On Sun, 19 May 2024, Lynne via ffmpeg-devel wrote:
This commit adds a decoder for the frequency-domain part of USAC.
[...]
+/* Finish later */
+static const enum AVChannel usac_ch_pos_to_av[64] = {
+ [0] = AV_CHAN_FRONT_LEFT,
+ [1] = AV_CHAN_FRONT_RIGHT,
+ [2] = AV_CHAN_FRONT_CENTER,
+ [3] = AV_CHAN_LOW_FREQUENCY,
+ [4] = AV_CHAN_BACK_LEFT, // unsure
+ [5] = AV_CHAN_BACK_RIGHT, // unsure
+ [6] = AV_CHAN_FRONT_LEFT_OF_CENTER,
+ [7] = AV_CHAN_FRONT_RIGHT_OF_CENTER,
+ [8] = 0, /* rear surround left is missing */
+ [9] = 0, /* rear surround right is missing */
+ [10] = AV_CHAN_BACK_CENTER,
+ [11] = AV_CHAN_SURROUND_DIRECT_LEFT,
+ [12] = AV_CHAN_SURROUND_DIRECT_RIGHT,
+ [13] = AV_CHAN_SIDE_LEFT, // fairly sure
+ [14] = AV_CHAN_SIDE_RIGHT, // fairly sure
+ [15] = AV_CHAN_WIDE_LEFT, // somewhat confident
+ [16] = AV_CHAN_WIDE_RIGHT, // somewhat confident
+ [17] = AV_CHAN_TOP_FRONT_LEFT,
+ [18] = AV_CHAN_TOP_FRONT_RIGHT,
+ [19] = AV_CHAN_TOP_FRONT_CENTER,
+ [20] = AV_CHAN_TOP_BACK_LEFT,
+ [21] = AV_CHAN_TOP_BACK_RIGHT,
+ [22] = AV_CHAN_TOP_BACK_CENTER,
+ [23] = AV_CHAN_TOP_SIDE_LEFT,
+ [24] = AV_CHAN_TOP_SIDE_RIGHT,
+ [25] = AV_CHAN_TOP_CENTER,
+ [26] = AV_CHAN_LOW_FREQUENCY, // actually LFE2
+ [27] = AV_CHAN_BOTTOM_FRONT_LEFT,
+ [28] = AV_CHAN_BOTTOM_FRONT_RIGHT,
+ [29] = AV_CHAN_BOTTOM_FRONT_CENTER,
+ [30] = 0, /* top left surround is missing */
+ [31] = 0, /* top right surround is missing */
+};
Some comment would be nice about the source of this table (which
document, which table).
It looks very similar to the ISO channel positons used in mov_chan. I
think we follow this mapping in most cases:
Left Surround is SIDE_LEFT
Right Surround is SIDE_RIGHT
Rear Surround Left is BACK_LEFT
Rear Surround Right is BACK_RIGHT
So in your table [4] and [5] should be SIDE, [8] and [9] should be
BACK. [26] can be AV_CHAN_LOW_FREQUENCY_2, we do have that.
Yes, Left/Right Surround and Left/Right Side Surround will be the same,
but those are not present in commonly used layouts at the same time.
Regards,
Marton
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