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

[Leo Izen <leo.izen@gmail.com>]

On 3/15/23 21:32, Andreas Rheinhardt wrote:
> 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@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?
> 

Good catch, I missed a call to bytestream2_get_bytes_left.

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