[FFmpeg-devel] [PATCH v2] swscale/unscaled: correctly round yuv2yuv when not dithering

[Niklas Haas]

On Tue, 17 Dec 2024 15:35:22 +0100 Niklas Haas <ffmpeg at haasn.xyz> wrote:
> From: Niklas Haas <git at haasn.dev>
>
> We should at least bias towards the nearest integer, instead of always
> rounding down, when not dithering. This is a bit more correct.
>
> Signed-off-by: Niklas Haas <git at haasn.dev>
> Sponsored-by: Sovereign Tech Fund

Ignore this one, it's wrong. I forgot to amend my local changes before sending
the patch. Will send a v3.

> ---
>  libswscale/swscale_unscaled.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/libswscale/swscale_unscaled.c b/libswscale/swscale_unscaled.c
> index c7ad6b014a..ea6fb038bc 100644
> --- a/libswscale/swscale_unscaled.c
> +++ b/libswscale/swscale_unscaled.c
> @@ -2075,17 +2075,18 @@ static int packedCopyWrapper(SwsInternal *c, const uint8_t *const src[],
>
>  #define DITHER_COPY(dst, dstStride, src, srcStride, bswap, dbswap)\
>      unsigned shift= src_depth-dst_depth, tmp;\
> +    unsigned bias = 1 << (shift - 1);\
>      if (c->opts.dither == SWS_DITHER_NONE) {\
>          for (i = 0; i < height; i++) {\
>              for (j = 0; j < length-7; j+=8) {\
> -                dst[j+0] = dbswap(bswap(src[j+0])>>shift);\
> -                dst[j+1] = dbswap(bswap(src[j+1])>>shift);\
> -                dst[j+2] = dbswap(bswap(src[j+2])>>shift);\
> -                dst[j+3] = dbswap(bswap(src[j+3])>>shift);\
> -                dst[j+4] = dbswap(bswap(src[j+4])>>shift);\
> -                dst[j+5] = dbswap(bswap(src[j+5])>>shift);\
> -                dst[j+6] = dbswap(bswap(src[j+6])>>shift);\
> -                dst[j+7] = dbswap(bswap(src[j+7])>>shift);\
> +                dst[j+0] = dbswap(bswap(src[j+0] + bias))>>shift);\
> +                dst[j+1] = dbswap(bswap(src[j+1] + bias))>>shift);\
> +                dst[j+2] = dbswap(bswap(src[j+2] + bias))>>shift);\
> +                dst[j+3] = dbswap(bswap(src[j+3] + bias))>>shift);\
> +                dst[j+4] = dbswap(bswap(src[j+4] + bias))>>shift);\
> +                dst[j+5] = dbswap(bswap(src[j+5] + bias))>>shift);\
> +                dst[j+6] = dbswap(bswap(src[j+6] + bias))>>shift);\
> +                dst[j+7] = dbswap(bswap(src[j+7] + bias))>>shift);\
>              }\
>              for (; j < length; j++) {\
>                  dst[j] = dbswap(bswap(src[j])>>shift);\
> @@ -2169,6 +2170,7 @@ static int planarCopyWrapper(SwsInternal *c, const uint8_t *const src[],
>                  uint16_t *dstPtr2 = (uint16_t*)dstPtr;
>
>                  if (dst_depth == 8) {
> +                    av_assert1(src_depth > 8);
>                      if(isBE(c->opts.src_format) == HAVE_BIGENDIAN){
>                          DITHER_COPY(dstPtr, dstStride[plane], srcPtr2, srcStride[plane]/2, , )
>                      } else {
> @@ -2248,7 +2250,7 @@ static int planarCopyWrapper(SwsInternal *c, const uint8_t *const src[],
>                          dstPtr2 += dstStride[plane]/2;
>                          srcPtr2 += srcStride[plane]/2;
>                      }
> -                } else {
> +                } else { /* src_depth > dst_depth */
>                      if(isBE(c->opts.src_format) == HAVE_BIGENDIAN){
>                          if(isBE(c->opts.dst_format) == HAVE_BIGENDIAN){
>                              DITHER_COPY(dstPtr2, dstStride[plane]/2, srcPtr2, srcStride[plane]/2, , )
> --
> 2.47.0
>