DRAW_VLC problem on certain Vectrex

[minsoft]

I have a strange problem. I'm using Malbans DRAW_VLC macro in place of Draw_VLc. As a rule it works perfectly, however on my spare Vectrex it displays one particular 'enemy' incorrectly. Specifically, one vector from the list of 9 vectors appear to be drawn wrong. My observations:

* It works fine on my other Vectrex

* The problem enemy vector list displays correctly with Draw_VLc bios routine

* The screen position the vector list is drawn at doesn't help/worsen the problem

* I am using a series of vector lists to animate the enemy, and all of them display incorrectly (at the same common vector)

* My other enemy etc vector lists display perfectly

This is an example problem vector list (SCALE_4 = 4 decimal):

DB 8
DB -12*SCALE_4, 10*SCALE_4
DB -11*SCALE_4, 20*SCALE_4
DB -15*SCALE_4,-30*SCALE_4
DB 15*SCALE_4,-30*SCALE_4
DB 11*SCALE_4, 20*SCALE_4   ; this vector is drawn too vertical and approx twice too long
DB 12*SCALE_4, 10*SCALE_4
DB -12*SCALE_4,-10*SCALE_4
DB -11*SCALE_4,-20*SCALE_4   ; (interestingly this is the inverse of the problem vector, and draws correctly)
DB -15*SCALE_4, 30*SCALE_4

Has anyone run into this problem before?

I will play around with the vector list and see if drawing it in a different order makes any difference.

[minsoft]

Interestingly drawing the vector list anti-clockwise instead of clockwise 'fixes' it. ie this in place of the vectorlist posted previously:

DB 8
DB -12*SCALE_4,-10*SCALE_4
DB -11*SCALE_4,-20*SCALE_4
DB -15*SCALE_4, 30*SCALE_4
DB 15*SCALE_4, 30*SCALE_4
DB 11*SCALE_4,-20*SCALE_4
DB 12*SCALE_4,-10*SCALE_4
DB -12*SCALE_4,-10*SCALE_4
DB -11*SCALE_4,-20*SCALE_4
DB -15*SCALE_4, 30*SCALE_4

[Malban]

Hi,
these "old" macros were written, when I had less experience in Vectrex peculiarities....

It might be the "problem", that your Vectrex is slightly "cranky" (see somewhere in my blog)...
Cranky Vectrex might be 1 out of 20 or so, we by now guess, that some internal components are from a different manufacturer, it is not that the vectrex is nearly "broken"... So don't despair...

If it is, and if this is the "same" situation that I encountered...

Than following will probably help:
- do not extend your vectors to fullest strength... I make a habit out of it to not extend vectors beyond a strength of 100
- use BIOS functions
- alter the Macro

The "crankyness" shows (at least) in two situations:

a) if the y on one vector is "very positive" and the y position of the following vector is very negative (a jump from e.g. +110 to -110), than the y length of the second vector can be "erratic"
b) switching from a "large" positive DAC value (which mostly was the last x position) to a "large" negative value (the next y position)

For both there is a solution (and that the BIOS works, is a sign for me, that the original developers new of this...):

a) can be remedied by putting additional cycles between the two <VIA_port_b accesses (the switching of Y/X integrators)
b) can be remedied by putting additional cycles between <VIA_port_b and <VIA_port_a access for setting the Y position.

So ... in my old MACRO, replace all occurrences of

  STD VIA_port_b

with

  STb <VIA_port_a  
  STa <VIA_port_b

And it will probably work fine.

Cheers

Malban

[minsoft]

Hi Malban,

Many thanks for your thorough reply!

The two situations you listed do not seem to match exactly where I am having the issue...but I have no doubt you are on the right track given that drawing the vectors in the reverse order has 'fixed' it.

I have not tried altering the macro as you suggest yet, but will try to do some and report back.

Thanks again!