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
<por...@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
<por...@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
<por...@gmail.com> escreveu:
Em sáb., 22 de jul. de 2023 às 18:39, Christof Ressi
<i...@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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