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avfilter/vf_scale: fix 4:1:0 interlaced chroma pos

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The current logic hard-coded a check for v_sub == 1. We can extend this
logic slightly to cover the case of interlaced 4:1:0 (which has v_sub ==
2).

Here is a diagram explaining this scenario (with center-siting):

a   a   a   a   a   a   a   a

b   b   b   b   b   b   b   b
      X               X
a   a   a   a   a   a   a   a

b   b   b   b   b   b   b   b

a   a   a   a   a   a   a   a

b   b   b   b   b   b   b   b
      Y               Y
a   a   a   a   a   a   a   a

b   b   b   b   b   b   b   b

a = even luma rows
b = odd luma rows
X = even chroma sample
Y = odd chroma sample

In progressive mode, the chroma samples sit at (384, 384) respectively.

Relative to the 8x4 grid of even luma samples (a), the X sample sits at:
  h_chr_pos = 384
  v_chr_pos = 192

Relative to the 8x4 grid of odd luma samples (b), the Y sample sits at:
  h_chr_pos = 384
  v_chr_pos = 576

The new code calculates the correct values in all circumstances.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
This commit is contained in:
Niklas Haas 2024-07-04 16:28:50 +02:00 committed by Paul B Mahol
parent 6f30aee978
commit 8c9327c119
1 changed files with 13 additions and 3 deletions
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16
libavfilter/vf_scale.c
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@ -643,9 +643,19 @@ static void calc_chroma_pos(int *h_pos_out, int *v_pos_out, int chroma_loc,
v_pos = v_pos_override;
/* Fix vertical chroma position for interlaced frames */
if (v_sub == 1 && index > 0) {
v_pos += 256 * (index == 2); /* offset by one luma row for odd rows */
v_pos >>= 1; /* double luma row distance */
if (v_sub && index > 0) {
/* When vertically subsampling, chroma samples are effectively only
* placed next to even rows. To access them from the odd field, we need
* to account for this shift by offsetting the distance of one luma row.
*
* For 4x vertical subsampling (v_sub == 2), they are only placed
* next to every *other* even row, so we need to shift by three luma
* rows to get to the chroma sample. */
if (index == 2)
v_pos += (256 << v_sub) - 256;
/* Luma row distance is doubled for fields, so halve offsets */
v_pos >>= 1;
}
/* Explicitly strip chroma offsets when not subsampling, because it