Document SEAM+ECM video mode

[dansanderson]

VIC-IV Super Extended Attribute Mode (SEAM) character graphics support a feature similar to the C64's Extended Color Mode (ECM) where bits of the screen code are repurposed to select different background colors. This mode can only access a portion of the full character set, in exchange for more color combinations. We currently have no documentation about the SEAM version of this feature.

See Gurce's summary and discussion on Discord: https://discord.com/channels/719326990221574164/720355877701550131/1334191693074792613

[gurcei]

Was pondering what the "win" use-case for this mode could be.

One suggestion gardners gave that it could be used to allow multiple players or enemies to have different colours.

I initially felt that the $ff pixel colour (to select foreground colour of char) would be better suited to this, as you can control where it is drawn pixel-by-pixel within the char.

Then again, maybe having ECM mode (using $00 pixel colour) allow us to choose a bg-colour across a 8x8 pixel char could still be put to use with clever sprite/pixie design, which would then allpw for 2 unique colours per player/enemy, while still using the same underlying chars to draw them.

Ah wait, I'm mistken in that last paragraph, the bg colour can be placed wherever you want with $00 pixels, so it's just as flexible as $00 pixels :-) (I was mistaking the bg limitation was as with the c64 ECM mode)

[gurcei]

Hmm, but then again, I'm kind right too, if you use a unique bg colour for your player/enemy, it means that within that 8x8 block, you won't be able to use the true/traditional background colour ($d021), so yeah, it does require more careful sprite/pixie design.

Aah, and I wonder how it would handle the seam transparency flag when drawn over a background... Would these extra bg colours then become transparent too? Maybe so...

Unless perhaps we assure that bg colour 0 is the only one that respects the trqnsparency?