From: rowlands@pocomoco.NoSubdomain.NoDomain (Jon Rowlands)
Subject: Re: More gray levels out of the screen
Nntp-Posting-Host: pocomoco.hc.ti.com
Reply-To: rowlands@hc.ti.com (Jon Rowlands)
Organization: Texas Instruments, SPDC, DSP Technology Branch, Dallas
Lines: 51

In article <1pp991$t63@cc.tut.fi>, jk87377@lehtori.cc.tut.fi (Kouhia Juhana)
writes:
>In article <1993Apr5.040819.14943@kpc.com> hollasch@kpc.com (Steve
>Hollasch) writes:
>>
>>    I think you're proposal would work to get an extra one, maybe two extra
>>bits of color resolution.  However, if you had a display that chould do only
>>zero or full intensity for each primary, I don't think you'd get great
>>equivalent 24-bit photographs.
>
>I have not suggested to do so; I wrote about problems, and the problem
>were clearly visible with 7 bit b&w images; not to mention 24 bit images.

[ description of experiment deleted ]

>If the 1 bit images are viewed quickly and in sync with screen,
>then 100 intensities could be better than we have -- I dunno.

[ more deleted ]

>In any case, getting black color with slow machines is problem.
>I could try it on our 8 bit screens but I don't know how to
>render pixels with X in constant time. I recall our double buffer
>has other image color and one b&w -- that doesn't help either.
>Maybe I should dump photos to screen with low level code; how?

A few years ago a friend and I took some 256 grey-level photos from
a 1 bit Mac Plus screen using this method. Displaying all 256 levels
synchronized to the 60Hz display took about 10 seconds. After
experimenting with different aperture settings and screen
brightnesses we found a range that worked well, giving respectable
contrast. The quality of the images was pretty good. There were no
visible contrast bands.

To minimize the exposure time the display program built 255
different 1 bit frames. The first contained a dot only for pixels
that had value 255, the second only for pixels that had value 254,
etc. These frames were stored using a sparse data structure that was
very fast to 'or' onto the screen in sequence. Creating these
frames sometimes took 5-10 minutes on that old Mac, but the camera
shutter was closed during that time anyway. And yes, we wrote
directly to the screen memory. Mea culpa.

Our biggest problem was that small images were displayed in the
top left corner of the screen instead of the center. It took
an extra week to have the film developed and printed, because the
processors took the trouble to manually move the all images into
the center of the print. Who'd have guessed?

regards,
Jon Rowlands
