Framing, Mounting, Postprocessing
All three terms involve more or less the same: the cutting and mutual alignment of the two stereo fields. Only the correct mounting of two single stereo fields yields a wholesome and valuable stereo image.
While in the anlogue aera we talked about framing or slide framing, we prefere today in the digital age the terms mounting, stereo mounting or stereo alignment.
In commercial cinema area there has always been the "Postpro" (post-processing), which includes all subsequent image editing processes. For 3D film, this Postpro must now be extended at least to the stereoscopic requirements and the mutual alignment of the images.
The stereo image alignment is made up of two components,
1.) the geometric image correction, and
2.) the stereoscopic image cropping at the left and right edges of the two individual images.
1. The Geometric Image Correction
The geometric image correction has the goal to adjust all corresponding pixels in the left and right field to the same height. If this requirement is not fulfilled, stereoscopic experts speak of height errors. Causes of height errors can be: a simple vertical offset of the images, different sizes or rotations of the images, but also different trigger times of the cameras (synchronous errors).
Figure 1: In a correctly mounted stereo image there must be only horizontal shifts of corresponding pixels. Here, the left image is clearly mounted too far upwards.
With the geometric image correction it is tried to equalize all height errors using all possible image manipulations as there are shifts, scalings, rotations, but also shears and keystone treatments.
2. The Stereoscopic Cropping
In a stereo projection the spatial image is perceived within an opening in an solid wall. This opening is called stereo window. By stereoscopic cropping of the left and right image fields, the position of the image space in relation to the stereo window is determined.
Whether an object can be seen behind the stereo window, in the stereo window or in front of the stereo window, is dependent entirely on the stereoscopic cropping and not from the shooting itself!
Figure 2 a,b,c: The same stereo image with three different stereo window mountings: behind the stereo window, in the stereo window and in front of the stereo window.
In deciding, where the motive is to be placed in relation to the stereo window, the term stereo window mounting has been established. Although naturally a creative act, this task can be taken over today by automatic mounting software.
3. Mounting Software
With appropriate software, both the geometric image correction as well as a proper stereo window setting can be carried out fully automatically. This is possible both for individual stereo images, complete directories of stereo images as well as for stereo video.
At this point, two programs shall be mentioned for these tasks:
The Stereophoto Maker is freeware and therefore especially suitable for entry into this wonderful world of stereoscopy. COSIMA represents COrrect Stereo IMAges and is a commercial software product, written by a member of the DGS. For the innovation of fully automatic stereo image correction, COSIMA was awarded by the DGS 2006 with the Deutsche Raumbildpreis.
Information: It is planned to work out the mathematical theory for stereo image matching in the SIG Stereobild- und Videokorrektur. This SIG has not yet begun.
Want to know more? Visit one of our regular meetings throughout Germany and discuss with us. Become a member of the DGS!
Photo credits: ©Gerhard P. Herbig