Thanks Mike I think I need to clarify what im trying to acheive.
Im actually printing with a 6 colour inkjet printer using disperse dyes with profiles i have made. The Gamut of disperse dyes is about 70% of standard CMYK except where we use spot colours. Therefore the RIP is setup using Perceptual Rendering intent. This is 'safe' but not good for spot matching. Since I need to match Spot colours accurately you would think I would use Relative Colourmetric intent. However I often have images and vectors that must use the same intent or they would look different. (ie Flattened PDF's) So I cant set vector to Relative Colorumetric and Raster to Perceptual. The way we solve this at the moment is to choose an RGB colour from a test print which is usually has a much stronger chroma than the resulting spot, However with Perceptual compression it will print accurately. This is a manual process of looking up colours on charts. My first email was a little inaccurate. What I would like to do is convert a CMYK/Lab colour via Relative Colourmetric intent to the printer profile, The resulting 6 colour formula is my 'ideal match'. However since the RIP is using Perceptuial Intent I need to do a reverse lookup to find an RGB colour which when convered RGB>Perceptual>Printer will give me the same 6 colour formula. I use RGB has a bigger gamut and gives room to compresate for perceptual compression. One consideration is that both Perceptual and Relative Colorumetric may not produce the same 'formulas' At the moment I calcualte a close RGB colour then iterate through a few thousand combinations to find the cloest match. What I would like to do is perform a reverse A2B lookup but NOT a B2A lookup as the printers B2A table only uses 5 data points vs the 33 datapoints for the A2B Table. Tricky? Glenn Mike Russell wrote: >> Hi Im hoping someone can help me, >> >> Im converting an CMYK colour via perceptual rendering intent to a >> printer colour, in this case its 6 colour printer (CMYK+2Spots) >> >> What I need to do is find an RGB colour that results in the same Printer >> Colour using Perceptual and Relative Colourmetric intents. This means I >> need to perform a reverse Lookup from the Printers Colour to RGB. >> >> Did I mention im using a Visual basic Class I wrote to interface with >> lcms.dll but can't directly access the internal procedures of Lcms? >> >> Currently Im doing a B2A transform then testing all combinations of RGB >> within a range to find the closest match. (Slow and not 100% accurate) > > Considering only the CMYK part of your problem, Create a transform > that uses your RGB profile as the source, and something like the SWOP > v2 transform for CMYK, and do a direct conversion. It will be > lightning fast. You will get somewhat brighter colors using Relative > Colorimetric intent instead of Perceptual. > > Re adding the two spot colors, this been done already with hexachrome > and hifi color. Use their inks and profiles, and you'll get a better > result than rolling your own. If you do step outside the hexachrome > inks, spot colors are not the same as a process inks. Some of them > are more opaque, for example, and mixing them will produce > unpredictable results. To predict what the screened versions will > look like, you must do your own measurements of various combinations > of the six colors screened together. If you do that, pick spot colors > that are reasonably transparent. > > Stepping back a bit from your question, instead of doing a six color > job, pay a professional to do the conversions and perhaps use better > stock. This will look better, even in four colors, than an automated > conversion to hexachrome. Post your question to a printing oriented > group, such as comp.publish.prepress for advice > > Mike Russell - www.curvemeister.com > ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
