[Libav-user] Questions with avf_showspectrum.c

[Ricky Huang]

Thank you Clement, your answers helped.

On May 15, 2014, at 12:05 AM, Clément Bœsch <u at pkh.me> wrote:

> On Wed, May 14, 2014 at 06:54:45PM -0700, Ricky Huang wrote:
>> Hello all,
>> 
>> I am currently examining avf_showspectrum.c and I have a few questions that I am hoping you guys can clarify for me:
>> 
>> (This is the command I am running, BTW: ./ffmpeg -i input.mp3 -filter_complex showspectrum=mode=separate:color=intensity:slide=0:scale=cbrt -y output.avi)
>> 
>> 1)  First it is about the output: I am assuming the plotted image has time horizontally, the frequency range vertically, and the intensity of color as the amplitude.  Is that correct?
>> 
> 
> Yes
> 
>> 2)  In plot_spectrum_column() function, the code copies some amount of the data to the rdft_data:
>>>  s->rdft_data[ch][start + n] = p[n] * s->window_func_lut[start + n];
>> 
>> and then performs a av_rdft_calc() on the data.  But it's never clear the s->rdft_data array, does this mean the plot is cumulative over time?
>> 
> 
> see s->filled, which represents the amount of data loaded in rdft_data. It
> doesn't need to be cleared, the buffer content will be replaced, and when
> it's fully replaced a new av_rdft_calc() will occur. Every sample will be
> used only once, no accumulation.
> 
>> 
>> Thanks in advance.
> 
> -- 
> Clément B.
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