Re: [FFmpeg-devel] [PATCH 2/2] swscale/output: Bias 16bps output calculations to improve non overflowing range for GBRP16/GBRPF32

[Mark Reid <mindmark@gmail.com>]

On Wed, Nov 2, 2022 at 2:03 PM Michael Niedermayer <michael@niedermayer.cc>
wrote:

> Fixes: integer overflow
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>  libswscale/output.c       | 25 +++++++++++--------------
>  libswscale/x86/output.asm | 16 +++++++++++++++-
>  2 files changed, 26 insertions(+), 15 deletions(-)
>
> diff --git a/libswscale/output.c b/libswscale/output.c
> index df4647adde..5c85bff971 100644
> --- a/libswscale/output.c
> +++ b/libswscale/output.c
> @@ -2372,18 +2372,15 @@ yuv2gbrp16_full_X_c(SwsContext *c, const int16_t
> *lumFilter,
>
>          Y -= c->yuv2rgb_y_offset;
>          Y *= c->yuv2rgb_y_coeff;
> -        Y += 1 << 13;
> +        Y += (1 << 13) - (1 << 29);
>          R = V * c->yuv2rgb_v2r_coeff;
>          G = V * c->yuv2rgb_v2g_coeff + U * c->yuv2rgb_u2g_coeff;
>          B =                            U * c->yuv2rgb_u2b_coeff;
>
> -        R = av_clip_uintp2(Y + R, 30);
> -        G = av_clip_uintp2(Y + G, 30);
> -        B = av_clip_uintp2(Y + B, 30);
> +        dest16[2][i] = av_clip_uintp2(((Y + R) >> 14) + (1<<15), 16);
> +        dest16[0][i] = av_clip_uintp2(((Y + G) >> 14) + (1<<15), 16);
> +        dest16[1][i] = av_clip_uintp2(((Y + B) >> 14) + (1<<15), 16);
>
> -        dest16[0][i] = G >> 14;
> -        dest16[1][i] = B >> 14;
> -        dest16[2][i] = R >> 14;
>          if (hasAlpha)
>              dest16[3][i] = av_clip_uintp2(A, 30) >> 14;
>      }
> @@ -2448,18 +2445,18 @@ yuv2gbrpf32_full_X_c(SwsContext *c, const int16_t
> *lumFilter,
>
>          Y -= c->yuv2rgb_y_offset;
>          Y *= c->yuv2rgb_y_coeff;
> -        Y += 1 << 13;
> +        Y += (1 << 13) - (1 << 29);
>          R = V * c->yuv2rgb_v2r_coeff;
>          G = V * c->yuv2rgb_v2g_coeff + U * c->yuv2rgb_u2g_coeff;
>          B =                            U * c->yuv2rgb_u2b_coeff;
>
> -        R = av_clip_uintp2(Y + R, 30);
> -        G = av_clip_uintp2(Y + G, 30);
> -        B = av_clip_uintp2(Y + B, 30);
> +        R = av_clip_uintp2(((Y + R) >> 14) + (1<<15), 16);
> +        G = av_clip_uintp2(((Y + G) >> 14) + (1<<15), 16);
> +        B = av_clip_uintp2(((Y + B) >> 14) + (1<<15), 16);
>
> -        dest32[0][i] = av_float2int(float_mult * (float)(G >> 14));
> -        dest32[1][i] = av_float2int(float_mult * (float)(B >> 14));
> -        dest32[2][i] = av_float2int(float_mult * (float)(R >> 14));
> +        dest32[0][i] = av_float2int(float_mult * (float)G);
> +        dest32[1][i] = av_float2int(float_mult * (float)B);
> +        dest32[2][i] = av_float2int(float_mult * (float)R);
>          if (hasAlpha)
>              dest32[3][i] = av_float2int(float_mult *
> (float)(av_clip_uintp2(A, 30) >> 14));
>      }
> diff --git a/libswscale/x86/output.asm b/libswscale/x86/output.asm
> index 84e94baaf6..f943a27534 100644
> --- a/libswscale/x86/output.asm
> +++ b/libswscale/x86/output.asm
> @@ -44,11 +44,13 @@ pd_yuv2gbrp_y_start:       times 8 dd  (1 << 9)
>  pd_yuv2gbrp_uv_start:      times 8 dd  ((1 << 9) - (128 << 19))
>  pd_yuv2gbrp_a_start:       times 8 dd  (1 << 18)
>  pd_yuv2gbrp16_offset:      times 8 dd  0x10000  ;(1 << 16)
> -pd_yuv2gbrp16_round13:     times 8 dd  0x02000  ;(1 << 13)
> +pd_yuv2gbrp16_round13:     times 8 dd  0xE0002000  ;(1 << 13) - (1 << 29)
>  pd_yuv2gbrp16_a_offset:    times 8 dd  0x20002000
>  pd_yuv2gbrp16_upper30:     times 8 dd  0x3FFFFFFF ;(1<<30) - 1
>  pd_yuv2gbrp16_upper27:     times 8 dd  0x07FFFFFF ;(1<<27) - 1
> +pd_yuv2gbrp16_upper16:     times 8 dd  0x0000FFFF ;(1<<16) - 1
>

pd_yuv2gbrp16_upper16 doesn't appear to be used anywhere


>  pd_yuv2gbrp16_upperC:      times 8 dd  0xC0000000
> +pd_yuv2gbrp_debias:        times 8 dd  0x00008000 ;(1 << 29 - 14)
>  pb_pack_shuffle8:       db  0,  4,  8, 12, \
>                             -1, -1, -1, -1, \
>                             -1, -1, -1, -1, \
> @@ -883,14 +885,26 @@ cglobal yuv2%1_full_X, 12, 14, 16, ptr, lumFilter,
> lumSrcx, lumFilterSize, chrFi
>          paddd G, Y
>          paddd B, Y
>
> +%if  DEPTH < 16
>          CLIPP2 R, 30
>          CLIPP2 G, 30
>          CLIPP2 B, 30
> +%endif
>
>          psrad R, RGB_SHIFT
>          psrad G, RGB_SHIFT
>          psrad B, RGB_SHIFT
>
> +%if  DEPTH >= 16
> +        paddd R, [pd_yuv2gbrp_debias]
> +        paddd G, [pd_yuv2gbrp_debias]
> +        paddd B, [pd_yuv2gbrp_debias]
> +
> +        CLIPP2 R, 16
> +        CLIPP2 G, 16
> +        CLIPP2 B, 16
> +%endif
> +
>  %if FLOAT
>          cvtdq2ps R, R
>          cvtdq2ps G, G
> --
> 2.17.1
>
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