Image RGB to CMYK conversion
Christophe
I have a number of images in RGB-format (not created by Matlab) that I need to convert to CMYK.
Is there a simple script to:
read image_in_RGB.jpg (or .png, or, ...)
convert RGB-to-CMYK and then save image_in_CMYK.jpg
Help would be very much appreciated...
PS: I only have the "standard" matlab and none of the toolboxes...
Bruno Luong
Anyway, the only output format that supports CMYK is tiff. See help of IMWRITE. I found some "generic" conversion formula between RGB and CMYK (both are 0-1 based):
% Black = minimum(1-Red,1-Green,1-Blue)
% Cyan = (1-Red-Black)/(1-Black)
% Magenta = (1-Green-Black)/(1-Black)
% Yellow = (1-Blue-Black)/(1-Black)
Up to you to make the conversion and invoke IMWRITE
Bruno
Dave Robinson
As far as I am aware, the primary use for CMYK colour space is in printing, and there the main reasons for its use is that Black (key) ink is much cheaper than the 3 sets of coloured ink that you would need to make up the black component; and also because of the poor colourimetric purity of the inks, equal quantities of CYM usually make a muddy brown and not black, so adding an extra black ink significantly improves the rendition of the picture, when printed.
The relationship that Bruno has given is, as he states, only one of the probably infinite number of of transforms. In the print industry they have another parameter which is called the under cover removal UCR) ratio, which represents the amount of black (key) that is to be used to replace equal quantities of CM & Y, thus having no key and having a murky brown image represents a UCR of 0, and the equation that Bruno has provided you is equivelent to a UCR of 1, where the maximum amount of coloured inks are exchanged for black (key). Invariably the value of UCR selected is very ink/printing machine dependent.
Hope that helps
Regards
Dave Robinson
ImageAnalyst
are
1. A nice collection of "book" formulas for a variety of color space
conversions http://www.easyrgb.com/index.php?X=MATH However, I warn
you that none of the "book" formulas will work accurately in a real
world environment because they just use default values. To get
accurate conversions you would have to accurately characterize your
imaging pipeline. Sometimes devices (displays, printers, scanners)
install their own custom ICC profiles and the operating system is
usually aware of those and will use them to try to do its best. You
can do even better if you really want to put the effort into it.
2. See Steve's blog where he just talked about this:
http://blogs.mathworks.com/steve/2009/09/09/many-steps-needed-for-some-color-space-conversions/
3. Maybe you should just use Photoshop. Why do you think MATLAB is
preferable for this task? I think far, far more people in the
printing/publishing industry use Photoshop for RGB/CMYK separations
than MATLAB. You can easily program up a macro in Photoshop to do
what you say.
4. Who (what program) is going to be the consumer of the cmyk file you
write out? If you read it back in with a program that expects jpg
images to be RGB then it will look very strange. The program has to
be aware that you wrote it out in that color space and not RGB.
Just some thoughts........
Christophe
A bit of background might help: I am a scientist by day and freelance cartoonist by night and my agent has suddenly asked me to provide my cartoons in CMYK instead of RGB as the newspapers are asking for it now (after 10 years without a problem :( ). My problem is that my version of Paint Shop Pro does not do the RGB to CMYK conversion in one operation. It splits the files in 4 files and it is ugly...
I thought about using Matlab to do a simple load, save as CMYK, voila...
Unfortunately, what I wrote is:
cartoon = imread('cartoon1401_colour.tif');
Output_File_Name = 'cartoon1401_colour_CMYK';
image(cartoon);
axis off
axis equal
axis tight
print('-dtiff','-cmyk','-r600',Output_File_Name);
imfinfo('cartoon1401_colour_CMYK.tif')
I was expecting the output to be CMYK as I specify the '-cmyk' flag, but the info from "imfinfo" says it is still RGB and this is what I don't understand:
ans =
Filename: 'cartoon1401_colour_CMYK.tif'
FileModDate: '15-Sep-2009 07:30:14'
FileSize: 21467516
Format: 'tif'
FormatVersion: []
Width: 4804
Height: 3602
BitDepth: 24
ColorType: 'truecolor'
FormatSignature: [73 73 42 0]
ByteOrder: 'little-endian'
NewSubFileType: 0
BitsPerSample: [8 8 8]
Compression: 'PackBits'
PhotometricInterpretation: 'RGB'
StripOffsets: [3602x1 double]
SamplesPerPixel: 3
RowsPerStrip: 1
StripByteCounts: [3602x1 double]
XResolution: 600
YResolution: 600
ResolutionUnit: 'Inch'
Colormap: []
PlanarConfiguration: 'Chunky'
TileWidth: []
TileLength: []
TileOffsets: []
TileByteCounts: []
Orientation: 1
FillOrder: 1
GrayResponseUnit: 0.0100
MaxSampleValue: [255 255 255]
MinSampleValue: 0
Thresholding: 1
ImageDescription: 'MATLAB Handle Graphics'
Mayby, as you say, I should just bite the bullet and buy photoshop... It is a $1000 though :(
Dave Robinson
Have another word with your publisher - In my experience most printers want the image in the form of 4 monochrome image separations exactly as Paintshop Pro is giving you.
Remember that these separations cannot be viewed in a combined form directly on your monitor which only deals with a three plane image which is based on R,G and B pixels, not a 4 plane image. (You would need some form of display with cyan, magenta and yellow pixels. I believe such monitors are available for specialist applications).
You can simulate the printing press by converting your black (key) plane to equal quantities of cyan, magenta and yellow - then add this to the respective planes, then subtract the result from 1 (if you are working in doubles) or 255 (if you still are in uint8 format), then display as an RGB image. A bit of a long winded operation when Paintshop Pro will recombine your CMYK image back to RGB.