Taking the Impossible Photograph

Written by Paul Bourke
October 2021
See also: Hosier Ln, Melbourne

Spanish translation by Chema Bescos.

The following presents an application of 3D reconstruction (photogrammetry, if you like) to take an otherwise impossible photograph, a photograph not otherwise possible. The first example is the alleyway below the left side of which has been painted. The alley is only about 2m wide and the painting runs for about 40m. Obviously one cannot stand back far enough to take a single "front-on" photograph of the mural.

Mural by Laeline Design Studio.

How to photograph the whole mural?

Option 1: Use a fisheye from a central location: Problem, the resolution towards the ends would be VERY low.

Option 2: Take a multiple shot panorama from a central location. Problem, the resolution still drops off towards the ends (although not as bad as 1). The view towards the ends would be highly oblique, one would prefer to be looking straight on to the painting.

Option 3: Take a series of photographs moving down the alley. Problem, there is enough 3D structure in this case that parallax issues would confound any feature point based stitching.

Solution: Capture a full 3D model using photogrammetric methods and then take a virtual photograph without the physical limitation of the opposite alley wall.

The other side of the alleyway. Both these "photographs" are orthographic projections (similar to a very long lens) from the reconstructed 3D model.

Perspective, askew "photographs" of the model are shown below, of course one can now position a virtual camera in any position with respect to the wall.

Navigable 3D model

In the image above and below the opposite wall has been removed so as not to obstruct the view of the wall of interest.

Navigable 3D model

The full 3D model is shown below.

Navigable 3D model

Example 2: Rock art cave

In this example the 3D model is used to create floor and ceiling plans of a relatively complex 3D cave system.

Plan of the floor.

Plan of the ceiling and rock art.

In both cases above one cannot photograph the entire floor or ceiling due not only to limited distance between the two, but also due to occluding rock columns.