[PD-dev] Fwd: help with some new multichannel tools

[Christof Ressi]

Forgot to send to the list...

---

Buffer aliasing means that the input and output signal might point to 
the same buffer. This is done to minimize memory usage and increase 
cache friendliness. In practice, this means that processing happens /in 
place/. You must not write to any output before reading the 
/corresponding /inputs. For most objects this is not a problem because 
for each sample you'd typically read the input(s) and then calculate the 
output(s).

Problems arise when you want to write a whole output /block /before 
processing/copying all the inputs. Here's a simple example: Say you send 
the list [2 1( to your [remap~] object. In your original "naive" 
version, you would copy the second input channel to the first output 
channel (2 -> 1). However, since input and output alias each other, you 
have in fact overwritten the first input channel! Now, when you go on to 
copy the first input channel to the second output channel (1 -> 2), you 
get unexpected values.

Side note: why do your channel indexes start at 1? Pd uses zero-based 
indexes for almost anything, with only few exceptions (ADC/DAC and MIDI 
channels, anything else?).

Christof

On 23.07.2023 05:59, Alexandre Torres Porres wrote:
>
>
> On Sun, 23 Jul 2023 at 00:10 Alexandre Torres Porres 
> <porres at gmail.com> wrote:
>
>     I see now I was just doing something silly thinking I was trying
>     to copy some sophisticated trick that would sove things.
>
>     I can't remember now which object I had to deal with something
>     like this.
>
>
>
> Found one, copied the structure, it works! I still have no good idea 
> on how things work, but hey, I got it working and I can sleep
>
>
>
> Thanks
>
>
>
>
>     anyway, in this last version, I have my own "perform" method that
>     fails equally as before in some cases, but it is not as weird as
>     before...
>
>     let me just copy it instead of linking to a github code that might
>     change, here's the code section
>
>     *static*t_int*remap_perform(t_int *w){
>
>     t_remap *x = (t_remap *)(w[1]);
>
>     t_int n = (t_int)(w[2]);
>
>     t_int nchans = (t_int)(w[3]);
>
>     t_sample *in = (t_sample *)(w[4]);
>
>     t_sample *out = (t_sample *)(w[5]);
>
>     *for*(*int* i = 0; i < x->x_n; i++){ // channels to copy
>
>     *int* ch = x->x_vec[i].a_w.w_float - 1; // get channel number
>
>     *if*(ch >= nchans)
>
>     ch = nchans - 1;
>
>     *for*(*int*j = 0; j < n; j++){ // j is sample number of each channel
>
>     *if*(ch >= 0)
>
>     *out++ = in[ch*n + j];
>
>     *else*
>
>     *out++ = 0;
>
>     }
>
>     }
>
>     *return*(w+6);
>
>     }
>
>
>     Em sáb., 22 de jul. de 2023 às 21:22, Alexandre Torres Porres
>     <porres at gmail.com> escreveu:
>
>         ok, I was able to compile a version that does not blow pd up
>
>         obviously I have no idea of what I am doing and I did some
>         things based on warnings I was getting, but then, I do copy
>         the input first and then reorder the channels but I do get the
>         exact same result as before, so it was all for nothing.
>
>         here's my new attempt, hopefully not too far from getting
>         things right
>         https://github.com/porres/pd-else/blob/master/Code_source/Compiled/signal/remap~.c#L34
>
>         cheers
>
>         Em sáb., 22 de jul. de 2023 às 21:02, Alexandre Torres Porres
>         <porres at gmail.com> escreveu:
>
>
>
>             Em sáb., 22 de jul. de 2023 às 18:39, Christof Ressi
>             <info at christofressi.com> escreveu:
>
>                 if the input and output signals have the same channel
>                 count, they will alias each other, just like in
>                 regular single-channel objects.
>
>             actually, funny stuff happens for 4 multichannel input and
>             3 multichannel output, but I've seen that issue with
>             single channel objects and I tried copying stuff from
>             other objects in ELSE into this multichannel idea and failed.
>
>                 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.
>
>
>             I wonder if you can give me a better detailed strategy/steps.
>             thanks
>
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