17 Oct
2008
17 Oct
'08
12:27 p.m.
In article <m1Kqa6m-000IxyC at p850ug1>,
ard at p850ug1.demon.co.uk (Tony Duell) writes:
The reason you don't see it is because you are torturing the
definitions in order to come up with some crazy example.
Characters are not pixels because you don't send characters to the
CRT. Eventually characters are turned into a bitmap pattern of pixels
Actually, I believe there was a CRT-like device (I forget the name --
'Charactertron'?) where you did sort-of send the characters to the CRT.
THe CRT had 3 deflection systems and an electode between them that
cotnained the patterns for all the xymbols (characters, etc) that the
device could display. You used the rearmost deflection system to direct
the beam from the electron gun at the right pattern and the front
deflection system to put it at the right place on the screen. No, I don;t
have one.
Characters are not pixels, so no, I'm not setting a constraing on the
'Pixel' == Picture Element, I beleive. Idon't see why a 'block graphics
character' can't be an element of a picture. and *that* is what is sent to the CRT. That's why
pixels are picture
And I don't send the bitmap directly to the CRT either. Typically you
take the bitmap from the memory, extract the approprtiate number of bits
(I assume you allow more than 1 bit/pixel), process those in some way
(perhaps by feeding them itno a colour lookup table, then to a DAC), and
only then feed them to the video amplifiers nad then to the CRT.
elements, but characters are not. I can't believe
I have to explain
this basic element of digital video scanout to you Tony, I know for a
fact you've delved into the circuitry of these kinds of subsystems
often enough to know exactly the difference between a character and a
pixel and why they are not the same.
My problem is that I've worked on plenty of things that fit _neither_
definition, and I want to know if oyu consider them to be graphics
displays or not.
Apart from the obvious vector displays (which you seem to ignore, for all
the 1950's grpahics displays are very likely to be of this type since
memory was expensive -- very -- back then), what about :
1)_ The DEC 'waveform generator' grapghics in the VT55 and VT105 (amongst
others). It had a fairly high resolution and limited memory, but you
could only display 2 points in each vertical column (or, IIRC, all the
points below the specified one in that column). It was thus useless for
displaying bitmapped images, but ideal for the intended application which
was displaying mathematical function grapbs and histograms.
2) A design that can either be bitmapped or character based without
changing the hardware. This was certainly used commercially in machines
like the Sirius (victor 9000) and Apricot. Let me explain how it works.
You have the conventional scan counters addressing 2K (or so) of 16 bit
memory, as in a character display. But 11 bits of of that 16 bit value
are used to address a programmable 'character generator' memory, the
output of which is serialised, combined with attributes (blinking, etc)
determined by the remaining 5 bits of the 16 bit 'character') and sent
to the CRT. Now, the point is that 11 bits is enough to have a different
'character pattern' at each location on the screen. So you can use it in
one of 2 ways :
a) As a character based display. Store the patterns for the ASCII (or
whatever) character in the second area of memory, store the character
codes yoy want to display in the first area.
b) as a bitmapped display. Initialise the first area of memory with a
difference code at each location, and then set/reset bits in the second
are to turn individual pixels on or off.
-tony