Pinhole Array Parallax Barrier:
Parallax barrier is a simple grid with a series of precisely placed holes which when placed on top of the image source, like LCD, allows for different set of pixels to be seen from different viewing angles. We can have this grid printed out on a transparency film with holes spaced apart by 5 pixels in both dimensions, hence blocking all but one pixel in every 5×5 pixel group, or “super-pixel.” After the view interlacing, each pixel in this super-pixel will correspond to a different viewpoint of the scene, hence creating a sense of depth through parallax as the viewer moves over the screen to see a different view.
Super-Pixel Zoomed In:
Figure below shows the side view of one of the display panel’s super-pixel with the pinhole barrier placed on top and the barrier pitch (size) equal to the super-pixel pitch. The Field of View of the pinhole is determined by the super-pixel pitch and the spacing between the panel and the barrier, which is given by the cellphone screen cover and the acrylic spacer.
Figure below, with true sub-pixel arrangement for iPhone 6 display panel, shows the top view of a single super-pixel composed of 5×5 pixel/view grid where the transparency barrier masks all but the center pixel. iPhone 6 screen has a 1334-by-750-pixel resolution at 326 ppi (or the pixel size of 78 micron (= 25.4 mm/inch / 326 pixels/inch)). Hence our super pixel size is 390-by-390-micron (78micron * 5 pixels = 390micron), with a total of 266-by-150 super-pixels across the screen.
Note: Using a Consumer Laser Jet Printer with 1200 dpi:
We conclude from our experience that consumer printers do not have high enough resolution to produce a workable parallax barrier. For the pinhole array to provide the proper parallax and hence depth perception:
- all barriers and their pinholes should have consistent pitch across the mask. This ensures a constant field of view across the screen and that the same views are seen from all parts of the screen at a given viewing angle.
- pinhole size should be equal to or smaller than the pixel pitch. This ensures that only 1 view is visible from any viewing angle and the cross-talk between views is minimized
- pinhole barriers should be properly aligned with the panel super-pixels. (proper alignment means that the field of views of all the barriers across the pinhole mask has a uniform directionality or has a directionality that makes them converge above the center of the screen (more on this at the end of this section)
These criteria are not satisfied if the barrier (printed super-pixel) pitch is not equal to the super-pixel pitch or if the pinholes’ size vary across the mask.
Our first attempt at creating the parallax barrier was to print one on a transparency using a household laser jet printer. Our laser printer had the reported resolution of 1200dpi (or the dot size of 21.16 micron (= 25.4 mm/inch / 1200 pixels/inch)), due to which the panel’s super-pixel pitch of 390micron is not exactly divisible by the printer dot size of 21.16micron. Thus the closest the barrier pitch comes to the super-pixel pitch is with 18 printer dots (380micron) which is still off by atleast 10 micron as shown below.
Nevertheless, we printed a pinhole mask with a repeating printed “super-pixels” comprised of 18 dots with the pinhole formed by 4 white printed dots. The following MATLAB code produces such a mask:
%% MASK with 18 pixels super-pixel iphone6_mask18 = zeros(4917,2764); %750iphone pixels* 78micron pixel size/ 21.16 printer pixel size for p=8:11 %4 printer pixel pinhole for q =8:11 iphone6_mask18(p:18:end,q:18:end) = 255; %390micron superpixel / 21.16micron printer pixel = 18pixels end end imwrite(iphone6_mask18, 'iphone6_printableMask_18.tif');
However, we were not able to see distinct views in the “numbers” sequence (described in the next section) with 18 dots barrier pattern. We then tried to make the barriers align with the super-pixels as closely as possible in the global sense across the entire display panel. We printed a mask with variable barrier size sequence that repeats after every 5 super-pixels, with a barrier of 423micron pitch being followed and trailed by 2 barriers each of 380micron pitch. In this way, the overall pitch (1946 micron) of the 5 barrier sequence would be off from the 5 super-pixel sequence pitch (1950 micron) by only 4 micron:
Failing to get any improvement in the observed parallax with this approach either, we tried to align the pinholes exactly at the expense of adapting variable pinhole sizes again because of the discretization introduced by the printer’s dpi of 21.16 micron which is not a factor of panel’s pixel pitch of 78 micron. To achieve this, we first formed the pinhole pattern for iPhone 6 screen resolution of 1334-by-750 pixels as described on the Alignment page. We then resized it to the equivalent printer dots resolution (i.e. 4917-by-2764 number of printer dots that can fit in the iPhone screen size). We used the nearest neighbor interpolation such that each of the 4917-by-2764 pixels of this new pattern take up a value of either 255 or 0 in grayscale. The resultant pinhole mask look like so when zoomed in:
Even though pinholes are now aligned with the super-pixels, because of the variable pinhole size and hence the field of view, we get severe cross-talk between views from pinholes through the screen and hence little to no depth perception.
Photographic Transparency Film Printing using PhotoPlotter:
The simplest way to design the pinhole mask is to have the pitch of each of the pinhole barriers equal to the super-pixel pitch. However, that is not possible as we just saw with the consumer printer resolution of 1200 dpi or 21.16micron wide print dot which is not exactly divisible by the pixel size. Thus, we need a super high resolution printer of atleast 5000 dpi or 5 micron wide print dot which would be a factor of super-pixel size of 390 micron and a factor of 75 micron which we can set as our pinhole size.
An even better approach is to go through the route of photoplotter.
Vector Graphics Format:
Photoplotters require the pinhole pattern to be provided in the vector graphics format. In particular, the local photoplotting vendor that we used, ArtNetPro Inc., required the mask to be in open ASCII vector format, Gerber, otherwise a one-time conversion fee would be charged to convert a postscript vector graphics format to Gerber format. Any vector graphics editor like Adobe Illustrator can be used to create the mask in .eps or .dpf format and these formats can then be converted to Gerber format by the vendor. Or the mask can be straight created in the Gerber format using the CAD or PCB design software, like Eagle.
Nevertheless, here is the file for our pinhole mask for iPhone 6 in the Gerber format which can be directly input to the photo plotter.
Make sure the file extension is GBR after you download it, not TXT.
Note: Accurate Selection of Barrier Pitch with respect to the Panel Super Pixel Pitch
Next, we’ll show you how to align and affix your parallax barrier.