Digital BW, The Print

John,

I think that your idea has merit from a technical point of view but I
suspect that in practice we would get lost at the point our eyes cannot
distinguish between Dmin and D'min. This would only seem to be of interest
if it was clearly visible to some degree. I think that in quality silver and
inkjet prints the difference between Dmin and D'min is lost to the eye, or
at least my eyes anyways.

Martin

----- Original Message -----
From: "John Brownlow" <lists@...>
To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
Sent: Thursday, April 04, 2002 7:27 AM
Subject: Re: [Digital BW] Some equations WAS Thoughts about Imaging


> I'm going to reply to my own post here because I think I was wrong, or not
> as right as I could have been.
>
> On 4/4/02 7:12 AM, "John Brownlow" <lists@...> wrote:
>
> > The second
> >
> > (2)    D2 = Cmax - Cmin
> >
> > where C is a log measure of contrast, measures the difference between
the
> > greatest contrast a printer can render, and the smallest. (In my opinion
> > this needs to include some criterion of how large an area we are talking
> > about, in order to take into account the coarseness or otherwise of the
> > dither, but that's a side issue).
> >
> > I'm assuming here that
> >
> > (3)    Cmax = Dmax - Dmin.
> >
> > I'm also assuming that
> >
> > (4)    Cmin = Dmin' - Dmin
> >
> > where Dmin' is the lightest tone the printer is capable of rendering
> > which is distinguishable above the 'noise level' or natural tonal
> > variation of the paper base.
>
> Actually, I think you can make this (sort of) independent of the paper.
>
> Equation (3) becomes:
>
> (3a)    Cmax = Dmax - Dmin'
>
> Where Dmin' is defined as above.
>
> Equation (4) becomes:
>
> (4a)    Cmin = MIN ( delta (D) )
>
> That looks a bit intimidating but all I mean is that it is the minimum
> possible value of (D1-D2) where D1 and D2 are two different tones output
by
> the printer. In other words the smallest possible difference in contrast
the
> printer is reliably capable of achieving (or the visual noise floor of the
> paper, whichever is larger).
>
> Actually measuring Cmin on an 8-bit printer is not that difficult. All you
> need is a 16x16 checkerboard representing brightness values 1-256. You
> measure them all and find the difference between the two whose density
> values are closest together.
>
> However I'm not sure what this would tell you. Wouldn't it be more useful
to
> see what the *maximum* density difference between two *adjacent* gray
cells
> was? This would genuinely be a measure of the coarseness of the contrast
> since any big jumps here would appear as posterisation in the final print.
> Relating this to Dmax is clearly relevant, too.
>
> Eg
>
> (4b)    Cmin = MAX ( delta (Dn, Dn+1) )
>
> Where Dn is the density of a patch of gray representing brightness level n
> (between 1 and 2**n in an n-bit grayscale image)
>
> > Plugging (3) and (4) into (2) gives
> >
> > (5)     D2 = Dmax - Dmin'
>
> Or in this case
>
> (5a)    D2 = (Dmax - Dmin') - MAX ( delta (Dn, Dn+1))
>
> This seems like an achievable measurement and definitely different from
the
> original D1 in equation (1). I wonder what, if anything, it really tells
us?
>
>
> --
> John Brownlow
>
> http://www.pinkheadedbug.com
>
>
>
> Please visit the Group Homepage to check the Files, Bookmarks, Polls and
other resources as they are often being updated. The page is at:
>
> http://groups.yahoo.com/group/DigitalBlackandWhiteThePrint
>
> Please follow these basic guidelines:
> - Include your full name with your message.
> - Include the address of your website, if you have one.
> - As threads develop, trim off excess portions of earlier messages to keep
them short.
> - As the topic of a thread changes remember to change the subject header.
> - Good manners are required at all time. No personal attacks or "flames."
> - Complete your Yahoo profile.
> - Before posting a question, search the message archives and the various
resources on the homepage.
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>