Passive 3D from the beginning – Part 2 – Line allocation

From the last post we know that with a passive 3D TV, the separation of left and right eye images is achieved by allowing the left eye to see only the odd-numbered display lines, and the right eye to see only the even-numbered display lines, taking the top line of the picture to be 1. The total number of horizontal lines in the display is 1080 (as of today; should 2160 line displays become available in the future, all problems will be solved). Given that, it is clear that the left eye sees only 540 lines, while the right eye sees the other 540 lines.

Note: while the filtering process of the display is totally passive, the TV’s electronics must do things with regard the source image whenever it is displaying a 3D image. In particular, it must work out which lines from the source are shown on which lines on the display.

If you take a typical Blu-ray 3D disc, this has 1080 horizontal lines in the left eye image, and 1080 for the right eye image. So the question is: how are the 1080 source lines for the left eye allocated to the 540 display lines for the left eye (and similarly for the right eye).

It seems to me that there are three somewhat sensible ways this could be done. I shall call these three ways: Same Line Allocation, Alternate Line allocation, Both Line Allocation.

In what follows I shall code lines as follows: L1, L2, L3 means the first, second and third left eye lines while R1, R2 and R3 are the same lines for the right eye. I will not discuss the visible implications of these, nor how to determine which is being used, in this post. They will come in future ones.

Same Line Allocation: With this the display simply throws out L2, L4 … L1080, and also throws out R2, R4 … R1080. In other words, all the even numbered lines are disposed of, leaving only the odd numbered lines (this could be reversed, but I’ll stick with this to keep the grammar clear). So L1, L3 etc are displayed on their intended odd numbered display lines while R1, R3 etc are shifted one display line down and shown on the even numbered display lines.

Alternate Line Allocation: With this the display throws out L2, L4 … L1080, and also throws out R1, R3 … R1079. Thus there is no shifting. With your left eye you see only odd numbered lines in their correct placement on the odd numbered display lines. Likewise with your right eye you see only even numbered lines, correctly placed on their even numbered display lines. So all lines that you see are in their correct places on the display. But, of course, half the lines are missing.

Both Line Allocation: With this the display does not throw out any of the lines. Instead it displays both L1 and L2 on display line 1, and it displays both R1 and R2 on display line 2. And so on for the L3, L4 pair, and the R3, R4 pair, all the way to the bottom. It can display two source lines on just one display line either by displaying them one after the other, or by electronically merging them and displaying this composite.

Next: Part 2 – Same Line Allocation in action

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