Re: [FFmpeg-devel] [PATCH] lavc/h274: transpose IDCT

[Niklas Haas <ffmpeg@haasn.xyz>]

On Thu, 28 Sep 2023 23:08:48 +0200 Niklas Haas <ffmpeg@haasn.xyz> wrote:
> From: Niklas Haas <git@haasn.dev>
> 
> This is mathematically equivalent to what we were doing before, but
> gives subtly different results due to rounding (rows first vs columns
> first). Doing it this way makes our film grain database generation match
> reference implementation and now produces bit-exact outputs in my
> testing.
> 
> Rename the transposed variables to be a bit less confusing.
> ---
>  libavcodec/h274.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/libavcodec/h274.c b/libavcodec/h274.c
> index a5caf09564d..5709200322e 100644
> --- a/libavcodec/h274.c
> +++ b/libavcodec/h274.c
> @@ -59,13 +59,13 @@ static void init_slice_c(int8_t out[64][64], uint8_t h, uint8_t v,
>      //
>      // Note: To make the subsequent matrix multiplication cache friendlier, we
>      // store each *column* of the starting image in a *row* of `out`
> -    for (int y = 0; y <= freq_v; y++) {
> -        for (int x = 0; x <= freq_h; x += 4) {
> +    for (int l = 0; l <= freq_v; l++) {
> +        for (int k = 0; k <= freq_h; k += 4) {
>              uint16_t offset = seed % 2048;
> -            out[x + 0][y] = Gaussian_LUT[offset + 0];
> -            out[x + 1][y] = Gaussian_LUT[offset + 1];
> -            out[x + 2][y] = Gaussian_LUT[offset + 2];
> -            out[x + 3][y] = Gaussian_LUT[offset + 3];
> +            out[l][k + 0] = Gaussian_LUT[offset + 0];
> +            out[l][k + 1] = Gaussian_LUT[offset + 1];
> +            out[l][k + 2] = Gaussian_LUT[offset + 2];
> +            out[l][k + 3] = Gaussian_LUT[offset + 3];
>              prng_shift(&seed);
>          }
>      }
> @@ -74,9 +74,9 @@ static void init_slice_c(int8_t out[64][64], uint8_t h, uint8_t v,
>  
>      // 64x64 inverse integer transform
>      for (int y = 0; y < 64; y++) {
> -        for (int x = 0; x <= freq_h; x++) {
> +        for (int x = 0; x <= freq_v; x++) {
>              int32_t sum = 0;
> -            for (int p = 0; p <= freq_v; p++)
> +            for (int p = 0; p <= freq_h; p++)
>                  sum += R64T[y][p] * out[x][p];
>              tmp[y][x] = (sum + 128) >> 8;
>          }
> @@ -85,8 +85,8 @@ static void init_slice_c(int8_t out[64][64], uint8_t h, uint8_t v,
>      for (int y = 0; y < 64; y++) {
>          for (int x = 0; x < 64; x++) {
>              int32_t sum = 0;
> -            for (int p = 0; p <= freq_h; p++)
> -                sum += tmp[y][p] * R64T[x][p]; // R64T^T = R64
> +            for (int p = 0; p <= freq_v; p++)
> +                sum += tmp[x][p] * R64T[y][p]; // R64T^T = R64
>              // Renormalize and clip to [-127, 127]
>              out[y][x] = av_clip((sum + 128) >> 8, -127, 127);
>          }
> -- 
> 2.42.0
> 

Merged as 22530ad1ce
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