Re: [stella] Grid demo

Subject: Re: [stella] Grid demo
From: Eckhard Stolberg <Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx>
Date: Thu, 29 Oct 1998 15:16:28 +0100
At 15:03 28.10.98 -0800, you wrote:

>> Look at trick18 or extra18.
>> The last hits to RESP0/RESP1 both generate three copies.
>Hmmm.... I don't think so.  I don't think it's possible for a RESPx
>other than the first one on a line to generate three copies.  Check out
>the attached latehit.bin.  It has two groups of three sprites and I
>don't think either of them overlap.  Yet only two sprites per group show

I made a small mistake, when I explained my theory in the last post,
so I'll try again. When you set the sprites to two copies close, you
will get an output like:


My theory is: if you hit RESPx during the s part (atleast certain
parts of it) you will get three copies. If you hit RESPx during the
b part you will get only the last two copies. 

Take a closer look at extra18.bin and see where the one pixel gaps
are. It displays 1 sprite on the left, a larger gap, 4 sprites, a
1 pixel gap, 4 sprites, a 1 pixel gap, 4 sprites, a 1 pixel gap
and 5 sprites. The leftmost sprite is trailing from the line before,
so that the last hits to RESP0 and RESP1 must be generating three
copies each.

In extra18.bin there are 11 cycles or 33 pixels between two hits
of RESP0, so that the second hit will happen after the first pixel
of the third sprite should have been displayed. The strange thing
is, that this first pixel is not displayed, but a one pixel gap.
In trick18.bin there are 12 cycles between two hits to RESP0 and
it genarates a four pixel gap between two groups of sprites.

Latehit.bin would fit in this theory, since the hits to RESPx for
one group of sprites are in the b phase of the other sprite.

>first RESPx of the current line.  Check out evenodd.bin.  The second set
>of RESPx is done on alternate lines.  So alternately you see three
>sprites in the first group, then two sets of two sprites.

This would fit my theory too. In the lines with only one hit to RESPx
this hit will be in the b phase of the last group of sprites of the
previous line and therefor result in only two copies. In the next line
the TIA will treat the first group of sprites as nomally positioned,
since the previous line contained only one normal hit to RESPx. 

Should my theory be true, than evenodd.bin would prove, that hits
to RESPx after the last copy of the previous occurance of spriteX
will position the sprites like a three copy version but only display
the last two copies.

>I guess I'm not surprised.  So I guess what you can see is 17 sprites. 

If I count the sprite, that is displayed on the right and left border
at the same time as only one, than I am getting only 16 sprites.

>BTW, I hope you forgive me for constantly hacking apart your program!

No problem. I just proves that someone can actually make use out
of my quick hacks, without me commenting them extensivly. :-)

Ciao, Eckhard Stolberg

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