On 21 Jun, Dr Andrew C Aitchison wrote: > On Thu, 20 Jun 2002, Christoph Koulen wrote: >> The delimiting factor, I agree, would be the human eye! I wonder, if it >> is capable of distinguishing between 1024 shades of a primary color?
Yes it is. There is scientific work on the eye-brain vision system and one of the results is a clear answer yes. At this level of detail you must also be precise about the meaning of the word "distinguish". I've experienced several definitions of it: 1) Able to differentiate two halves of a split circle contrast target on a neutral background. 2) Able to correctly identify the polarity of a 3x3 checkboard target. I.e., correctly identify the center square as darker or lighter. 3) Able to correctly locate a contour line at a "flat spot" in an image. The tests I worked with were done in grey and blue (because those are what radiology works in). When alert and wearing glasses my vision quits somewhere around 1500-2000 levels. In the semi-random sample of several thousand field service engineers we found none that were below 300 levels. Most were 500 or better in the mid-greys. > > Probably not, especially since there are colours too bright and too dim > for a monitor to show. However with only 256 shades the steps between > adjacent colors are not always even (gamma mapping can reduce this problem) > and it isn't difficult to find single steps which are very obvious, > especially on a gray ramp. 1024 shades makes it easer to make the steps > even, and maybe allow all of them to be invisible. > The luminance (the kind in cd/m^2) of image and environment are key parameters in defining the number of visible levels. The key parameters when the display covers the full field of view are the brightness of "black" (which includes reflection of ambient lighting) and the brightness of "white". Typically lit ordinary CRT monitors are often in a range where the eye is limited to under 256 levels. As you mentioned, constraining these 256 levels to be points of equal voltage to a monitor further eliminates levels because these levels are not placed uniformly in perceptual space. The natural CRT gamma curve is a fair approximation to the eye's response, which is why CRTs have been successful. It is not perfect. Increasing the DAC resolution to 10-bits voltage puts sufficient adjustment into the exact positioning of luminance levels so that all of the roughly 256 visible levels can be displayed. This is one of the major reasons for the need for 10-bit video output resolution. Note that a 10-bit DAC is enough. 8-bit RGB pixel storage remains sufficient because the purpose of the 10-bit DAC is presentation using the eye's response curve rather than the CRT's gamma curve. For specific examples of how this can be used, see http://medical.nema.org/dicom/2001.html/01_14PU.PDF or Barten's book Contrast Sensitivity of the Human Eye and Its Effects on Image Quality Then follow the references to track down other major researchers on this topic. R Horn _______________________________________________ Xpert mailing list [EMAIL PROTECTED] http://XFree86.Org/mailman/listinfo/xpert
