn (say bottom-left, top-left, top-right, bottom-right);
- in Paths dialog, rename path as "active";
- in Paths dialog, right-click and select New Path ..name it "ref";
- click on each corner of the rectangle (or other shape) that you want to transform the portico to, in the same sequence as used for the "active" path;
- run Filters>Distorts>Align to path, select the "active" and "ref" paths respectively, then hit OK.

Just before running the plugin:


After running the plugin and then Fit-canvas-to-layers:

okyl168 & jontait2,

Many thanks for your help and advice. Paths have been something of a mystery until now! There is so much to learn, but many helpful people to smooth the progress.

david.

Sorry about the confusion, I do admitted that I didn't spend time on thinking the term in use. Initially, it was designed to align 2 or more images to the reference image. To implement the affine transformation in gimp which required to specify the control points in both images. I borrow path's mechanism to implement it, hence the name align to path evolved.

This plugin not only perform the perspective transformation or the reshape function, it also allow me to specify the position on where it goes in the target/to image. And is this feature that makes image alignment a lot easier. The gimp path mechanism allow me to undo and go back to fine tune the points in the path and re-apply the script.

Paths are the current topic of the 8 Feature Tutorials on the Tutorials Index.

okyl168 & jontait2,

Many thanks. I followed your instructions and it worked like a charm! I have previously used Hugin, which is no doubt mathematically correct but can be long-winded.

Now you have set me some more homework - learning about paths!

Thanks again,
david.

okyl168: looks like a useful plug-in, thanks for sharing.

Got an example I can show what it does, here I got 2 images taken at few seconds apart without tripod at a busy cross junction.

after applied align to path where the fixing is the building
The result in difference mode, as you can see only the building is aligned.

Then apply layer mask and paint the area you want from another image, you may need to color curve align between 2 images if they were taken at different exposure.
The merged result