[FFmpeg-devel] Gamma function (was Re: [PATCH] lavc/pngenc: Support writing colorspace tags.)

[Kevin Wheatley]

On Sat, Feb 28, 2015 at 1:50 AM, Niklas Haas <git at nand.wakku.to> wrote:

> +static int png_get_gama(enum AVColorTransferCharacteristic trc, uint8_t *buf)
> +{
> +    double gamma;
> +    switch (trc) {
> +        case AVCOL_TRC_BT709:
> +        case AVCOL_TRC_SMPTE170M:
> +        case AVCOL_TRC_SMPTE240M:
> +        case AVCOL_TRC_BT1361_ECG:
> +        case AVCOL_TRC_BT2020_10:
> +        case AVCOL_TRC_BT2020_12:
> +            /* these share a segmented TRC, but gamma 1.961 is a close
> +              approximation, and also more correct for decoding content */
> +            gamma = 1.961;
> +            break;
> +        case AVCOL_TRC_GAMMA22:
> +        case AVCOL_TRC_IEC61966_2_1:
> +            gamma = 2.2;
> +            break;
> +        case AVCOL_TRC_GAMMA28:
> +            gamma = 2.8;
> +            break;
> +        case AVCOL_TRC_LINEAR:
> +            gamma = 1.0;
> +            break;
> +        default:
> +            return 0;
> +    }
> +
> +    AV_WB32_PNG(buf, 1.0 / gamma);
> +    return 1;
> +}
> +


Co-incidentally I was thinking of submitting a patch to write the old
QuickTime gama atom and most of this function would be useful if
extracted to a common place (everything but the AV_WB32_PNG() as
.mov's need a different scaling). Where should it go? libavutil?

As part of it I would also allow manually specifying a specific value
to encode on the command line, but I wondered about how to form the
command line options...

Case 1

User wants to write gamma based on TRC settings, using the above (now
extracted) function.

+write_gama on movflags?

Case 2

User wants to specify the actual gamma on the command line

Same as above, plus a  -gamma <double>

I'd suggest to require +write_gama on the command line as the gama
atom is deprecated and should really be only needed for backwards
compatibity/broken tools.

Comments?

Kevin