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2021-08-04 08:02

ChristopherJam

Registered: Aug 2004
Posts: 1377

Release id #204427 : Cloud Face

Interesting conception.

One could enumerate all the possible images by noting that the internal LFSR for the noise generator contains only 23 bits of state, and thanks to the SEI the only other confounding factor is the start time relative to DMA - only 18581 possible values there (63*312-43*25). So - a grand total of well under 155,868,725,248 possible outputs.

But, LFSR is never all zeros IIRC, and also all the start times outside the display area are equivalent, thanks to zeroing d011. So, probably more like (195*63-25*43+1)*(2**23-1) = 94,044,673,077 possible outputs if I have my maths right.

(the 195 is the number of lines from $30 (too late to cancel DMA that frame) to $f3 (the line of the last DMA at default scroll position). I'm probably out by a few cycles because I haven't looked up where on line $2f the last chance to disable DMA is)

Even at 50Hz update it would take roughly 60 years to cycle through displaying all of them. I wonder if there are any image processing/AI metrics that could be used to search for more "interesting" outputs, or if they all just look look like noise...

2021-08-04 09:55

Frantic

Registered: Mar 2003
Posts: 1627

One of them is probably a crystal clear image of Satan's face

2021-08-05 14:23

Copyfault

Registered: Dec 2001
Posts: 466

Interesting point, especially from a mathematical point of view...

@CJam: regarding the question wether there are "interesting outputs" among the ~ 94 * 10^9 possible ones, I'd tend to simplify it a little as follows:

- the influence of the colours on the picture our brain forms of what it is fed with is by far higher than the actual pattern definitions; thus let us assume we can ignore the character definition of the output and stick to the col output only (i.e. what gets written to $d8xx)

- Let's say we have a fixed background colour (be it black = $00), every character can be assigned 15 different values. Thus, for a screen filled with reverse spaces consisting of 40x25 chars we get (40x25)^15 = 1000^15 = 10^45 possible colourings

- comparing these values, we see that the cloud face-routine creates a really tiny subset of the total set of colourings; so the usual "law of large numbers"-arguments won't apply here, i.e. no satanic faces, neither any excerpts from whatever novel (c.f. infinite monkey theorem, you know what I mean)

Nontheless, it might be worth it to break down the cloud face routine to single char colours, i.e. to find a formula that gives the possible colours for a given char that are possible with the routine. Since the code goes

...
lda $d41b
sta screen,x
(4 times)
...
lda $d41b
sta col_ram,x
(4 times)
...,

I guess the values that are stored to the colour ram could be understood. Didn't travel further on this track, but it might be a way to find "interesting outputs".

2021-08-05 14:32

Copyfault

Registered: Dec 2001
Posts: 466

Oops, checked wikipedia once again, it's the "law of truly large numbers" I was referring to. Sorry for missing out this seemingly important extra word ;)

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