From c8bc6fabd79ee19df93f2486b8a6ebeedb7cd27e Mon Sep 17 00:00:00 2001 From: Niklas Haas Date: Thu, 4 Jul 2024 16:28:50 +0200 Subject: [PATCH] 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. --- libavfilter/vf_scale.c | 16 +++++++++++++--- 1 file 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