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spectrogram: add -n flag to normalise the output to maximum brightness

This change adds a "normalize" flag, -n, to sox spectrogram to adjust
the spectrogram gain such that the highest values get the brightest
colours, allowing one to get uniform spectrograms regardless of
different volumes in music files.

Tested-by: Eric Wong <normalperson@yhbt.net>

--- a/sox.1

+++ b/sox.1

@@ -3235,6 +3235,10 @@

 .I num

 effectively increases the `brightness' of the spectrogram display,

 and vice versa.

+.IP \fB\-n\fR

+Sets the upper limit of the Z axis so that the loudest pixels

+are shown using the brightest colour in the palette - a kind of

+automatic \fB\-Z\fR flag.

 .IP \fB\-q\ \fInum\fR

 Sets the Z-axis quantisation, i.e. the number of different colours (or

 intensities) in which to render Z-axis

--- a/src/spectrogram.c

+++ b/src/spectrogram.c

@@ -61,7 +61,7 @@

   double     pixels_per_sec, window_adjust;

   int        x_size0, y_size, Y_size, dB_range, gain, spectrum_points, perm;

   sox_bool   monochrome, light_background, high_colour, slack_overlap, no_axes;

-  sox_bool   raw, alt_palette, truncate;

+  sox_bool   normalize, raw, alt_palette, truncate;

   win_type_t win_type;

   char const * out_name, * title, * comment;

   char const *duration_str, *start_time_str;

@@ -115,7 +115,7 @@

   char const * next;

   int c;

   lsx_getopt_t optstate;

-  lsx_getopt_init(argc, argv, "+S:d:x:X:y:Y:z:Z:q:p:W:w:st:c:AarmlhTo:", NULL, lsx_getopt_flag_none, 1, &optstate);

+  lsx_getopt_init(argc, argv, "+S:d:x:X:y:Y:z:Z:q:p:W:w:st:c:AarmnlhTo:", NULL, lsx_getopt_flag_none, 1, &optstate);

   p->dB_range = 120, p->spectrum_points = 249, p->perm = 1; /* Non-0 defaults */

   p->out_name = "spectrogram.png", p->comment = "Created by SoX";

@@ -136,6 +136,7 @@

     case 'a': p->no_axes          = sox_true;   break;

     case 'r': p->raw              = sox_true;   break;

     case 'm': p->monochrome       = sox_true;   break;

+    case 'n': p->normalize        = sox_true;   break;

     case 'l': p->light_background = sox_true;   break;

     case 'h': p->high_colour      = sox_true;   break;

     case 'T': p->truncate         = sox_true;   break;

@@ -559,6 +560,7 @@

   int         i, j, k, base, step, tick_len = 3 - p->no_axes;

   char        text[200], * prefix;

   double      limit;

+  float       autogain = 0.0;	/* Is changed if the -n flag was supplied */

   free(p->shared);

   if (p->using_stdout) {

@@ -585,8 +587,22 @@

     png_rows[rows - 1 - j] = (png_bytep)(pixels + j * cols);

   /* Spectrogram */

+

+  if (p->normalize)

+    /* values are already in dB, so we subtract the maximum value

+     * (which will normally be negative) to raise the maximum to 0.0.

+     */

+    autogain = -p->max;

+

   for (k = 0; k < chans; ++k) {

     priv_t * q = (priv_t *)(effp - effp->flow + k)->priv;

+

+    if (p->normalize) {

+      float *fp = q->dBfs;

+      for (i = p->rows * p->cols; i > 0; i--)

+	*fp++ += autogain;

+    }

+

     base = !p->raw * below + (chans - 1 - k) * (p->rows + 1);

     for (j = 0; j < p->rows; ++j) {

       for (i = 0; i < p->cols; ++i)

@@ -653,7 +669,7 @@

     step = 10 * ceil(p->dB_range / 10. * (font_y + 2) / (k - 1));

     for (i = 0; i <= p->dB_range; i += step) {           /* (Tick) labels */

       int y = (double)i / p->dB_range * (k - 1) + .5;

-      sprintf(text, "%+i", i - p->gain - p->dB_range);

+      sprintf(text, "%+i", i - p->gain - p->dB_range - (int)(autogain+0.5));

       print_at(cols - right + 1, base + y + 5, Labels, text);

     }

   }

@@ -694,6 +710,7 @@

     "\t-Y num\tY-height total (i.e. not per channel); default 550",

     "\t-z num\tZ-axis range in dB; default 120",

     "\t-Z num\tZ-axis maximum in dBFS; default 0",

+    "\t-n\tSet Z-axis maximum to the brightest pixel",

     "\t-q num\tZ-axis quantisation (0 - 249); default 249",

     "\t-w name\tWindow: Hann(default)/Hamming/Bartlett/Rectangular/Kaiser/Dolph",

     "\t-W num\tWindow adjust parameter (-10 - 10); applies only to Kaiser/Dolph",