Path: utzoo!attcan!uunet!ncrlnk!ncrcae!hubcap!gatech!purdue!decwrl!sgi!andru@rhialto.SGI.COM From: andru@rhialto.SGI.COM (Andrew Myers) Newsgroups: comp.graphics Subject: Re: Luminance from RGB (Was "intensity" from RGB) Summary: More information Message-ID: <21843@sgi.SGI.COM> Date: 9 Nov 88 03:08:33 GMT References: <76353@sun.uucp> Sender: daemon@sgi.SGI.COM Organization: Silicon Graphics, Inc., Mountain View, CA Lines: 38 In article <76353@sun.uucp>, poynton%vector@Sun.COM (Charles Poynton) writes: > In Comp.windows.x article <8811011523.AA02242@LYRE.MIT.EDU>, Ralph R. > Swick comments: > > > I'd like to suggest that (.39R + .5G + .11B) is not a good choice for > > "intensity" in the realm of computer graphics. ... > > > > A better choice in computer graphics is to equally weight the colors: > > ((R+G+B)/3.0). Let white be white. > > Equal weighting of the primaries is NOT the right thing to do, unless the > viewers of your images are members of some species that has uniform > response across the visible spectrum, unlike homo sapiens. > From Foley and Van Dam, page 613, we have the relationship between the YIQ and RGB color systems: Y 0.30 0.59 0.11 R I = 0.60 -0.28 -0.32 . G Q 0.21 -0.52 0.31 B From this, we can see that the luminous reponse of the eye (Y) is exactly that described by Mr. Poynton, albeit with less precision. What the other poster was being confused by was the *inverse* of this process. That is, when I,Q=0 (white light), we have R=G=B=Y. This can easily be seen from the inverse matrix: (Again, with 2 digits of precision) R 1.00 0.95 0.62 Y G = 1.00 -0.28 -0.64 . I B 1.00 -1.11 1.73 Q The fact that Y = 0.30R + 0.59G + 0.31B has little to do with white being made up of equal components of red, green, and blue. Or at least the connection is through a matrix inverse, unintuitive at best. Hope this clarifies more than it confuses. Andrew