Stereo photography for lenticular print
Updated: Dec 30, 2020
Revised Nov. 15, 2020
Introduction
The 3D lenticular print method can be used to print stereo photographs and present them as true 3D pictures in full color and without any visual aids. However, not every stereo photograph can be printed as a 3D lenticular picture. Stereo photography must be specially staged for such an application.
Printing of stereo-pair consists of two steps:
Computation of intermediate frames,
Lenticular printing.
The main challenge is to achieve success in the first step. The success of this step cannot be guaranteed because it involves "guessing" of missing information. Moreover, it is impossible to formulate general guidelines because the success depends on the scene characteristics, not only on the photography. Therefore, in many cases, an experimental "try-and-error" loop may be inevitable. Such a procedure is suggested below.
The second step, lenticular printing, imposes additional constraints. The design rules in this step are well established, and if followed, a high-quality lenticular picture can be produced with certainty.
This post relates to a stereo photograph of an object place against a background. We use an example of a portrait stereo photograph (figure 1) to present the various considerations involved. The general principles can be deduced, to a certain extent, from this discussion.
The photograph shown below was taken from 1m with a camera separation of 15cm.
Figure 1: Stereo portrait used as an example in this post
Computation of intermediate images
A stereo photograph consists of two images only, which are projections of a scene from two adjacent points. The lenticular picture needs more than two images, typically 10. This set of images is called "lenticular sequence."
The first step in producing a lenticular picture from a stereo pair is to generate the additional images required for the lenticular sequence. These images are projections of the scene from a series of equally spaced points on the line connecting the two shooting points of the stereo pair.
The computation of the intermediate images involves "guessing" of missing information, and, therefore, its success cannot be guaranteed. Understanding the failure mechanisms will help the photographer reduce their effect and increase the chances of success. The good news is that the success of the computation does not depend on the sequence length. If it succeeds for a certain number of images, it will succeed with any number of images.
There are two main failure mechanisms: excessive differences and occultations.
The success of the lenticular sequence computation depends critically on the differences between the two stereo images. For small differences, the success probability is high, and it diminishes as these differences increase. The parallax concept is used to quantify these differences (see below).
Normally, any given object point is present in both images of the stereo pair. But sometimes, a certain point in one image has no corresponding point in the other image. Such a condition is called "occultation." Occultations lead to visual distortions and artifacts in the computed lenticular sequence. In some cases, these defects may be acceptable; they may render the whole sequence useless in others.
Avoiding occultations
The image window is a common cause of occultations. Objects near the image edge may be seen in one image but not in the other and vice-versa. In the figure below, the window occultation regions in the background are denoted by vertical green bands. These occultations may cause defects in the computed lenticular sequence.
Figure 2: Window occultations in the stereo images
In the present example, the background is a vertical plane. In such a case, the window occulted regions are vertical rectangles adjacent to the stereo pair images edges (figure 2). Such obscurations can be easily cropped out from the sequence. For this reason, a vertical plane background is preferred.
Scene occultations occur due to the scene geometry can be avoided only by modifying the subject or the photography angle. In the present example, scene occultations are present in the background near the subject's face. These occultations can also cause disturbing defects in the computed frames. Although the defects may be barely visible, they may be conspicuous in the 3D picture.
A simple way to avoid the effects of scene occultations in the background is to use uniform or almost uniform backgrounds, in which occultations defects will be unnoticed both in the images and in the 3D picture. The stereo image shown in figure 1 has such a background.
A stereo photograph with a uniform color background can be used to create occultation-free sequences with any background using the chroma-key method. This requires two stereo photographs: one of the background only (without the subject) and the other of the subject with a uniform color background. Next, both stereo pairs are converted to lenticular sequences, and the two sequences are combined into a single sequence using the chroma-key method.
The parallaxes of a stereo image pair
The parallax concept quantifies the differences between a pair of stereo images. Each point on the photographed object has an associated parallax. The parallax is the distance between the given object's positions in the two images (when they are superimposed one on the top of the other).
Let us consider a pair of stereo images shown in figure 1. The pixel width of the images is 800 each.
The parallaxes can be conveniently measured using Stereo Photo Maker (SPM). The SPM parallax adjustment window for this photograph is shown below:
Figure 3: Parallax adjustment window of SPM
Of special interest are the parallaxes of the extreme points of the scene. These are the closest and farthest points from the camera (distance is measured along the camera axis). The parallaxes corresponding to these points are called "front parallax" and "back parallax." In this case, the nearest point is the model nose tip. Its parallax can be measured by moving the 'H Position' slider at the top until the nose tip appears grey. The result is shown in the figure below:
Figure 4: Measurement of the front parallax
The front parallax value, in this case, is 24 pixels. To measure the back parallax, the horizontal slider is moved until the background appears grey. Since, in the present example, the background is almost uniform, this method cannot be used. Instead, one can rely on the furthest viewable objects of the subject, which are, in this case, the shoulder and the neck silhouettes. The result is shown in the figure below:
Figure 5: Measurement of back parallax
The back parallax, in this case, is -19 pixels. Note that the back parallax is smaller than the front parallax, both in magnitude and in (algebraic) value. According to the SPM sign convention, the algebraic value of the back parallax is always smaller if the left image (of the SPM side-by-side display, shown in figure 1) corresponds to the left view.