Convert a text file with R G B values [0-255] separated by whitespace, one triplet per line, to Photoshop's ACT format.

actout.py

#!/usr/bin/env python3
import os, struct, sys

if len(sys.argv) < 3:
    sys.exit(
        "Convert a text file with up to 256 RGB values to Photoshop's ACT format\n\n"
        "   Usage: " +sys.argv[0] + " <infile> <outfile>\n"
        "       <infile> is a text file with R G B values [0-255] separated by\n"
        "           whitespace, one triplet per line\n"
        "       <outfile> will be written in Photoshop's .ACT color format\n"
    )

infile, outfile = sys.argv[1], sys.argv[2]
transparencyColorIndex = 0

if os.path.exists(outfile):
    sys.exit("Cowardly refusing to overwrite " + outfile + "\n")

act = bytes(0)
with open(infile, "r") as f:
    for line in f:
        r, g, b = line.split(None,2) # Python's split is a bit odd.
        rgb = bytes([ int(r)%256, int(g)%256, int(b)%256 ])
        act += rgb

colorCount = len(act)//3
if colorCount > 256: # were there extra lines?
    colorCount = 256
if colorCount < 256: # do we need to pad the result with zeroes?
    act += bytes(768 - len(act))

with open(outfile, "wb") as f:
    f.write(act)
    f.write(bytes(struct.pack(">H", colorCount)))
    f.write(bytes(struct.pack(">H", transparencyColorIndex)))