On Thu, 28 Sept 2023 at 17:25, David Miguel Susano Pinto
<carandr...@gmail.com> wrote:
> [...]
> Assuming YCbCr, where with the [-128 127] range black is:
>
> ycbcr([-128 0 0])
>
> I was expecting that zeroing all coefficients of all components and
> setting the DC of the first component to:
>
> -CENTERJSAMPLE / comp_info.quant_table[0].quantval[0]
>
> was the way to go but that's not it (I just get a slightly darker
> grey).
>
I found the following on jdct.h:
/*
[...]
* The DCT inputs are expected to be signed (range +-CENTERJSAMPLE).
* The DCT outputs are returned scaled up by a factor of 8; they therefore
* have a range of +-8K for 8-bit data, +-128K for 12-bit data. This
[...]
*/
and now, setting the DC of the first component to:
(-CENTERJSAMPLE * 8) / comp_info.quant_table[0].quantval[0]
seems to works fine but this hardcoded 8 seems a bit odd and I'm
wondering if it's specified somewhere else.
Many thanks
David
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