FFT questions

beginners
#1

Could I please ask a few questions about fft?

#1. when i draw the spectrum with this in draw()
plot(fft.getBins(), 128);

and in plot()
for (int i=0; i <n; i++) {
ofVertex (i, buffer[i]);

this is drawing all the frequency data as it is coming in, right?

and if I do like this and change the variable n to a # (like 200)
for (int i=0; i <200; i++) {
ofVertex (i, buffer[i]);

that is going to draw the frequency data coming in from a portion of the bins, in this case 0-200, right?

#2 If I want to get the amplitude of a bin, and save that number as a variable to use in drawing a shape I can do like this:
for (int i = 0; i<200; i++) {
float a =fft.fft->getAmplitudeAtBin(buffer[i]);}

and then ‘a’ will give me some number that is the amplitude of bin 0-200, right?

Because i tried that but when i try to draw a shape, using ‘a’ as a point on a vertex, it seems like ‘a’ is always 0. is that what is happening? I guess I’m not sure about if I do something like this what range of number can I expect to be in a?

#3 what do you recommend for other ways to get a series of variables that come from the sound input that i can then use to draw shapes?

ideally i want to have a series of floats (a,b,c) that I can fill with numbers from the sound input and fft and then i can use those to create shapes that will move (as the sound input changes)

I saw in a post that I can also use getBinFromFrequency and GetAmplitudeAtFrequency, do you think those would be helpful for my purpose? If so, I would need a little help to figure out how to syntax it correctly and figure out what kind of range of number to expect.

thank you very much

#2

for #1 it should look something like:

  
  
	ofBeginShape();  
	for (int i = 0; i < n; i++) {  
		ofVertex(i, buffer[i] * scale);  
	}  
	ofEndShape();

this comes from example-basic inside ofxFft.

for #2, you should say something like:

  
  
float a = fft->getAmplitudeAtBin(100); // get amplitude at bin 100

or if you have an amplitude buffer like in example-basic, you would say:

  
  
float a = buffer[100]; // get amplitude at bin 100

in response to #3, the only suggestion i have is to use small numbers for bins, like 0-50. everything else is higher frequency, and doesn’t correspond to the sounds we normally are thinking about.