On 9/11/04 14:32, Bob Frost wrote: > But if you look on Bruce Lindblooms website at his RGB color space > information, you will see that Adobe98 has a coding efficiciency of 100%, > which means that all the rgb triplets are real colors.
Hi Bob, As I am not a profile building expert, your post prompted me to do some more research, which included an email discussion with Steve Upton to seek clarification. In his usual helpful manner he gave me this reply: SNIP OK, a couple of points. - Lab is a model of all colours visible to humans. The Lab used by the ICC and Photoshop is limited to -128->127 in the a and b axes and that does not in fact contain all colours visible to the eye. There are a number of very saturated colours that exist outside of those limits BUT none of those colours are "reproducible" by manmade systems (multi-colourant systems like RGB, CMYK, etc). SO, the Lab we use in the PCS and Photoshop is sufficient for our task. - Working spaces that are matrix-based (rather than lookup table, which are very rare) are comprised of 3 RGB coordinates (in XYZ) 3 tone curves (or one for all channels) and a white point. Very simple and very small on disk. When you graph the XYZ values on the horseshoe diagram (which is in Yxy space, a derivative of XYZ) you will find that some of the synthetic spaces such as Adobe RGB and ProPhoto RGB have blue or green coordinates that are, in fact, outside of the "horseshoe" itself. That is, they are outside of the range of human vision - this does not mean beyond ultraviolet or infrared but instead, made of numbers that simply do not correspond to colours. This is a reasonable and necessary thing to do if you want the "triangle" to be large enough to encompass your desired colours. What is not reasonable is to create a 255 blue in Adobe RGB or ProPhoto and expect it to reproduce accurately. The print profile will map it to some printable colour but comparing it to an original is folly. One other thing. There are number values in all colour coordinate systems that correspond to non-colors. Try Lab 0,50,50 or Lab 100,50,50. not possible. Lab and other colour systems are rectangular blocks that contain the blob that is our perceptible colours and a bunch of wasted space. Nature of the beast. SNIP He is going to take a look at Bruce's site with regard to the Coding Efficiency to clarify what it means in this context. I hope this is useful - it has certainly helped me! Best wishes, Jack -- :: Jack Lowe Studio :: :: +44 (0)191 224 5150 :: =============================================================== GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
