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Subject: Re: [cgmo-webcgm] RE intensity
At 03:02 PM 7/13/2004 -0400, Benoit Bezaire wrote: >Hi all, > >I think we should go with this approach. The main reason is that it >is simpler. The HLS approach does work in an absolute scale, but not >very well on a relative scale. I also recall people saying we wanted >to 'fade' the colors. We should just add wording to say that the new >color values are clamped to [0..255]. If we go with Forrest's equations (below), the clamping caveat is not a necessary part of the definition. With the intensity parameter in the range [0,1], each component will be in the range [old_color,0xffff]. The only way that a component could go out of range (i.e., out of [0,0xffff]) is if the parameter is out of range. That said, whether or not to say anything about clamping depends on what's the error philosophy -- the equations can only yield out-of-range results for illegal parameter values. >[...] >Tuesday, July 13, 2004, 1:34:49 PM, Forrest wrote: > >FC> All, > >FC> My understanding was that we wanted to make some APS fade in color >FC> (go toward white) so other objects would stand out more in the image. >FC> So an intensity of 1 would not change the color and an intensity of 0 >would >FC> make it white. I can live with it. It is an odd transformation, at least relative to its name ("relative intensity"). If adapted to a color component range of [0,255], then for any component x (R, or G, or B), the equations produce x' as follows: for i=1: x' = x' (all colors are unchanged) for i=0: x`=255 (all colors become white) for i=0.5: x' = 127 + 0.5*x -- black becomes gray, white is fixed -- red becomes 255,127,127 -- etc. Am I the only one that thinks "intensity" is a bad term for it? It is very counter-intuitive to me, that *minimum* intensity (=0) is white. (White = (255,255,255), and I think of this as maximum "intensity", in all components; minimum "intensity" in all components I would think of as black.) >Internally in our applications we convert all colors to a range >FC> from 0 to 0xffff where (0xffff,0xffff,0xffff) is white. If we are >going the >FC> other way >FC> how do you make red more intense, make it black? I guess when I think of "intensity", I imagine something like the "V" in HSV (or alternately "B" in HSB, for "brightness"). [See Foley & Van Dam, 2nd edition, plate II.7]. It doesn't really matter -- if a transformation like this does what people want, we should go with it, and be as clear as possible about what it does -- the equations (-- and think of a good name for it). -Lofton. >Tuesday, July 13, 2004, 1:34:49 PM, Forrest wrote: > >> new_color.red = 0xffff - intensity*(0xffff - old_color.red); > >> new_color.green = 0xffff - intensity*(0xffff - old_color.green); > >> new_color.blue = 0xffff - intensity*(0xffff - old_color.blue);
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