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Transposition

Ledcat can shuffle pixels around before outputting them. This can be useful if the arrangement of pixels in the output display does not match that of the input. With the --transpose option, you can set one or more operations to apply.

Because some operations are designed to work on 2 dimensional images, Some operations need to know the dimensions of the display they are operating on. Such operations require the number of pixels to be specified by the --geometry option as WIDTHxHEIGHT.

Reverse

DEVICE <- X <- X <- X <- X <- X <- X <- X <- X

Applying --transpose reverse simply reverses the output image, meaning that pixel 0 in the input becomes pixel N-1 in the output, 1 becomes N-2, etc.

Reversing a 2D image is equivalent to rotating by 180 degrees.

Zig Zag

X axis:

DEVICE -> X -> X -> X -> X -> X -> X -> X -> X
                                             v
          X <- X <- X <- X <- X <- X <- X <- X
          v
          X -> X -> X -> X -> X -> X -> X -> X
                                             v
          X <- X <- X <- X <- X <- X <- X <- X

Y axis:

DEVICE -> X    X -> X    X -> X    X -> X    X
          v    ^    v    ^    v    ^    v    ^
          X    X    X    X    X    X    X    X
          v    ^    v    ^    v    ^    v    ^
          X    X    X    X    X    X    X    X
          v    ^    v    ^    v    ^    v    ^
          X -> X    X -> X    X -> X    X -> X

When building a display out of a single LED-strip, it is not uncommon to arrange the strips in a repeating pattern that runs from left to right on the first row and right to left on the next row.

Ledcat can transpose an input image with a regular left to right to match a display with such zigzagged wiring. Setting Either --transpose zigzag_x or --transpose zigzag_y will perform a zigzag transposition over the X- or Y-axis respectively.

It is not possible to zigzag a 1D image, thus requiring the display size to be configured using --geomety instead of --num-pixels.

Mirror

Using --transpose mirror_x or --transpose mirror_y will mirror the output image of the respective axis.