[Simh] Improved CRT simulation

[Phil Budne]

> On 14 Dec 2018 at 09:24:42 -0500, Paul Koning wrote:
> > On Dec 14, 2018, at 1:17 AM, Lars Brinkhoff <lars at nocrew.org> wrote:
> > I'm entertaining the idea of improving the CRT display simulation in
> > SIMH......
> > 
> .....
> > If you read the fine print in the 340 documentation, you'll find that an
> > a point is .03"-.015" depending on the intensity, compared with the grid
> > spacing .0091".

I created the XY display code that SIMH for the PDP-1 and Type 340
displays (and is also used for TX-0). It's been my suspicion for a
long time that a REALLY good simulation of the user experience would
require MANY more pixels than I've ever had available to me (I don't
yet own a 4K resolution display, and it looks like even that wouldn't
be enough, see below).

xdpyinfo describes the laptop I'm writing this on as 96 DPI or
0.01042" which isn't very different from the 0.0091 grid spacing
above, BUT simulating the 0.03 (low intensity) point size above with
square pixels would require greater resolution to draw circular points.

A 204PPI 3840 x 2400 "4K" display only has enough resolution to draw a
1024x1024 area with "points" that are two pixels wide, and 2x2 points
which just halves your resolution!  Unless someone who understands
graphics (and visual trickery) better than I (which is hardly at all)
can figure something out.

An "8K UHD" 7680 x 4320 resolution device could draw a 1024x1024
raster with 4x4 "points" which I imagine with anti-aliasing might make
it possible to draw "points" which look circular.

I've also suspected that "high intensity" on the radar display tube
that the Type 30 (PDP-1) and Type 340 displays used was more eye
searingly bright than anything that "modern" 24-bit color can manage
(which is only 256 levels of intensity for a given color).

> Not SIMH, but I wrote a CDC DD60 simulation for use with DtCyber that attempts to model the CRT and spot behavior.  It does that by computing a 2-D Gaussian intensity distribution around the spot center, then sums that into the saved screen pixel value (with saturation).  And then that value is decayed exponentially.

Paul,

What is the resolution of the DD60 display?  I seem to recall the CDC display
used analog circuitry for letter forums, as opposed to drawing them with 
discrete dots, as the DEC Type 340 does.

Thanks!
Phil