Combining 4 separate monochrome tiffs into 1 CMYK tiff

Question

I have tiff images stored in such a way that i have each plane(color) stored in a separate file. Each file(C,M,Y,K) is a chunky tiff stored as a monochrome 8 bits per pixel file.

I want to combine these 4 files into one CMYK colored tiff using the python Imaging library(PIL)

This is the code I have so far but the output tiff produced is not correct, the tiff is being combined into a file that is mostly just black. I have merged these files with another utility and the result is correct, so I know there isnt a problem with the input files.

This is the code I have so far:

if len(sys.argv) <= 1:
    print "ERROR: Usage !"
    exit(1)

try:
    cFile = str(sys.argv[1])+"Cyan.tif"
    mFile = str(sys.argv[1])+"Magenta.tif"
    yFile = str(sys.argv[1])+"Yellow.tif"
    kFile = str(sys.argv[1])+"Black.tif"

    print "Opening files:"
    print cFile
    print mFile
    print yFile
    print kFile

    c_img = Image.open(cFile)
    c_img = c_img.convert("L")

    m_img = Image.open(mFile)
    m_img = m_img.convert("L")

    y_img = Image.open(yFile)
    y_img = y_img.convert("L")

    k_img = Image.open(kFile)
    k_img = k_img.convert("L")

except Exception, e:
    print "ERROR: Unable to open file..."
    print str(e)
    exit(1)
try:
    mergedRaster = Image.merge('CMYK', (c_img, m_img, y_img, k_img))
    mergedRaster = mergedRaster.convert("CMYK")

except Exception, e:
    print "ERROR: Merging plates"
    print str(e)
    exit(0)
#exit(0)
try:
    mergedRaster.save("output.tif", format="TIFF")

except Exception, e:
    print "ERROR: Writing tiff"

NOTE: I have done the same without any of the .convert functions and found the result to be the same.

Answer 1

I found the solution to be that all the values in the separated files needed to be inverted, i.e. 255 - value.

Converting each file to a numpy array and then subtracting it from 255.\

try:
    cArray = numpy.array(c_img)
    mArray = numpy.array(m_img)
    yArray = numpy.array(y_img)
    kArray = numpy.array(k_img)
except Exception, e:
    print "ERROR: Converting to numpy array..."
    print str(e)
    exit(1)

try:
    toSub = 255
    cInvArray = toSub - cArray
    mInvArray = toSub - mArray
    yInvArray = toSub - yArray
    kInvArray = toSub - kArray

except Exception, e:
    print "ERROR: inverting !"
    print str(e)

try:
    cPlate = Image.fromarray(cInvArray)
    mPlate = Image.fromarray(mInvArray)
    yPlate = Image.fromarray(yInvArray)
    kPlate = Image.fromarray(kInvArray)

except Exception, e:
    print "ERROR: Creating image from numpy arrays"
    print str(e)
    exit(1)

try:
    mergedRaster = Image.merge('CMYK', (cPlate, mPlate, yPlate, kPlate))

I don't know why this was required, but if someone can explain it that would be great.