[FFmpeg-user] Need more info on blend filter

[compname at verizon.net]

At 6/21/2015 11:11 AM, you wrote:
>On 6/21/15, compname at verizon.net <compname at verizon.net> wrote:
> > I'm trying to overlay a spectrum analyzer display over another
> > video.  I want the analyzer bars to show on top of the other video,
> > but nothing else.  The spectrum analyzer video consists of blue bars
> > on a black background.  I tried the following filter:
> >
> > 
> [base_vid][spec]blend=all_expr='if(gte(B,10),B,A)':all_opacity=1,format=rgba[out]
> >
> > This "almost" works in that I get the desired effect more or less,
> > but there are some pixels on the edge of the blue bars that turn
> > different colors.  I think this is because the blend filter makes its
> > comparison on each channel individually.  The pixel format for
> > base_vid & spec is rgba.  In order to have a successful transparent
> > overlay, I need to have the comparison on B made on the luminance
> > only.  So I changed the pix_fmt for base_vid & spec to yuvj444p,
> > which then lets me compare c0 (luminance) in the blend filter.  The
> > problem is I need to have the luminance of B control the other 3
> > channels as well.  I don't see how this can be done given what's
> > given in the ffmpeg-filters manual.  What I really need is something like
> > this:
> >
> > 
> [base_vid][spec]blend=c0_expr='if(gte(B,10),B,A)':c1_expr='if(gte(c0,10),B,A)':c2_expr='if(gte(c0,10),B,A:c3_expr=(gte(c0,10),B,A:all_opacity=1,format=yuvj444p[out]
> >
> > (yes I did try it - doesn't work :(  )
> >
> > Is there a way to do this, or is blend limited to intra-channel
> > operations?  Thanks.
>
>Currently it is limited to intra-channel operations, maybe i will add
>feature to access other channels data too.

OK thanks.  I was beginning to suspect such, & started pursuing a 
different path: using the geq filter on the spectrum overlay video to 
selectively change the alpha on the black pixels to 0.

geq=r='r(X,Y)':g='g(X,Y)':b='b(X,Y)':a='if(gte(r(X,Y)+g(X,Y)+b(X,Y),50),255,0)'

Then I can replace the blend filter with a simple overlay.

Some simplification may be possible by using YUV space instead of 
RGB, but I wasn't sure if the alpha channel was available for the YUV 
formats.  I'll probably experiment more later, but in the meantime 
this seems to work great.