by Mike McNamee Published 01/10/2015
The Horizon Line
In perspective the Horizon Line is the imaginary line that is in the far distance but level with the eyes of the observer. In this instance the observer can be a human or a camera. For a standing adult it is at eye level but a camera could be higher or lower on a tripod, which would raise or lower the horizon line accordingly. Everything about perspective is referenced to the horizon line - thus if a railway track is going into the distance it will appear to converge to a point at the horizon line. Do not confuse the horizon line with the everyday horizon line where the sea meets the sky - that only applies when the camera is level and the boundary between sea and sky is right through the middle of the frame. If the camera is pointing down, the horizon line is high in the frame; if the camera is pointing up, the horizon line is low in the frame. For many images this is regardless of whether a horizon is even visible - in that instance you have to look for other clues to guide you to where the horizon is placed (see later for good video references of the effect).
In the image here the landing stage decking can be seen to converge just above the horizon line - the camera was 19" above ground level. Note when we warp the boards upwards so that they converge above the horizon line the decking appears to be thrown upwards. This upward shift survives through to the final composition even after many layers of waves have been added.
Other than things that are square-on to the camera, everything converges to a point somewhere. This is very easy to figure out with boxes and cubes, and progressively more difficult as you move through towards complex shapes such as faces and bodies. Most man-made objects can be divided down into rectangular shapes or within rectangular shapes and this can be used as guides. Even so, an understanding of what is happening helps when guessing how much perspective distortion to put on a placed element within a composite image. As with so many things Photoshop, it is best to learn your craft on simple stuff such as moving windows, doors and re-cladding buildings before tackling people and animals.
Photoshop to the Rescue
Although the issue of setting vanishing points and perspective planes is complicated, it is because it is amenable to computer calculation that Photoshop is able to provide considerable assistance. The Vanishing Point Filter is the go-to place for this help. From there you can allow Photoshop to compute the new shape of say a window moved from one wall to another around a corner but if you need full control you can make a new layer, set up the grid using the Vanishing Point Filter and then click the 'Render Grids to Photoshop' in the 'Settings and Commands' drop-down list. This draws the grid on its own layer and this may be used when distorting a shape to fit the correct perspective of its new surroundings. Introduced in CS2, this ability to draw perspective girds had long been asked for and remains a powerful aid to the serious montager! We can barely scratch the surface of what this filter can do but there is a wealth of material available on the internet.
When montaging the subjects have to share the same horizon line or they will look false. This may be difficult to determine from the original - there might not be any visual clues. Additionally you have to be able to decide if the camera was pointing upwards or downwards as this shifts the horizon line above or below any real horizons in the image - pointing the camera down moves the horizon line up in the frame and vice versa.
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