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Old 08 June 2011, 02:53   #114
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Join Date: Sep 2007
Location: Melbourne/Australia
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One of the most interesting things about the next Indivision AGA will be it's support of Graffiti, for those that don't know much about this graphics mode I've found some blurb about the original implementation here.

1. What it does

Graffiti changes the Amiga bitplaned graphics into a chunky
pixel mode. I.e. one byte in memory represents a single
pixel. The value of a byte selects it's colour. The colour
palette is 256 out of 262144 at a time. Possible
resolutions are 640x256, 320x256, 160x256 and 80x256. In
each mode, 256 colours are available, interlacing doubles
the vertical resolution.

3. System requirements, compatibility, limits

For full speed advantages, AGA Chipset are
recommended. For more speed, accelerator cards with
burstmode fastmem are recommended (ACA?).

Due to the limited bandwidth of the chipmemory, the frame
rate of a motion sequence is limited. At about 7MB/s on
AGA-chipsets using 4 Bpls hires, with longword-access about
74 frames/s can be transferred - more than required. Using
word access, it's reduced to about 37 frames/s, and with
byte access only 18 frames/s are possible, so setting up
longwords in the processor registers or in fastmem is
recommended. (Numbers assuming a 384x256 window). Using
the 160x256 pixel mode or the 80x256 pixel mode, the chunky
buffer is completely linear, so you can access four pixels
in a row with a single longword move.

Colour #0 of the Graffiti (external palette, see further
down) must always be set to black (R=G=B=0), because of some
monitors trying to adapt to a 0- level after Hsync. This
causes vertical brightness fading in the first lines if
colour#0 is not set to black. It's different on different
monitors, so this cannot be used as an effect.

4. Activating the card, word definitions

To activate the card, switch to a hires or SHires screen at
1, 2 or 4 bitplanes, depending on the resolution you want to
display. Set the genlock audio bit to 1, otherwise the
standard Amiga bitplaned graphics will be displayed.

The Graffiti card has got it's own color palette that is
called "external palette" from now on. The Amiga-palette
that is set in denise/lisa will be called "internal

Choosing 4 bitplanes hires or 2 bitplanes SHires sets the
Graffiti to 320x256 pixel. Setting up 2 bitplanes hires or
1 Bitplane SHires selects 160x256 pixel, and a single
bitplane hires selects the 80x256 mode. The highest
resolution available is 4 Bitplanes SHires, resulting in a
display of 640x256 pixel. The Graffiti is
overscan-compatible, so up to 768 pixel and more than 512
lines are possible (depends on PAL or NTSC modes, interlaced
or non-interlaced).

Last edited by NovaCoder; 08 June 2011 at 07:42.
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