I've seen that many people around here have liked very much these 105 color picture tricks for MSX1. Makes me wonder how about MSX2... Maybe people would like to see beautiful pictures on this platform as well?
So here we go... My idea of cute picture is not interlaced SCREEN 7 picture with 512 colors. How ever because I'm not too familiar with color reduction algorithms, dithering etc. stuff I'll leave this side open to anyone who is interested, I'll just show my idea of the way how to do it. So here we go...
When thinking about source format I suggest reading 24 bit BMP picture, because it is ideal for this kind of stuff. Just throw away header and you have RAW data that has one byte for each R,G & B value. For this purpose it is enough to take just 3 most high bits of each byte. Remember anyway that in BMP file the picture is upside down, so last line is actually first.
Here are the steps how I think conversion should be implemented: (Not 100% accurate, but I'm sure you'll get the idea)
- Select 16 colors that are best for representing the last line in picture (512 dots)
- Select 12 colors from generated pallette that are best for representing second last line in picture
- Select 4 colors that would be best with the existing 12 colors to form the second last line
- Check that these new selected colors are not same as the colors you did not select. If the colors are same, make sure they end up with same color number they were previously using. Mark these colors also with "no change" flag or something similar.
- Repeat last 3 steps untill you are in top of picture.
- Save the pallette you ended up (first 16 * %0RRR0BBB, %00000GGG of data in example display routine)
- Convert the first line of picture without using the colors that are going to be changed for next line. Only if these colors are marked with "no change" flag you can use them.
- For example display routine save pallette changes in following format after the exisiting pallette data: %0000CCCC (C=color number), %0RRR0BBB, %00000GGG
- Repeat 2 last steps untill you are in last line. In last line you can freely use all 16 colors.
What we end up is a picture where each line contains 12-16 unique colors. The total number of colors depends of the picture. Basic idea is, that the pallette is newer visually changed on color that the VDP is displaying.
If you want to improve this idea even further I think that you can define even 5 colors / line, but I think you'll end up just having more "no change" flags.
I think better idea is to split the picture in right and left side and think only 2 different changeing colors. This way you can define a color for example in left side of screen and use it on right side. You can go even further... In theory it should be possible to have even more than 16 colors in one line still without flickering!
BTW even that I've been talking about 16 colors all the time, you may want to keep one color solid in order to have one color backround.
If you like interlaced pictures then you can run this algorithm for odd and even lines separately. You can check wich pallette set should be used by reading VDP status register 2 bit 1.
You may also want to generate more colors using similar trick that was used in those MSX1 demos. Feel free, if you are skillfull enough...
Anyway... I'll leave future development for you... Here is the very simple example display routine for these kind of pictures:
DEFB #FE ; BASIC header.
DEFW BEGIN
DEFW END
DEFW START
ORG #D000
DATA: EQU #B000 ; Put your pallette data in this address
; 16 * %0RRR0BBB, %00000GGG (32 bytes of first line pallette)
; lines * 4 * %0000CCCC, %0RRR0BBB, %00000GGG (12 bytes / line)
BEGIN:
START:
LD A,(#2D)
CP 3
LD A,#80
CALL Z,#180 ; Change to Z80 if MSX tR
DI
IN A,(#AA)
AND #f0
or 8
OUT (#AA),A
LD A,#90 ; PALETTE register
OUT (#99),A
LD A,#91
OUT (#99),A
AGAIN:
XOR A
OUT (#9B),A
LD HL,DATA
LD BC,#209A
OTIR ; First palette
LD A,0
OUT (#99),A
LD A,#8F
OUT (#99),A
WAITLINE0:
IN A,(#99)
RRA
JP NC,WAITLINE0
LD A,2
OUT (#99),A
LD A,#8F
OUT (#99),A
LD D,212 ; Number of lines
WAIT:
IN A,(#99)
AND 32
JP Z,WAIT
INC C
OUTI
DEC C
OUTI
OUTI
INC C
OUTI
DEC C
OUTI
OUTI
INC C
OUTI
DEC C
OUTI
OUTI
INC C
OUTI
DEC C
OUTI
OUTI
DEC D
JP NZ,WAIT
NEXT:
IN A,(#A9)
AND 1
JP NZ,WAITLINE0 ; SPACE pressed?
EXIT:
XOR A
OUT (#99),A
LD A,#8F
OUT (#99),A
EI
RET
END:
