Introduction
Lens curvature is a major obstacle in making large-format lenticular pictures. This defect limits the viewing angle, and in extreme cases the picture cannot display a clean image at all.
Lens manufacturers invest much efforts to produce lenticular sheets free of curvature. However, due to technological hurdles, this defect can only be reduced to a certain degree, and not completely eliminated. The magnitude of this defect is random; in some sheets it is acceptable, and in others not. As a result, many lens sheets are rejected by the printers, causing a significant capital loss. Fortunately, there is an easy and effective method to overcome this problem.
Correction of lens curvature
Lens curvature can be compensated by warping the interlaced image with the same curvature as the lens. When this is done, the picture will behave as if the lens is perfectly straight.
The lens curvature correction is done in two steps:
1. Measurement of the lens curvature,
2. Warping the interlaced image according to the lens curvature.
The lens curvature can be measured using a pattern interlaced from a sequence of uniformly colored images. Such pattern is called "fringe pattern," after a similar phenomenon observed in optical interferometry.
A 25lpi lens of size 100x100cm was used to photograph the fringe pattern in the left of figure 1. The interlaced sequence had 5 images, in colors R, G, B, W, and K. All five colors can be seen in the picture, so the lens, as is, is unusable.
The lens curvature can be measured from this pattern, and the interlaced image can be warped to compensate for these curvature. After applying this correction to the RGBWK interlace that was used to create the fringe pattern in the left of figure 1, the fringe pattern shown in the right of this figure is obtained. The corrected picture displays a single solid color, like a perfectly straight lens. Both figures 1 and 2 were photographed with the same curved lens sheet.
A photograph of the final picture with the corrective warping applied to its interlace is shown below:
This procedure allows making large-format lenticular pictures regardless of the lens curvature.
The UnCurve software
The UnCurve software from Pop3DArt is dedicated to the correction of lens curvature. It is used both to measure the lens curvature from a fringe pattern and to apply the correction to a given interlaced image.
Earlier research
Some earlier research on this subject in the context of electronic lenticular displays is cited in the bibliography below.
Bibliography
Yun-Gu Lee and Jong Beom Ra, "Reduction of the distortion due to non-ideal lens alignment in lenticular 3D displays," SPIE vol. 5664, p. 506 (2005).
Silvio Jurk et al., "Correction of misalignments between optical grid and pixel panel on autostereoscopic displays at future increasing display resolution," Electronic Displays Conference paper (2016).
Fringe patterns in optical interferometry: