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Here is the second approach to correction of the tilted horizon problem that was first described this post.
A time intensive technique is, after straightening the image, to build out the blank sections. Here is an image after straightening. Corrected image size increases with degree of tilt, measured by the degrees of correction with the area of size increase being blank. It is this blank space that is filled by this technique.
The technique is to open a copy of the image, leaving the original unchanged, then, with Photoshop Lasso tool, select a portion to copy to clipboard, then to the blank space. A great deal of time is consumed by the trail-and-error approach required.
The sky was the simplest, the cliffs most difficult. The grass and flowers were not a difficult as they first appear. I varied feathering with sky the highest (30 pixels), grass and flowers at 10 pixels. Another general maxim is to run the lasso line through pixels the most similar to each other. After pasting use the Move tool to place the new layer.
A common effect to avoid is duplication of image elements. For example, after copying there are duplicates of flowers. This can be corrected through use of the Erase tool to reveal original image elements under the copied layer.
The cliffs on the upper left were impossible to correct. I solved this corner by running the Lasso line through the sky, water and grass. As a result the space between Rathlin and the “mainland” is wider.