Re: [FFmpeg-devel] [PATCH] avutil/pixfmt: fix AV_PIX_FMT_RGB8 description

[Jeffrey Knockel <jeff@jeffreyknockel.com>]

On 1/14/24 6:32 AM, Stefano Sabatini wrote:
> On date Monday 2024-01-08 09:53:37 -0500, Jeffrey Knockel wrote:
>> Previously the description was partially mistaken, explaining the format
>> as RGB 3:3:2, (msb)2R 3G 3B(lsb).  While the RGB 3:3:2 part is correct,
>> the latter part should be: (msb)3R 3G 2B(lsb). The corresponding bit
>> masks are red: 0xE0, green: 0x1C, blue: 0x03.
>>
>> Signed-off-by: Jeffrey Knockel <jeff@jeffreyknockel.com>
>> ---
>>  libavutil/pixfmt.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/libavutil/pixfmt.h b/libavutil/pixfmt.h
>> index 58f9ad28bd..9c87571f49 100644
>> --- a/libavutil/pixfmt.h
>> +++ b/libavutil/pixfmt.h
>> @@ -83,7 +83,7 @@ enum AVPixelFormat {
>>      AV_PIX_FMT_BGR8,      ///< packed RGB 3:3:2,  8bpp, (msb)2B 3G 3R(lsb)
>>      AV_PIX_FMT_BGR4,      ///< packed RGB 1:2:1 bitstream,  4bpp, (msb)1B 2G 1R(lsb), a byte contains two pixels, the first pixel in the byte is the one composed by the 4 msb bits
>>      AV_PIX_FMT_BGR4_BYTE, ///< packed RGB 1:2:1,  8bpp, (msb)1B 2G 1R(lsb)
> 
>> -    AV_PIX_FMT_RGB8,      ///< packed RGB 3:3:2,  8bpp, (msb)2R 3G 3B(lsb)
>> +    AV_PIX_FMT_RGB8,      ///< packed RGB 3:3:2,  8bpp, (msb)3R 3G 2B(lsb)
> 
> This is not consistent with pixdesc definition:
>     [AV_PIX_FMT_RGB8] = {
>         .name = "rgb8",
>         .nb_components = 3,
>         .log2_chroma_w = 0,
>         .log2_chroma_h = 0,
>         .comp = {
>             { 0, 1, 0, 6, 2 },        /* R */
>             { 0, 1, 0, 3, 3 },        /* G */
>             { 0, 1, 0, 0, 3 },        /* B */
>         },
>         .flags = AV_PIX_FMT_FLAG_RGB,
>     },
> 
> so either the documentation or the pixdesc description is wrong.
> 
> Do you confirm "(msb)3R 3G 2B(lsb)" is the correct definition?

Yes, I created three solid-colored RGBA raw samples filled with either #ff0000ff, #00ff00ff, or #0000ffff, and converted them using `ffmpeg ... -pix_fmt rgba ... -pix_fmt rgb8 ...` and inspected the output.  I also tested with `ffmpeg ... -pix_fmt rgba ... -pix_fmt yuv420p ...` followed by `ffmpeg ... -pix_fmt yuv420p ... -pix_fmt rgb8 ...`.  In these tests the outputs were consistent with "(msb)3R 3G 2B(lsb)".

> In this case pixdesc should be fixed accordingly.

Got it, will send an updated patch!

Thanks,
Jeff
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