Newsgroups: comp.sys.apple2
Path: news.weeg.uiowa.edu!news.uiowa.edu!uunet!convex!convex!darwin.sura.net!
howland.reston.ans.net!sol.ctr.columbia.edu!news.kei.com!yeshua.marcam.com!
zip.eecs.umich.edu!destroyer!nntp.cs.ubc.ca!newsserver.sfu.ca!sfu.ca!vantune
From: vantune@fraser.sfu.ca (Joseph Van Tunen)
Subject: Apple II/IIGS graphics & converting to Mac
Message-ID: <vantune.758910946@sfu.ca>
Summary: How does the IIGS convert Hi or Double Hi-Res to RGB?
Keywords: RGB, NTSC, Mega II, VGC, Hi-Res, Double Hi-Res, graphics, converting,
Sender: news@sfu.ca
Organization: Simon Fraser University, Burnaby, B.C., Canada
Date: Tue, 18 Jan 1994 16:35:46 GMT
Lines: 525
This message is a rewritten version of a question I posted before that Michael
Foegelle (foegelle@ccwf.cc.utexas.edu) responded to (thanks). I added more
technical info, fixed some mistakes, clarified things and explained more.
I want to know how the IIGS converts hi-res and double hi-res into RGB. The
IIGS Hardware Reference pg 56 and Call-A.P.P.L.E. April 1987 pg 12 say that
the Mega II, after reading the graphics information, outputs a 4 bit code (at
pins 30-32,34 I guess) representing one of the 16 standard Apple colors (eg the
colors for the IIGS border, background and text, or the lo-res or double hi-res
colors). The value is input into the Video Graphics Controller (VGC at pins
20-23) and is used to lookup a 12 bit RGB value that is passed (out of pins
62-68,1-5) to 3 digital to analog converters (one each for red green, and blue).
****** The manual doesn't say how the Mega II gets the 4 bit code!
What I need to know is how does the Mega II gets the 4 bit code, and how often
it gets a code. (What digital algorithm does it use to convert hi-res and double
hi-res bits into 4 bit color codes?) I want to duplicate the process in software
to convert Hi-Res graphics on to the Mac without ignoring some of the subtle
quirks / overlooked, little known side effects.
-Maybe somebody out there knows...
-Or someone could find out...
-Or build some kind of breadboard electronic thing that checks what the Mega
II is outputing to the 4 pins (at 14MHz I guess)...
-Or maybe the patent says something (Call-A.P.P.L.E mentioned it... I know
nothing of patents, what information they hold, what accessibility a person
has to them or what you're allowd to do with them... What is "patent pending"
anyway?)
-Doesn't SHR Convert do Hi-Res? I know it does Double Hi-Res but it does it
the simple way (4 bit pixel boundaries. eg. 4 bits is white here: 1111 but
not here: 01111).
The following is what I could figure out (a whole bunch of technical stuff
(some might be irrelevant but still interesting) and the method explained in
the IIGS Hardware Reference and tn-aiie-003):
--------------------------------------------------------------------------------
references used:
technote tn-iigs-063 "Master Color Values"
technote tn-aiie-003 "Double High-Resolution Graphics"
Call-A.P.P.L.E. April 1987 pp. 10-13
COMPUTE! August 1983 pp. 172-177
Apple ][ Reference Manual (c1979, 1981)
Apple IIe Technical Reference Manual (c1986)
Apple IIGS Hardware Reference 2nd Edition (c1989)
--------------------------------------------------------------------------------
Table 1. Color Values reference: technote tn-iigs-063 "Master Color Values"
Color Color Register LR HR DHR Master Color R,G,B
Name Value # # # Value
----------------------------------------------------
Black 0 0 0,4 0 $0000 (0,0,0)
(Magenta)Deep Red 1 1 1 $0D03 (D,0,3)
Dark Blue 2 2 8 $0009 (0,0,9)
(Violet) Purple 3 3 2 9 $0D2D (D,2,D)
Dark Green 4 4 4 $0072 (0,7,2)
(Gray 1) Dark Gray 5 5 5 $0555 (5,5,5)
(Blue) Medium Blue 6 6 6 C $022F (2,2,F)
(Cyan) Light Blue 7 7 D $06AF (6,A,F)
Brown 8 8 2 $0850 (8,5,0)
Orange 9 9 5 3 $0F60 (F,6,0)
(Gray 2) Light Gray A A A $0AAA (A,A,A)
Pink B B B $0F98 (F,9,8)
(Green) Light Green C C 1 6 $01D0 (1,D,0)
Yellow D D 7 $0FF0 (F,F,0)
(Aqua) Aquamarine E E E $04F9 (4,F,9)
White F F 3,7 F $0FFF (F,F,F)
LR: Lo-Res HR: Hi-Res DHR: Double Hi-Res
--------------------------------------------------------------------------------
Method for getting 4 bit color code:
technote, tn-aiie-003 "Double High-Resolution Graphics", explains:
"An NTSC color monitor decides what color to display based on its view of four bit
windows in each line, starting from the left edge of the screen. The monitor looks
at the first four bits, determines which color is called for, then shifts one bit
to the right and determines the color for this new four- bit window. But remember,
the color depends not only on the pattern, but also the position of the pattern.
To compensate for relative position from the left edge of the screen, the monitor
keeps track of where on each line each of these windows start. (For those of you
of the technical persuasion, this is done through the use of the color burst signal,
which is a 3.58 MHz. clock)."
Which basically says that every sequence of 4 bits in DHR is used for a color (bits
0 to 3, 1 to 4, 2 to 5, etc) with the shifting of bits taken into consideration.
This suggests that the RGB value that the Mega II outputs is changed at a frequence
of 14M (The rate at which DHR bits are output to the screen).
This method is also explained in the IIGS Hardware Reference pg 88. I can't see how
this method results in what is displayed on the screen. I'll explain at the end...
--------------------------------------------------------------------------------
Table 2. Transfer of info to the screen (sizes and frequencies)
DHR bits | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
14M -----------------------------------------------------------------------
HR bits | LSB 0 | bit 1 | bit 2 | bit 3 | bit 4 | bit 5 | bit 6 |
7M -----------------------------------------------------------------------
colrburst| cycle or pixel 0 | pixel 1 | pixel 2 | pixe
3.5M -----------------------------------------------------------------------
DHR bytes| (auxilary memory) byte 0 | (main memory) byte 1 |
2M -----------------------------------------------------------------------
HR byte | byte 0 |
1M -----------------------------------------------------------------------
notes:
LSB = Least Significant Bit of the byte (bit 0).
MSB = Most Significant Bit (bit 7) is not shown because it is not displayed.
A 14M clock crystal (14.31818MHz) is used to get the frequencies.
7M = 1/2*14M 3.5M = 1/4*14M 2M = 1/7*14M 1M = 1/14*14M
7.159MHz 3.580MHz 2.045MHz 1.023MHz
On the IIGS a 28M crystal is used (28.636360MHz)/10=2.86MHz=fast speed of IIGS.
1M is the same frequency used by the Apple ][, //e, //c and IIGS in normal speed.
3.5M is used for the color burst at the beginning of each scan line of an NTSC (or
composite) monitor. A pixel is 3.5M for a reason described below.
dot/pixel/bits: (convention I try to use to avoid confusion..)
double hi-res hi-res
dot (B&W) 1 bit 1 bit (a double hi-res dot is a hi-res half dot)
pixel (color) 4 bits 2 bits
--------------------------------------------------------------------------------
NTSC standard: reference "Call-A.P.P.L.E April 1987 pp. 10-13"
(Apple IIGS Hardware Reference pg 87)
At the beginning of each scan line output to the monitor is the horizontal sync and
color burst, (burst phase, color subcarrier). The rest of the line used for display
includes:
Luminance: Brightness information sent to the monitor (proportional to the voltage
amplitude of the composite video signal). eg: Black has less luminance than white.
Chrominance (chroma): color info that rides on top the luminance signal, and is of
the same frequency as the color burst. Chroma includes Hue and Saturation:
Hue: Phase shift of chroma (relative to burst phase) eg: Green is phase shifted 180
degrees relative to Purple. (I don't know what the phase shift is for the colors
relative to the burst phase) More explanation later.
Saturation: Amplitude of chroma (relative to luminance level). eg: Pink is less
saturated then red. A saturation of zero eliminates the chroma, ie. no amplitude
means no wave, therefore no phase shift or hue or color, just gray level or
luminance.
Figure 1. NTSC stuff. Two colored pixels side by side with color phase above.
(Imagine the waves as being cureved (sinusoidal or sine wave shape))
3.5M /\ /\ /\ /\ /\ /\ /\
color phase / \ / \ / \ / \ / \ / \ / \
\ /| \ / \ / \ / \ / \ / \ /
\/ | \/ \/ \/ \/ \/ \/
|
white level-> |hue |
(phase shift|
270 degrees)| _
/\ | /\ } saturation
luminance_> / \ |/ \ _} (amplitude
(average | \ / \ /| of wave > 0)
voltage 50%)| \/ \/ |
| |
| |
black_level______| |____________
A B&W monitor ignores color burst and chroma (because of there high frequency, I
guess) and use only the luminance (which is the average of the chroma relative to
the black level).
--------------------------------------------------------------------------------
Apple ][ NTSC output
After an Apple ][ outputs the horizontal sync and color burst, they output the
pixels on the graphics line.
The Apple IIGS uses the RGB anolog signals to create a real NTSC signal.
The other Apple ][s use a simpler method that output a compatible NTSC signal.
For HR, a byte is loaded into a shift register and each bit is shifted out the
right side (LSB first) directly into the NTSC signal (1 is high voltage or bright
dot, 0 low). The bytes are taken at a frequency of 1M, and the bits are shifted
at 7M so that only 7 of the 8 bits are used. 2 bits are output at 3.5M. This is
very important for color!
The colors are a seqence of ones and zeros that make a 3.5M square wave:
black white
________________----------------
purple: --__--__--__--__
green: __--__--__--__--
Note: the green wave is 180 degrees phase shifted relative to purple. On a
monochrome monitor, this would be white and black dots. On a B&W TV (with less
bandwidth than a computer monitor), this would be 50% gray (luminance is average
of chroma, (white 100% + black 0%)/2=50%). Although the signal is not exactly
NTSC, the circuitry of a color monitor still manages to interpret the 3.5M square
wave as color.
After the original Apple ][ (Revision 0 Board), the colors orange and blue were
added. The serialized output stream of a byte would be delayed a half of the 7M
clock cycle (the size of a DHR dot) if the byte's high bit was set. This has the
effect of adding an additional 90 degrees phase shift to purple and green.
0 1 2 3 4 5 6 0 1 2 3 4 5 6
--__--__--__--__--__--__--__
black white
______________--------------
black2 white2
_______________-------------
purple: --__--__--__--__--__--__--__
blue: _--__--__--__--__--__--__--_
green: __--__--__--__--__--__--__--
orange: ___--__--__--__--__--__--__-
(note: while the byte is being delayed, the last dot (black or white) is still
being drawn. See the section on converting HR to DHR)
Now, look at the Macintosh color wheel (which shows hue, saturation, and
brightness (the HSV model where V is for value which is brightness. Brightness
is not the same as lightness though. I think the HSL model is better for NTSC
comparison) and note the positions of the 4 colors on the circle:
Green Orange The green hue is 90 degrees away from orange (and blue) and 180
\ / from purple. Purple is 90 from blue (and orange) and 180 from
__\/__ green.
/\
/ \
Blue Purple
--------------------------------------------------------------------------------
Double Hi-Res Apple ][ NTSC output
Bytes loaded at 2M into a shift register, alternating between auxillary memory
first and main second, bits shifted out at 14M, and high bit ignored.
Purple, Green, Orange, and Blue are a sequence of 2 ones and 2 zeros which make
a 3.5M square wave again (because 4 bits are drawn at 14M/4=3.5M). These colors
have a luminance of 50%(approx). The other colors are a repeating pattern of 1
one and 3 zeros (25%), or 3 ones and 1 zero (75%). All of those colors have a
pattern that repeats at 3.5M.
Black is 4 zeros (0%) and White is 4 ones (100%). Light gray and dark gray are
repeating one and zero (a 14M signal that the monitor circuitry interprets as
gray because it can't find any 3.5M pattern). This would mean a luminance of 50%
for both but on the IIGS, the grays are different. The Other Apple's don't have
the ability to output any color they want like the IIGS (The IIGS could have any
RGB value for the grays). I don't remember, but maybe on the other Apple ][s, the
grays are the same. This would explain why the grays are called gray 1 and gray
2 in the Apple books (except the IIGS books). On the other hand, the composite
monitor's circuitry might output a different color for the grays because of their
different phase shifts (if the circuitry notices it) which might make the
luminance appear different........
--------------------------------------------------------------------------------
Converting Hi-Res to Double Hi-Res (by example)
Symbols:
'|' = End of byte or pixel; screen boundary
'#' = White dot, set bit
' ' = Black dot, cleared bit
Hi-Res Symbols:
notes: The left and right borders of the screen are considered to have no shift.
Previous byte means byte associated with previous bit.
Next byte means byte associated with next bit.
A Shifted byte has the high bit set.
'.' = Black dot. The previous byte is shifted (hi bit set) and half of bit 6 is
not drawn because the next byte is not shifted and is drawn on top.
'`' = Black dot. The next byte is shifted and the previous byte's last bit (bit
6) is clear.
'*' = White dot. Bit 6 of the previous byte is set and the next byte is shifted
so that bit 6 is extended a half HR dot while the next byte is being
delayed.
'?' = White dot if next byte is shifted, or the next byte is not shifted and the
first bit is set; black otherwise.
'!' = White dot if next byte is not shifted and first bit is set; black dot
otherwise.
Shown below are the DHR colors
1) sorted by number
2) grouped by luminance and sorted by shift
3) grouped by color and sorted by luminance
|0123456012345601234560123456012345601234560123456012345601234560123456 : bit
| ab0 | mb1 | ab2 | mb3 | ab4 | mb5 | ab6 | mb7 | ab8 | mb9 | : byte
| | | | | | | | | | | | | | | | | | : pixel
| Black # # Magenta # # Magenta
| # # # # # # # Magenta # # Brown # ## # Purple
| # # # # # # # Brown # # Dark Green # ### ## Pink
| ## ## ## ## ## ## ## Orange # # Dark Blue
| # # # # # # # Dark Green ## ## Orange # # Brown
| # # # # # # # # # # # # # # Dark Gray ## ## Green ## ## Orange
| ## ## ## ## ## ## ## Green ## ## Blue ### ### Yellow
| ### ### ### ### ### ### ### Yellow # ## # Purple
|# # # # # # # Dark Blue ### ### Yellow # # Drk Green
|# ## ## ## ## ## ## # Purple ### ### Aqua ## ## Green
|# # # # # # # # # # # # # # Light Gray ## ### # Light Blue ### ### Aqua
|# ### ### ### ### ### ### ## Pink # ### ## Pink
|## ## ## ## ## ## ## Blue Black # # Dark Blue
|## ### ### ### ### ### ### # Light Blue # # # # Light Gray ## ## Blue
|### ### ### ### ### ### ### Aqua # # # # Dark Gray ## ### # Lite Blue
|############################ White ######## White
Shown below are the HR colors (converted to DHR, each # is a HR half dot). They
line up with the DHR colors above. Note that the DHR screen is 1 HR half dot right
of the HR screen (compare position of purple in HR and DHR at left side of screen).
|--==--==--==--==--==--==--==--==--==--==--==--==--==--== : a bit is two chars
| --==--==--==--==--==--==--==--==--==--==--==--==--==--= : shifted byte's bits
|0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 : bit
| even byte 0 | odd byte 1 | even byte 2 | odd byte 3 | : byte
| | | | | | | | | | | | | | | : pixel
|## ## ## ## ## ## ## ## ## ## ## ## ## ## ! purple
|`## ## ## #* ## ## ## `## ## ## #* ## ## ## ! blue
| ## ## ## ## ## ## ## ## ## ## ## ## ## ##? green
|` ## ## ## `## ## ## #* ## ## ## `## ## ## #? orange
|
|########################################################?
|`#############*#############*#############*#############?
If you have a color monitor, you can see how each color is shifted a half a dot
from the one above. You can see the individual dots better in monochrome. The IIGS
has a B&W mode for hi-res but it eliminates the shifting. Use an NTSC monochrome
monitor instead of the RGB display..
The following shows other colors you can get in HR. It shows that 2 bits side by
side don't always give white! 2 bits are set at either side of an even/odd or
odd/even byte boundary or 1 bit is set at the end of a byte boundary. The colors
that the bits line up with are abbreviated P, B, G, O. s means high bit set, c
means high bit clear.
(these line up with the tables above also)
|0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 : bit
| even byte 0 | odd byte 1 | even byte 2 | odd byte 3 | : byte
Aqua ### BG
Dark Blue #. Bc
Light Blue ##* Ps
OP ### Pink
Oc #. Brown
Gs ##* Yellow
The applesoft program below draws some color lines on the HR screen and shows
all the colors you can get (you have to press a key alot. 80 columns is used for
labeling the colors):
10 PRINT CHR$ (4)"PR#3"
20 TEXT : HOME : VTAB 20: HGR
30 REM ****** COLOR BANDS PURPLE BLUE GREEN ORANGE (ORDERED BY HALF DOT
40 REM POSITIONS FROM LEFT SIDE OF SCREEN).
50 HCOLOR= 2: HPLOT 0,10 TO 279,10
60 HCOLOR= 6: HPLOT 0,11 TO 279,11
70 HCOLOR= 1: HPLOT 0,12 TO 279,12
80 HCOLOR= 5: HPLOT 0,13 TO 279,13
90 HCOLOR= 3: HPLOT 0,14 TO 279,14
100 HCOLOR= 7: HPLOT 0,15 TO 279,15
110 GET A$
120 HGR
130 C(0) = 2:C(1) = 6:C(2) = 1:C(3) = 5
140 REM ****** DRAW 2 COLORS SIDE BY SIDE.
150 REM LEFT COLOR ON EVEN BYTE, RIGHT ON ODD.
160 XL(0) = 6:XL(1) = 6:XL(2) = 5:XL(3) = 5
170 XR(0) = 8:XR(1) = 8:XR(2) = 7:XR(3) = 7
180 FOR CL = 0 TO 3: FOR CR = 0 TO 3:X = (CL * 4 + CR) * 14
190 HCOLOR= C(CL): HPLOT X + XL(CL),0 TO X + XL(CL),79
200 GET A$
210 HCOLOR= C(CR): HPLOT X + XR(CR),0 TO X + XR(CR),79
220 GET A$
230 NEXT : NEXT
240 REM ****** DRAW 2 COLORS SIDE BY SIDE.
250 REM LEFT COLOR ON ODD BYTE, RIGHT ON EVEN.
260 XL(0) = 12:XL(1) = 12:XL(2) = 13:XL(3) = 13
270 XR(0) = 14:XR(1) = 14:XR(2) = 15:XR(3) = 15
280 FOR CL = 0 TO 3: FOR CR = 0 TO 3:X = (CL * 4 + CR) * 14
290 HCOLOR= C(CL): HPLOT X + XL(CL),80 TO X + XL(CL),159
300 GET A$
310 HCOLOR= C(CR): HPLOT X + XR(CR),80 TO X + XR(CR),159
320 GET A$
330 NEXT : NEXT
340 REM ****** DRAW DARK BLUE, BROWN, LIGHT BLUE, YELLOW LINES.
350 HCOLOR= 7: HPLOT 230,0 TO 230,79: HPLOT 237,80 TO 237,159
360 HCOLOR= 3: HPLOT 244,0 TO 244,79: HPLOT 251,80 TO 251,159
370 GET A$
380 HCOLOR= 4: HPLOT 245,0 TO 245,79: HPLOT 252,80 TO 252,159
390 GET A$
400 REM ****** LABEL THE COLOR LINES WITH THE COLORS USED ON THE LEFT AND
410 REM RIGHT SIDE OF THE BYTE BOUNDARY. ALSO LABEL OTHER COLORS.
420 C$(0) = "P":C$(1) = "B":C$(2) = "G":C$(3) = "O"
430 VTAB 21
440 FOR CL = 0 TO 3: FOR CR = 0 TO 3: PRINT " "C$(CL)C$(CR);: NEXT : NEXT
450 PRINT " BcOcPsGs"
460 PRINT "TOP ROW: EVEN BYTE/ODD BYTE ";
470 PRINT "BOTTOM ROW: ODD BYTE/EVEN BYTE"
480 REM ****** CLEAR AND SET HIGH BITS OF SOME ROWS OF EVEN/ODD COLOR LINES.
490 FOR I = 0 TO 279 STEP 14
500 HCOLOR= 0: HPLOT I,30 TO I,39: HPLOT I + 13,30 TO I + 13,39
510 HCOLOR= 4: HPLOT I,40 TO I,49: HPLOT I + 13,40 TO I + 13,49: NEXT
520 REM ****** CLEAR AND SET HIGH BITS OF SOME ROWS OF ODD/EVEN COLOR LINES.
530 FOR I = 7 TO 266 STEP 14
540 HCOLOR= 0: HPLOT I,110 TO I,119: HPLOT I + 13,110 TO I + 13,119
550 HCOLOR= 4: HPLOT I,120 TO I,129: HPLOT I + 13,120 TO I + 13,129: NEXT
--------------------------------------------------------------------------------
Line addresses: reference COMPUTE! August 1983 pp. 172-177
Brief explanation of line number (eg 0-191) to line start address conversion
(eg $2000-$4000):
For text mode, the display is divided in 3 and each third is split in 8 rows. A row
is a character's vertical position. For HR and DHR, these are split into 8 character
lines (the height of a character is 8 pixels or hi-res lines).
third: 00,01,10__ ___character line 000..111
bit: 76543210 text line number: 000ABCDE
HGR line # bits: ABCDEFGH lo-res line #: 00ABCDEF
^^^row in third 000..111 even/odd row, lo, hi nibble^
high byte 76543210 76543210 low byte
HGR line start address bits: 0xxFGHCD EABAB000
xx = 01 for page 1, 10 for page 2.
text or lo-res line start address bits: 0000xxCD EABAB000
xx = 01 for page 1, 10 for page 2.
For lo-res, if F is 0 then top even line of row, low nibble $0x for byte, else
bottom odd line of row, high nibble $x0 for byte.
--------------------------------------------------------------------------------
The problem with the method in tn-aiie-003 and IIGS Hardware Reference is:
(example)
Take a white DHR pixel at bit 4 of first byte:
|0123456012345601234560123456
| ab0 | mb1 | ab2 | mb3 |
| ####
Take 4 bit windows at every bit (| | is a 4 bit window). Look in the DHR table
at the window's postion and match the 4 bits with a color:
DHR color table
|0123456012345601234560123456
| Black
| # # # # # # # Magenta
| ## ## ## ## ## ## ## Orange
| ### ### ### ### ### ### ### Yellow
|# # # # # # # Dark Blue
|## ## ## ## ## ## ## Blue
|### ### ### ### ### ### ### Aqua
|############################ White
V-bit position of moving window
Vvvvvvvvvvvvvv-bits in the window
Vvvvvvvvvvv- color the window represents
V- abbreviation
_|______________|___________|_
0| | Black K
1|| #| Dark Blue D
2| | ##| Blue B
3| | ###| Aqua A
4| |####| White W
5| |### | Yellow Y
6| |## | Orange O
7| |# | Magenta M
8| | | Black K
Using this method, the white dot is output not as WWWW but as DBAWYOMK (in very
small color pixels). But this doesn't seem it would look white on the IIGS's screen
(and it won't look white on a Mac screen). It's supposed to be a white dot. Maybe
the colors just blend together to make it look more white? (because of the size of
the phosphors compared to the size of the pixels..) Maybe it's like dithering in
640 Super Hi-Res mode? (DHR is 560).............
Does the Mega II actually use this method for it's DHR conversion to RGB? It
definitely has to do more than just say 010 is green and 101 is purple and any
2 adjacent 1's is white.. etc.