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Apple II Colour Graphics

Kris Kennaway

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Apple II Colour Graphics

How colours work in Hi-Res and Double Hi-Res

in 5 minutes

Why all of those crazy rules?

e.g. Violet pixel then Green pixel → turns White?!

...and even when you follow them, don’t get what you expect on the screen

fringing, interference

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Dots

Monochrome display
Start with Double Hi-Res

It’s simpler!

(said no-one else, ever)

560 horizontal dots per line
High bit of screen byte is ignored
7 bits in memory map to 7 dots on screen
Alternating bytes from AUX, MAIN memory

40 + 40 bytes gives 560 dot line

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Double Hi-Res dots

0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	..

Main (offset N)
7	6	5	4	3	2	1	0

Aux (offset N)
7	6	5	4	3	2	1	0

Aux (offset N+1)
7	6	5	4	3	2	1	0

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Hi-Res dots

Hi-Res has 280 horizontal resolution, right?
Nope, also 560

but can’t control every dot independently

Bits 0..5 turn on 2 dots
Bit 6 turns on 3 dots
Third dot may be overwritten by next byte
Bit 7 (palette bit) shifts dots right by 1 position

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Hi-Res dots

1	1	0	0	1	1	0	0	1	1	0	0	1	1	1

P	6	5	4	3	2	1	0
0	0	0	0	0	0	0	0

P	6	5	4	3	2	1	0
0	1	0	1	0	1	0	1

0	0	0	0	0	0	0	0	0	0	0	0	0	0

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Hi-Res dots with palette shift

1	1	0	0	1	1	0	0	1	1	0	1	1	1

P	6	5	4	3	2	1	0
1	0	0	0	0	0	0	0

P	6	5	4	3	2	1	0
0	1	0	1	0	1	0	1

0	0	0	0	0	0	0	0	0	0	0	0	0	0

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Artifact colours

Think like a TV
Scan each line, left to right
Colour reference signal, 1 complete cycle in same time as displaying 4 dots
Relative phase of dots determines colour
Messy and analogue, but simple digital approximation
Colour signal sees a sliding 4-bit window of dots

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4-bit colour dot patterns

Hires Colours
0	0	0	0
0	0	0	1
0	0	1	0
0	0	1	1
0	1	0	0
0	1	0	1
0	1	1	0
0	1	1	1


1	0	0	0
1	0	0	1
1	0	1	0
1	0	1	1
1	1	0	0
1	1	0	1
1	1	1	0
1	1	1	1

Double Hires Colours
0	0	0	0
0	0	1	0
0	1	0	0
0	1	1	0
1	0	0	0
1	0	1	0
1	1	0	0
1	1	1	0

(left-shifted)
0	0	0	1
0	0	1	1
0	1	0	1
0	1	1	1
1	0	0	1
1	0	1	1
1	1	0	1
1	1	1	1

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Example

Clock	0	1	2	3	0	1	2	3	0	1
Bits	0	0	0	0	1	1	0	0	0	0

Clock	0	1	2	3
Bits	0	0	0	0
Shifted	0	0	0	0
Colour

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Example

Clock	0	1	2	3	0	1	2	3	0	1
Bits	0	0	0	0	1	1	0	0	0	0

Clock	0	1	2	3	0
Bits		0	0	0	1
Shifted	1	0	0	0
Colour

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Example

Clock	0	1	2	3	0	1	2	3	0	1
Bits	0	0	0	0	1	1	0	0	0	0

Clock	0	1	2	3	0	1
Bits			0	0	1	1
Shifted	1	1	0	0

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Example

Clock	0	1	2	3	0	1	2	3	0	1
Bits	0	0	0	0	1	1	0	0	0	0

Clock	0	1	2	3	0	1	2
Bits				0	1	1	0
Shifted	1	1	0	0

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Example

Clock	0	1	2	3	0	1	2	3	0	1
Bits	0	0	0	0	1	1	0	0	0	0

Clock	0	1	2	3	0	1	2	3
Bits					1	1	0	0
Shifted	1	1	0	0

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Example

Clock	0	1	2	3	0	1	2	3	0	1
Bits	0	0	0	0	1	1	0	0	0	0

Clock	0	1	2	3	0	1	2	3	0
Bits						1	0	0	0
Shifted	0	1	0	0

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Example

Clock	0	1	2	3	0	1	2	3	0	1
Bits	0	0	0	0	1	1	0	0	0	0

Clock	0	1	2	3	0	1	2	3	0	1
Bits							0	0	0	0
Shifted	0	0	0	0

Oh look, we’ve discovered a Hi-Res Violet pixel (with fringing)

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Clock	0	1	2	3	0	1	2	3	0	1	2	3
Bit	0	0	0	0	1	1	1	1	0	0	0	0
Colour										?	?	?

Clock	0	1	2	3	0	1	2	3	0	1	2	3
Bit	0	0	0	0	0	0	1	1	0	0	0	0
Colour										?	?	?

Clock	0	1	2	3	0	1	2	3	0	1	2	3
Bit	0	0	0	0	1	1	0	0	0	0	0	0
Colour										?	?	?

Fringing

Violet

Green

White

Interference

=

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Everyone knows there are only 6 Hi-Res Colours

unless you read Sather, “Understanding the Apple IIe”
Remember the funny business with the palette bit shifting dots by 1 position, and how this extends/truncates dot patterns at the byte boundary?
Can get 14 of 16 colours at byte boundaries

Plus the usual fringing

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Hi-Res Yellow?!

Bit Pos	4	4	5	5	6	6	6	0	0	1	1
Clock	2	3	0	1	2	3	0	1	2	3	0
Bit	0	0	0	0	1	1	1	0	0	0	0
Colour									?	?	?

Odd offset
0	1	2	3	4	5	6	P
0	0	0	0	0	0	1	0

Even offset
0	1	2	3	4	5	6	P
0	0	0	0	0	0	0	1