[FFmpeg-devel] [PATCH v2 1/2] avcodec/pngdec: support sBIT chunks

[Andreas Rheinhardt]

Leo Izen:
> Add support for reading sBIT chunks, which mark the significant
> bit depth of the PNG file. This passes the metadata using the field
> bits_per_raw_sample of AVCodecContext.
> 
> Signed-off-by: Leo Izen <leo.izen at gmail.com>
> ---
>  libavcodec/pngdec.c  | 45 ++++++++++++++++++++++++++++++++++++++++++++
>  libavcodec/version.h |  2 +-
>  2 files changed, 46 insertions(+), 1 deletion(-)
> 
> diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
> index 9403d72f2f..3c91896c9b 100644
> --- a/libavcodec/pngdec.c
> +++ b/libavcodec/pngdec.c
> @@ -98,6 +98,7 @@ typedef struct PNGDecContext {
>      int bpp;
>      int has_trns;
>      uint8_t transparent_color_be[6];
> +    int significant_bits;
>  
>      uint32_t palette[256];
>      uint8_t *crow_buf;
> @@ -716,6 +717,14 @@ static int populate_avctx_color_fields(AVCodecContext *avctx, AVFrame *frame)
>      avctx->colorspace = frame->colorspace = AVCOL_SPC_RGB;
>      avctx->color_range = frame->color_range = AVCOL_RANGE_JPEG;
>  
> +    /*
> +     * tRNS sets alpha depth to full, so we ignore sBIT if set.
> +     * As a result we must wait until now to set
> +     * avctx->bits_per_raw_sample in case tRNS appears after sBIT
> +     */
> +    if (!s->has_trns && s->significant_bits > 0)
> +        avctx->bits_per_raw_sample = s->significant_bits;
> +
>      return 0;
>  }
>  
> @@ -996,6 +1005,38 @@ fail:
>      return ret;
>  }
>  
> +static int decode_sbit_chunk(AVCodecContext *avctx, PNGDecContext *s,
> +                             GetByteContext *gb)
> +{
> +    int bits = 0;
> +    int channels;
> +
> +    if (!(s->hdr_state & PNG_IHDR)) {
> +        av_log(avctx, AV_LOG_ERROR, "sBIT before IHDR\n");
> +        return AVERROR_INVALIDDATA;
> +    }
> +
> +    if (s->pic_state & PNG_IDAT) {
> +        av_log(avctx, AV_LOG_ERROR, "sBIT after IDAT\n");
> +        return AVERROR_INVALIDDATA;
> +    }
> +
> +    channels = ff_png_get_nb_channels(s->color_type);
> +
> +    for (int i = 0; i < channels; i++) {
> +        int b = bytestream2_get_byteu(gb);

What ensures that there is enough input left?

> +        bits = FFMAX(b, bits);
> +    }
> +
> +    if (bits < 0 || bits > s->bit_depth) {
> +        av_log(avctx, AV_LOG_ERROR, "Invalid significant bits: %d\n", bits);
> +        return AVERROR_INVALIDDATA;
> +    }
> +    s->significant_bits = bits;
> +
> +    return 0;
> +}
> +
>  static void handle_small_bpp(PNGDecContext *s, AVFrame *p)
>  {
>      if (s->bits_per_pixel == 1 && s->color_type == PNG_COLOR_TYPE_PALETTE) {
> @@ -1440,6 +1481,10 @@ static int decode_frame_common(AVCodecContext *avctx, PNGDecContext *s,
>  
>              break;
>          }
> +        case MKTAG('s', 'B', 'I', 'T'):
> +            if ((ret = decode_sbit_chunk(avctx, s, &gb_chunk)) < 0)
> +                goto fail;
> +            break;
>          case MKTAG('g', 'A', 'M', 'A'): {
>              AVBPrint bp;
>              char *gamma_str;
> diff --git a/libavcodec/version.h b/libavcodec/version.h
> index da54f87887..39dbec0208 100644
> --- a/libavcodec/version.h
> +++ b/libavcodec/version.h
> @@ -30,7 +30,7 @@
>  #include "version_major.h"
>  
>  #define LIBAVCODEC_VERSION_MINOR   6
> -#define LIBAVCODEC_VERSION_MICRO 100
> +#define LIBAVCODEC_VERSION_MICRO 101
>  
>  #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \
>                                                 LIBAVCODEC_VERSION_MINOR, \