The 'axis' field of the 'imir' item property is often misinterpreted.
In HEIF (ISO/IEC 23008-12:2017(E)) it is specified as follows:

    6.5.12.2 Syntax

    aligned(8) class ImageMirror
    extends ItemProperty('imir') {
          unsigned int (7) reserved = 0;
          unsigned int (1) axis;
    }

    6.5.12.3 Semantics

    axis specifies a vertical (axis = 0) or horizontal (axis = 1) axis for the mirroring operation.

We believe it should be interpreted as follows (I quote the comments from libavif's avif.h header):

typedef struct avifImageMirror
{
    // 'imir' from ISO/IEC 23008-12:2017 6.5.12:
    // "axis specifies a vertical (axis = 0) or horizontal (axis = 1) axis for the mirroring operation."
    //
    // Legal values: [0, 1]
    //
    // 0: Mirror about a vertical axis ("left-to-right")
    // 1: Mirror about a horizontal axis ("top-to-bottom")
    uint8_t axis;
} avifImageMirror;