[PD-dev] help with some new multichannel tools

[Christof Ressi]

Hi,

if the input and output signals have the same channel count, they will 
alias each other, just like in regular single-channel objects. In that 
case, your code may accidentally overwrite parts of the input before it 
is read. You first need to copy the whole input signal to a temporary 
buffer (probably allocated in your object) and then copy the 
corresponding channels to the output signal.

> ps. While we're at it, I don't get why we have both sp[0]->s_length 
> and sp[0]->s_n, aren't both the same?
s_length is just a more descriptive alias for s_n.

Christof

On 22.07.2023 20:52, Alexandre Torres Porres wrote:
> Hi, I'm creating new fun objects that are tools to deal with 
> multichannel signals to include in the next update of ELSE.
>
> I have a working "pick~" object that picks a single channel from a 
> multichannel connection. Of course you can set which channel with a 
> message.
>
> I'm now expanding this concept to remap or reorder a multichannel 
> signal, much like how you can reorder a list message in Pd with 
> dollar symbols ($1, $2, etc). It is called "remap~" for now and takes 
> floats or lists. A "0" means "none", so no channel is picked and we 
> get zeros. With a float input or argument it is just like "pick~".
>
> It is working well depending on the list I am sending, but somehow it 
> doesn't work for other lists.
>
> here's the "dsp" code.
>
> *static* *void* remap_dsp(t_remap *x, t_signal **sp){
>
> signal_setmultiout(&sp[1], x->x_n);
>
> *int* length = sp[0]->s_length;
>
> *for*(*int* i = 0; i < x->x_n; i++){
>
> *int* ch = x->x_vec[i].a_w.w_float;
>
> *if*(ch > sp[0]->s_nchans)
>
> ch = sp[0]->s_nchans;
>
> *if*(ch >= 0)
>
> dsp_add_copy(sp[0]->s_vec + ch*length, sp[1]->s_vec + i*length, length);
>
> *else*
>
> dsp_add_zero(sp[1]->s_vec + i*length, length);
>
> }
>
> }
>
>
>
> If I print the "ch" values and ch*length or i_lenght I get the 
> expected values, so I can't understand what goes wrong.
>
> For instance, if I have a 4 channel connection input with values "1 2 
> 3 4" and send it the list "1 2 3 2 1" I get the correct expected 
> output of "1 2 3 2 1". If I send it "2 1", output is the same! Single 
> float values also work as mentioned, now for some bugged outputs...
>
> "3 2 1" gives me "3 2 *3*"
> "1 1 2" gives me "1 1 *1*"
> "1 2 2 3" gives me "1 2 2 *2*"
>
> funny enough, something like "1 2 2 2 3" gives me the same correct output.
>
> I can't see a pattern here. I need help from the wizards to please 
> save me. Test patch attached. Code lives here 
> https://github.com/porres/pd-else/blob/master/Code_source/Compiled/signal/remap~.c
>
> thanks
>
> ps. While we're at it, I don't get why we have both sp[0]->s_length 
> and sp[0]->s_n, aren't both the same?
>
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