Using non-standard parts of characters

[Sdw]

I guess this is common knowledge, but I have been experimenting with putting other values than the "correct" (char * 8 ) - (Y / 2) in the character pointer, allowing me to display inbetween-parts of teh ROM chars.
This is a neat trick, and can be useful for a case I have. However, I get some really weird false overlapping of characters, ie. when I move another character in the vicinity (but not on top of) of my "special" character, I get the strange bugs that you get when overlapping normal characters.
Anyone know the particulars on how this works, and how to mitigate it?

[LD]

From my tests, you can overlap 2 pixels of any direction (up,down,left and right) without problems, more than that you have the overlap glitch.
The problem, even the blank pixels overlap the graphics (see the image, green pixels is the overlapped ones).
I've talk with Kevtris about it (because he implement O2 in a FPGA, so he knows the internals timings) and he suggest to not do it as the timings are very sensible and the game could crash.
I recommend you to read his doc about hardware timings, there is a lot of useful information if you want to push the hardware to it limits.

Kevtris Doc :
http://videopac.nl/forum/index.php?topic=1751.0

I think even invisible chars still collide as it not based on pixels, but on X & Y position on screen.

[Rafael]

From my tests, you can overlap 2 pixels of any direction (up,down,left and right) without problems, more than that you have the overlap glitch.

How I remember it´s possible overlap 2 pixels left or rigth, but it´s not possible to do up and down (considering the char are on the same X pos)

[Sdw]

Perhaps a bit of a misunderstanding - I was talking about the size of the "non-standard" characters when it comes to overlapping. After some more experimenting it seems like the actual character output varies in size, I assumed it always showed 8 pixel in height, but from what I gather now, depending on what values you feed into the char-ptr you can actually get characters that are both larger and smaller than 8 pixels, and of course then adding or subtracting from the y-area that collision covers.

[LD]

You're right. Change the height and the collision changes too. See Attack of TimeLord where the saucers are 4 (?) scanline height and flies at 1 pixel distance each other, any direction, without problems.