Re: Graphics Hacks, Part II

[Pim Blokland <pDOTblokland@spcgroup.nl>]

Michael A. Balfour wrote:
 
> How the heck does hi-res graphics work???
> 
> Case 1:  poke a 127 into memory, with a 255 underneath it.  You see the
> following, where each X represents a half-pixel.
> XXXXXXXXXXXXXX
>  XXXXXXXXXXXXX

[very interesting stuff deleted; see original message]

It takes a bit of experimenting to find out, but the rules of the half
pixel in between two bytes are as follows:

- If a byte is less than 128, it starts on a whole pixel boundary
- If a byte is 128 or greater, it starts a half pixel to the right,
extending the last half pixel of the previous byte, regardless of
whether this byte was greater than 128.

So, representing a half black pixel by . and a half white pixel by X, a
value of 64 followed by a value of 0 looks like this:
............XX..............

and a value of 64 followed by 128 looks like this:
............XXX.............

Similarly, if you poke 192 and 0, and underneath it 192 and 128, you
get:
 ............X..............
 ............XX.............

The only way you can get a half pixel sized gap, is to shift the left
byte, not to shift the right byte, and leave the rightmost pixel of the
left byte black. E.g. poke 160 and 1:
 ..........XX.XX............

That's about it. Hope this clarifies things a bit. If you have any other
questions, just ask. But please don't ask me to explain the 16 color
scheme of double hires. ;-)

-- 
Pim Blokland
if replying by e-mail, replace the DOT in my name with a dot.