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Message: 5391

Author: Igor Makovsky

Date: 03/01/2006

Subject: Rope-teleports (Re: Amazing new quirky feature ??? !!!)

 

Although ropes are an area of JSW that I don't understand 100%, I
don't see why it should have anything to do with having jumped
through an ILB.

If a rope collides with a pixel that was drawn before it (i.e. a
cell, Willy, or a guardian which precedes it in the room's guardian-
instance list), then the rope picks Willy up at the position of the
collision, no matter where Willy is at the time of the collision.

So if the collision happens near the top of the screen, Willy will
be `teleported' to the room above.

I have seen this bug in J4's "Burning Bridge" on my real Spectrum,
but am unable to replicate it for that room under RealSpectrum.

This bug can also be caused by placing the rope in an `invalid'
column, as I found when writing the room "WE PRETTY" and Party
Willy's "La Maison de Jet Set Dick", both of which feature non-
swinging ropes. I eliminated the bug by trial and error, without
understanding the rule for a valid rope-column.

In "The Pool Table" and "Burning Bridge", where I'm unable to
replicate the effect, the only thing that occurs to me is that a
memory-leak could have corrupted the room-data, with erroneous
pixels causing a collision?

"The Pool Table" and "Burning Bridge" have this in common: that the
rope is the /eighth/ guardian-instance in the room.

When you enter a room and it is copied into the current-room buffer
(32768-33023), the guardian-classes of the guardian-instances are
copied into an 8x8-byte guardian-buffer at 33024-33087.*

But a rope overwrites the /ninth/ byte of its guardian-class in this buffer. So if the rope is the room's 8th guardian-instance, it will
overwrite the byte at 33088,* which is off the end of the buffer!

Poor Daniel, his amazing new quirky feature appeared to be an old
kind "rope-teleport" effect. Yep, all the new is quite good forgotten
old things.. =) I remember the same effect in "Finders Keepers" in my
ZX Heroes. Interesting, that sometimes it happens, and sometimes not.
Have it a try if you have some time. Don't understand what triggers
Willy to be teleported to the room above. Though rope looks rather
unusual there, it doesn't collide with anything, but the effect still
exists.

 

 

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