On Sun, Feb 24, 2019 at 8:30 AM Julien Hardelin <jm.h...@wanadoo.fr> wrote:
> Hi devs,
> I can't understand the Inverse Transform option in Little Planet and
> Panorama Projection filters, and can't find any explanation about that
> on the Internet.
> UI pop help tells "useful for touching up zenith, nadir of other parts
> of panorama": how to manage that, I hardly recognize anything in the
> inverse transformed image.

Both of these operations only work on equirectangular panoramas that
are 360x180 degrees. If you have an incomplete panorama, or want to
edit out a tripod in nadir - straight down it is difficult directly on
the panorama since the bottom is very distorted and distributed all
along the bottom of the panorama image. To create a patch for the
panorama image with retouched/edited content with the
gegl:panorama-projection operation in GIMP you can do the following

1. duplicate layer
2. panorama-projection, adjust it to point straight down with desired
zoom (or in some other direction)
3. perform edits to the transformed view of the projection
4. panorama-projection, but this time recall the previous parameter
settings from the history list in the operation dialog, and tick the
"inverse transform" option
5. optional, merge/down, flatten image

This inverse transform in the panorama projection op the transform
that happens from lens-normalized input images to merged panorama when
creating a panorama with for example hugin. For little-planet it might
be a little less useful; but having an inverse transform here serves a
similar purpose.

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