Lenticulars - Lenticular Interlacing

This is my attempt at trying to explain how lenticular interlacing works and, most importantly, why it works.

First thing first, let's figure out the number of frames needed for a given printer and a given lenticular pitch (lenticules per inch aka lpi). The formula is simple: number of frames = printer's resolution / lenticular's LPI (pitch). So, if you have an HP printer with a resolution of 600 dpi and you have a 60 lpi lenticular, then the number of frames is 600/60=10. Here, I am using dpi (dots per inch), but it really should be the ppi (pixels per inch). People have a tendency to use the dpi instead of the ppi, so I am doing the same here (to add to the confusion, I guess). You may use fewer or more although I don't guarantee anything good will come out if you use more. Note that this number of frames corresponds to what you can put under each lenticule (the individual lens of a lenticular sheet).

Now that we have figured out the number of images (frames) to have on our interlaced image, let's have a look at what is actually printed on the interlaced image:

For simplicity, you can think of a what I call a column as a column of pixels, although its width doesn't have to be one pixel.

Let's have a look at what the eyes see when one is looking straight down at the lenticular:

Let's have a look at what the eyes see when one is not looking straight down at the lenticular (head is shifted):

It's important to note that the eye never sees an image in full resolution (the image one would see if it were not interlaced) but a sample of it. The image is seen through a mask with regularly cut vertical slits. In our case of 10 image interlacing, the vertical slit is placed every 10 columns. Whether the head looks straight down at the lenticular sheet or shifts to the left or right, you always experience a 3D effect but the scenes you are seeing are not the same (the vertical slits in the mask are not at the same place). It's a bit like virtually exploring the scene by simply shifting your head, or tilting the lenticular (which is basically the same thing, geometry wise). When you make a lenticular with just two images (the left and right image of a stereo pair), you don't get that effect and the head must be looking at the lenticular sheet straight on to get the 3D effect.

If you want to make your own interlaced image with Photoshop or The Gimp, I suggest you read 3D Lenticular Printing Interlacing Algorithm Illustrated Using Photoshop.