Legacy ImageMagick Discussions Archive

Thanks to all. The displacement map optimization, making a 1xheight DM and replicating it to widthxheight, reduced the processing time from 504 seconds to 66 for real-world images.

For those who are interested, the modified Makefile fragment is

time sndfile-spectrogram --dyn-range=$(DYN_RANGE ...

convert -size $(WIDTH)x$(LOG_HEIGHT) xc: pattern.png \
-virtual-pixel White \
-interpolate NearestNeighbor \
-fx "freq = $(MIN_FREQ_OUT) * pow($(MAX_FREQ_OUT) / $(MIN_FREQ_OUT), ($(MAX_Y_OUT) - j) / $(MAX_Y_OUT)); \
yy = $(MAX_Y_IN) - freq * $(MAX_Y_IN) / $(MAX_FREQ_IN); \
v.p{i,yy ...

Yes. log(freq-50) goes to log(0), not log(1).

I'll spare you the workings, but this seems to be the correct reverse mapping:

y_in = (((exp(y_out / max_y_out) - 1) / (base - 1)) * (max_freq_out - min_freq_out) + min_freq_out) * max_y_in / max_freq_in

for which a test run from 0 to 1023 step 93 ...

Oh. I just tried that:
$ cat >> logaxis.c << \EOF
#include <stdlib.h>
#include <stdio.h>
#include <math.h>

main(int argc, char **argv)
{
int max_freq_in = 22050;
int max_y_in = 8191;
double min_freq_out = 50;
double max_freq_out = 3150;
int max_y_out = 1024;
int y;

printf(" Out In\n ...

[quote="martinwguy"]i.e. to map from input pixel coords to output pixel coords it would be

Inlining freq and rearranging that...

y_out = log(((y_in * max_freq_in / y_max_in) - 50) * base / (3150 - 50)) * y_max_out
exp(y_out / y_max_out) = ((y_in * max_freq_in / y_max_in) - 50) * base / (3150 - 50 ...

i.e. to map from input pixel coords to output pixel coords it would be

freq = y_in * max_freq_in / y_max_in
if freq < 50 or freq > 3150 skip
y_out = log((freq - 50) * base / (3150 - 50)) * y_max_out

?

ok, (50 gives 1 and 3150 gives base) logs to 0..1

try
(freq - 50) * base / (3150 - 50)

when freq == 50, this gives 0
when freq == 3150, this gives base * (3150 - 50) / (3150 - 50)
(3150 - 50) / (3150 - 50) = 1 so that times base gives base.

Does that sound right?

> y_out = (log(freq/50)/log(base * freq/3150)) {gives 0..1

this was also garbage. the top of the division is 0 at f=50 and the bottom is 1.0 at f=3150, but the result of the division is eomething else.

flail, flail

> going from pixel 0 to pixel y_max_out, each step should add (log(freq/50)/log()/y_max_out

This was garbage, please ignore

M

OK, the math is this:

the input is a borderless WxH spectrogram as per above, where the vertical axis represents from 0 to 22500Hz linearly.
the output should have a logarithmic vertical axis from 50Hz to 3150 (well, log_min and log_max, let's say, and min_freq_in and max_freq_in for those others ...

Thanks. I'm still deciding whether it's best to do this or modify the spectrogram program, which already has an unimplemented --log-freq option.
But I'll take your idea into account. I had wondered if such a thing were doable, having seen passing references to it in the docs

Cheers, I'll let you ...

Many thanks for the reply and pointers into the docs.

The x axis of the spectrogram represents time progressing through the piece of music (2m56s or 176 seconds in the example image) and the y axis is frequency from 0 (at the bottom) to 22050Hz (at the top) while the brightness of each pixel in any ...

Hi!
Can anyone suggest how to perform the following distortion with IM?
I have a spectrogram of a piece of music (produced by sndfile-spectrogram) that has a linear frequency axis, which means that adjacent semitones are close and closer together as you go into the bass range, while the top half ...