[devel] Revised commentary about png_rgb_to_gray coefficents

and fixed spelling (truely -> truly) in comments.

pngrtran.c

@@ -3093,6 +3093,11 @@ png_do_gray_to_rgb(png_row_infop row_info, png_bytep row)
  *
  *     Y = (6968 * R + 23434 * G + 2366 * B)/32768
  *
+ *  Starting with libpng-1.5.4, if the image being converted has the
+ *  sRGB chunk, then the sRGB numbers are used by default:
+ *
+ *     Y = 0.33000*R + 0.60000*G + 0.06000*B
+ *
  *  The calculation is to be done in a linear colorspace.
  *
  *  Other integer coefficents can be used via png_set_rgb_to_gray().

pngset.c

@@ -99,7 +99,7 @@ png_set_gAMA_fixed(png_structp png_ptr, png_infop info_ptr, png_fixed_point
     * possible for 1/gamma to overflow the limit of 21474 and this means the
     * gamma value must be at least 5/100000 and hence at most 20000.0.  For
     * safety the limits here are a little narrower.  The values are 0.00016 to
-    * 6250.0, which are truely ridiculous gammma values (and will produce
+    * 6250.0, which are truly ridiculous gammma values (and will produce
     * displays that are all black or all white.)
     */
    if (file_gamma < 16 || file_gamma > 625000000)

pngvalid.c

@@ -2104,7 +2104,7 @@ modifier_progressive_read(png_modifier *pm, png_structp pp, png_infop pi)
       png_error(pp, "store state damaged (progressive)");
 
    /* This is another Horowitz and Hill random noise generator.  In this case
-    * the aim is to stress the progressive reader with truely horrible variable
+    * the aim is to stress the progressive reader with truly horrible variable
     * buffer sizes in the range 1..500, so a sequence of 9 bit random numbers
     * is generated.  We could probably just count from 1 to 32767 and get as
     * good a result.