Manual horizon correction is used when the result of the auto step is unsatisfactory and as a preliminary to a manual correction perspective.

The difficulty is to find the right element truly horizontal on which to rely
In our contemporary environment, vertical lines are most common.
Finding a vertical passing through the middle of the photo is relatively easy, especially in urban photography or monuments.


Verticals are more common

Successfully correcting the horizon

It is important to determine which reference elements can be used in the image.
Using a horizontal element is not always possible, since the perspective effect quickly loses the horizontality of the lines. It is often better to rely on the vertical elements of the image.

Using a vertical element

Using a vertical element

Rigorous photographers look for the median vertical element as close as possible to the vertical axis of the image. If there is not exactly one in the middle, it is often possible to interpolate a virtual line between two elements both sides. 



ViewPoint: in palette “Horizon”       
– “Vertical level” button – The tool is positioned in the middle of the image
– Adjust each end. Press SHIFT simultaneously to slow down

PhotoLab: in palette “Geometry / Horizon” OR the upper toolbar       
– “Horizon” button – The tool, a horizontal line, is centered on the image.       
Ignore the displayed level line and click on the first reference point 
Hold down the mouse button and set the other end of the new level line that appears. ViewPoint understands that this is a vertical level and considers it as such!




Complex Image

The reflection on a horizontal plane

An object and its reflection in a waterbody are strictly located on the same vertical. The horizon setting needs to be perfect.
The principle is identical to the previous examples. Point an element towards the middle of the image, not necessarily a line, and connect it to its reflection.       

In almost all cases, this operation must be followed by astraightening of perspective so that all objects and their reflections are vertical.

The reflection on a horizontal plane

Using a horizontal element

  In palette “Horizon”
– “Horizontal level” button – same procedure as previously



 Failed Auto Skyline

Determine the horizon line


Defining the horizontal of the photo consists in aligning with a support line which, in reality, is exactly on the skyline. This support line can occupy any position on the height of the photo but that has consequences on the vertical lines.

This position depends on the down / up orientation of the line of sight:
* Case 1- If it is pointing down (downward shot), the line of sight rises to the top of the picture and the vertical lines point naturally downwards
* Case 2- If the line of sight is horizontal (strictly vertical case), the horizon line passes exactly in the middle of the photo 
* Case 3- If it points upward, the horizon goes down. The low angle shot is the most frequent case for buildings / monuments. Note the verticals that point upward.
   
The difficulty is to find the correct really horizontal element which is often more complicated to discover than a vertical one.

Definition of the skyline

Identify the skyline



The rules of the skyline

Most of the time, the horizon line is not directly visible. We must first identify its position in the image    

* The simplest case is a building seen strictly from the front. In this situation, all facade horizontal lines are parallel to one another and to the horizon line.
Any of these lines can be used to correct the horizon.



Front building

  When the skyline is not visible, locate it.


Locating the skyline 1

* Locate the skyline using the parallel planes of the steps of a staircase.


Locating the skyline 2

Locate the skyline on topics of varying shape.


Locating the skyline 3


However, it must be ensured that the contiguous elements are on the same horizontal level, which is not always the case on historic buildings.


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