diff options
| author | Niklas Haas <git@haasn.dev> | 2024-07-04 16:28:50 +0200 |
|---|---|---|
| committer | Niklas Haas <git@haasn.dev> | 2024-08-16 11:43:37 +0200 |
| commit | c8bc6fabd79ee19df93f2486b8a6ebeedb7cd27e (patch) | |
| tree | 5adab4346d67313664a48670801bb5ef3177d543 | |
| parent | 15a67c0947d6fa8ff50607ef26185986813a6fd6 (diff) | |
| download | ffmpeg-c8bc6fabd79ee19df93f2486b8a6ebeedb7cd27e.tar.gz | |
avfilter/vf_scale: fix 4:1:0 interlaced chroma pos
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.
| -rw-r--r-- | libavfilter/vf_scale.c | 16 |
1 files changed, 13 insertions, 3 deletions
diff --git a/libavfilter/vf_scale.c b/libavfilter/vf_scale.c index ea77bac1c3..afa159914b 100644 --- a/libavfilter/vf_scale.c +++ b/libavfilter/vf_scale.c @@ -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 |