Mesh alignment for stereoscopic projection

Written by Paul Bourke
November 2011


Stereoscopic projection requiring two projectors, such as polaroid and infitec, require that the two images align perfectly. Any misalignment will distort the depth field and increase the chance of eyestrain. Projectors with high quality (matched) optics can generally be closely aligned if they have vertical lens shift, but this can be a time consuming task and perfect alignment is not assured. Some projectors have per corner keystone but except for a few models this is normally the reserve of the higher end products. Using commodity projectors and careful mechanical alignment might result in an alignment as shown below, often much worse.

This careful mechanical/optical alignment may be worth the effort for fixed installations but is not practical for portable or quick-setup scenarios. The approach used/proposed here is to do the alignment in software. More precisely, one plays all content back through a mesh made up of vertices and texture coordinates. The exact vertex positions of this regular mesh are the result of a software based alignment phase. During alignment one interactively moves the vertices of the corners, and optionally additionally the midpoints of the edges, until the left and right projector mesh align perfectly. This mesh is then exported and all subsequent software plays it's content through that mesh. The following is an example of the same image above after this alignment process.

Notes

  • Since there is little support for content playback through a mesh, the software may need to be developed for the application in mind. In the case here only movie playback is required, this can be achieved with the authors "warpplayer", or using the "pbmesh" that provides similar support for Quartz Composer based applications.

  • The software alignment process can be very fast, in the order of a minute or so.

  • Software: pbmesh, warpplayer. Game engines such as Unity3D can readily play content through such meshes.

  • There is some loss of image quality but it should be noted that this is also the case for projector based keystone alignment. The goal is certainly to get the mechnical/optical alignment as good as possible, for example, to within at least 5%. In general there will be other factors that are limiting the quality at the 5% level.

This is certainly not a new idea, using a mesh to warp geometry is fairly common in displays that require geometry correction, such as cylindrical or spherical displays. A similar process is also often used for edgeblended tiled displays, for example it was employed here at the Pausiris gallery at MONA (Museum of Old and New Art).