Transform perspective mode BR offset Y

[donkeyhead]

I don't quite understand what the BR offset Y does when perspective mode is used on transform.

I have a square and I want to apply a perspective transform to it.

square_not_transformed.png (22.73 KiB) Viewed 84767 times

Using BR offset X gives me the expected result: increasing the X gives the effect that the front edge of the square moves closer.

square_perspective_mode_br_offset_x_increased.png (35.62 KiB) Viewed 84767 times

However, using BR offset Y similarly applies some kind of squish or an rotation to the square?
(Kind of like the two opposite corners are moved up?)

square_perspective_mode_br_offset_y_increased.png (41.68 KiB) Viewed 84767 times

What does the BR offset Y do with the perspective mode applied?

[u2bleank]

hi donkeyhead,

the "BR" parameter mean "Bottom Right offset". This parameter add an offset to the Bottom Right corner of the transform Box. which is useful when you want to do a "free" quad transformation.

[donkeyhead]

That I understand, but why does it behave differently from BR offset X when perspective mode is on?

When using BR offset X with perspective mode on horizontal lines stay as horizontal lines.
Using BR offset Y with perspective mode I would expect similar behaviour with vertical lines staying vertical lines.
However, the vertical lines in the middle of the square are angled.

[u2bleank]

The algorithm try to "guess" the right perspective to match given points.
If you change some orientation or "order" it can generate slightly different results ( which points are closer and wich one are farther is a unknow for example )