[FFmpeg-devel] [PATCH] avcodec/s3tc: fix decoding when dimensions are not a multiple of 4

[Michael Niedermayer]

On Thu, May 07, 2015 at 04:17:10PM +0100, Tom Butterworth wrote:
> Image dimensions were rounded down causing rows to wrap early if dimensions were not a multiple of 4.
> ---
>  libavcodec/s3tc.c | 23 +++++++++++++----------
>  1 file changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/libavcodec/s3tc.c b/libavcodec/s3tc.c
> index 4743d78..9886b1d 100644
> --- a/libavcodec/s3tc.c
> +++ b/libavcodec/s3tc.c
> @@ -26,6 +26,7 @@
>  #include "s3tc.h"
>  
>  static inline void dxt1_decode_pixels(GetByteContext *gb, uint32_t *d,
> +                                      unsigned int w, unsigned int h,
>                                        unsigned int qstride, unsigned int flag,
>                                        uint64_t alpha) {
>      unsigned int x, y, c0, c1, a = (!flag * 255u) << 24;
> @@ -62,14 +63,16 @@ static inline void dxt1_decode_pixels(GetByteContext *gb, uint32_t *d,
>      colors[2] = rb2 + g2 + a;
>  
>      pixels = bytestream2_get_le32(gb);
> -    for (y=0; y<4; y++) {
> -        for (x=0; x<4; x++) {
> +    for (y=0; y<h; y++) {
> +        for (x=0; x<w; x++) {
>              a        = (alpha & 0x0f) << 28;
>              a       += a >> 4;
>              d[x]     = a + colors[pixels&3];
>              pixels >>= 2;
>              alpha  >>= 4;
>          }
> +        for (; x<4; x++)
> +            pixels >>= 2;
>          d += qstride;
>      }
>  }

> @@ -77,21 +80,21 @@ static inline void dxt1_decode_pixels(GetByteContext *gb, uint32_t *d,
>  void ff_decode_dxt1(GetByteContext *gb, uint8_t *dst,
>                      const unsigned int w, const unsigned int h,
>                      const unsigned int stride) {
> -    unsigned int bx, by, qstride = stride/4;
> +    unsigned int x, y, qstride = stride/4;
>      uint32_t *d = (uint32_t *) dst;
>  
> -    for (by=0; by < h/4; by++, d += stride-w)
> -        for (bx = 0; bx < w / 4; bx++, d += 4)
> -            dxt1_decode_pixels(gb, d, qstride, 0, 0LL);
> +    for (y=0; y < h; y += 4, d += stride-w)
> +        for (x = 0; x < w; d += FFMIN(4, w-x), x += 4)
> +            dxt1_decode_pixels(gb, d, FFMIN(4, w-x), FFMIN(4, h-y), qstride, 0, 0LL);
>  }

please dont rename variables in patches which contain functional
changes

also, can you share the sample that this patch fixes decoding for ?

Thanks

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Good people do not need laws to tell them to act responsibly, while bad
people will find a way around the laws. -- Plato
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