Despite reading several posts in the archive and the source code in
g_all_guis.c I am embarrassed to say that I am completely stumped by the
problem of extracting the individual RGB components from the IEM saved
color value in the .pd file, as ints between 0 and 255.
as a reference, [#to_iem] supports both iem colour formats, the 18-bit
format for files, and the 24-bit format for messages. Such a patch is very
But it does the opposite conversion of what you are trying to do.
What I am doing (in the case where the saved value is negative) is:
iemcolor = -1 - iemcolor;
r = (iemcolor & 0x3f000) >> 14;
g = (iemcolor & 0xfc0) >> 4;
b = (iemcolor & 0x3f) << 2;
this is the 18 bit iem format that you are trying to convert to a 3*8 bit
component format. You are correctly extracting blue and green, where the
shift is 0*6-2 for blue, 1*6-2 for green, but it has to be 2*6-2 for red.
For the 24 bit iem format, instead, you have to shift by 0*8, 1*8, 2*8.
-2 is the difference between 6 bits per component and 8 bits per
Note that those conversions scale in a crude way, such that 63 is scaled
to 252, instead of the maximum 255. If you want to scale more
appropriately to fill the whole range, you need to post-process those
components a little bit(s) :