I don't see a way to specify the matrix used to perform a desired perspective transformation manually. Of course, I can adjust it by dragging handles, but it would be much easier for me to calculate what I need and then enter the elements of the matrix by hand.

Needless to say, a typical gimp user would not be able to do this, but I am a mathematician who has taught geometry many times during my professional life, so I know what to do. I don't need an explanation of how the matrix is formed and what it does since I can work that out by experimentation.

The gimp manual is not helpful. It refers the reader to the section on convolution filters, but that appears to have to do with modifying pixels in place rather than moving the pixels---a rather misleading reference. Perhaps something entirely different happens if you use the full matrix, but the documentation is silent on that matter.

Of course, I can try to produce what I want by moving the handles, but for slight adjustments of large images, that is difficult if not impossible. It helps if I increase the magnification enormously, but it is still awkward. It would be a lot simpler to be able to enter the matrix elements directly.

On Mon, 2008-03-10 at 18:22 -0500, Leonard Evens wrote:

I don't see a way to specify the matrix used to perform a desired perspective transformation manually. Of course, I can adjust it by dragging handles, but it would be much easier for me to calculate what I need and then enter the elements of the matrix by hand.

I've gotten one response so far, and while interesting, it didn't work out.

Is there some way to do what I want using a script?

Is there software except gimp which runs under Linux which I could use instead?

If someone could direct me to the section of the gimp code which performs perspective transformations, I might be able to extract it and write my own program to do what I want, but from past experience I don't place much hope in that. Gimp code forms a complex network and it is usually impossible to isolate one particular part and make much sense of it. Of course it would be nice if the gimp programmers added the capability I want to a future version of gimp, but I won't hold my breath waiting for that.

Needless to say, a typical gimp user would not be able to do this, but I am a mathematician who has taught geometry many times during my professional life, so I know what to do. I don't need an explanation of how the matrix is formed and what it does since I can work that out by experimentation.

The gimp manual is not helpful. It refers the reader to the section on convolution filters, but that appears to have to do with modifying pixels in place rather than moving the pixels---a rather misleading reference. Perhaps something entirely different happens if you use the full matrix, but the documentation is silent on that matter.

Of course, I can try to produce what I want by moving the handles, but for slight adjustments of large images, that is difficult if not impossible. It helps if I increase the magnification enormously, but it is still awkward. It would be a lot simpler to be able to enter the matrix elements directly.

On Tue, 2008-03-11 at 21:56 +0100, Sven Neumann wrote:

Hi,

On Mon, 2008-03-10 at 18:22 -0500, Leonard Evens wrote:

I don't see a way to specify the matrix used to perform a desired perspective transformation manually. Of course, I can adjust it by dragging handles, but it would be much easier for me to calculate what I need and then enter the elements of the matrix by hand.

There's a PDB procedure that allows you to specify the corners of the perspective transformation, but it doesn't allow you to give the matrix directly. It wouldn't be difficult to add this functionality to the Procedural Database. We would certainly consider a patch to do this.

I'm not familiar with gimp code, so I don't understand what this means, but I think I can probably figure it out. I will give it a try. In any case, it would be even better for my purposes if I could specify where the corners of the handles go and let gimp calculate the matrix for me and apply it.

By the way, I don't really understand what the purpose of showing the matrix is if you can't change it directly. Its meaning is far from transparent to a naive user, and even someone who is sophisticated about the math, like me, would have to do some calculations to figure out what the transformation will do given the matrix. It would be more useful to know where the corners of the handles will go under the transformation. But in the present situation, it is all I have, so I don't propose removing it.

If you want to give it a try, please have a look at the code in tools/pdbgen/pdb/drawable_transform.pdb. From this source the wrapper code for the procedure is generated into app/pdb/drawable_transform_cmds.c. As you can see in drawable_transform.pdb, all you need to do is to initialize the transformation matrix with values taken from the procedure call.

Sven