On Thu, Nov 26, 2020 at 04:41 AM, Etienne Monneret (Perso) wrote:

You are speaking about horizontal disparity. But, on some stereos, you
may have a well-aligned whole image having some local deformations.

If they have local deformations they are not very well aligned.

It depends on the quality of the source.

If this is a cha-cha, you may have important local deformations.

With stereo dedicated hardware, you may have time shift, lens differences, bad aligned mirrors, focal or zoom differences.

There are many reasons why an original may not be perfect!

When zooming on these details, you may need to completely re-align the
zoomed sub-part. This is here where the "Automatic alignment" button takes a
great part of its interest!

OK, but it only makes sense if the original SBS are poorly aligned

Or badly shots.

For example, in the Spider-Man case, you may have a well-align whole
video, but it's very hard to get it properly with a well-aligned
Spider-Man (in fact, the background seems to be a simple flat
wall-paper!).

Spider-Man is a composition of two stereo pairs: the background and the
Spider-Man proper. If each pair were perfectly aligned in the origin, then
the composition of the man overlaying the background would be perfectly
aligned as well.

The background is a flat wallpaper, and yes, Spider-Man itself is not well aligned.

The whole image can be "at-best" aligned, but it's impossible to get it "perfectly-aligned" without doing a complex morphing.

The composition would surely require adjusting the depth of the Spider-Man
respect the background, because it would be bad if it appeared behind the
background, and it would also be bad if it appeared too much in front.

The original is "at it is", and you can't change the relative stereo position of the foreground with the background without doing a morphing. Whatever you do with simple transformations (translation, rotation, scale, width, height, etc), Spider-Man will always stay in front of the background.

This
is where I believe your "X" control in "Alignement" makes sense. They also
make sense the "X", "Y" and "D" of "Placement" because they do not spoil the
alignment, they only control the position and the zoom of the complete overlay.

But frankly, I would not use the "Y", "L", "H" and "R" of "Alignement"
because they spoil the vertical alignment, and it is very difficult to assess
the error visually.

It's completely dependant on the source.

Of course, if you have a perfect stereo original, you won't have to use them.

In fact, there is one case where you would need it, even with a perfect hardware: with an hyper-stereo near first plan. When shooting from too near positions for the first plan regarding the space between lens (too large for that distance), parts of the image will be nearer on one side than the other side, and when say nearer say larger. You will then be obliged to balance the alignment to keep with these larger/smaller parts, as we did with the Spider-Man example.

;-)