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palette: more accurate color conversion from 8 to 4 bits channels

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set_rgb8(), color8to4(), channel8to4()
This commit is contained in:
Irmen de Jong 2023-11-14 20:40:48 +01:00
parent 159f80d629
commit 86c6530e46
2 changed files with 19 additions and 9 deletions
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27
compiler/res/prog8lib/cx16/palette.p8
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@ -42,19 +42,30 @@ palette {
ubyte red
ubyte greenblue
repeat num_colors {
red = @(palette_bytes_ptr) >> 4
palette_bytes_ptr++
greenblue = @(palette_bytes_ptr) & %11110000
palette_bytes_ptr++
greenblue |= @(palette_bytes_ptr) >> 4 ; add Blue
palette_bytes_ptr++
cx16.vpoke(1, vera_palette_ptr, greenblue)
cx16.r1 = color8to4(palette_bytes_ptr)
palette_bytes_ptr+=3
cx16.vpoke(1, vera_palette_ptr, cx16.r1H) ; $GB
vera_palette_ptr++
cx16.vpoke(1, vera_palette_ptr, red)
cx16.vpoke(1, vera_palette_ptr, cx16.r1L) ; $0R
vera_palette_ptr++
}
}
sub color8to4(uword colorpointer) -> uword {
; accurately convert 24 bits (3 bytes) RGB color, in that order in memory, to 16 bits $GB;$0R colorvalue
cx16.r1 = colorpointer
cx16.r0 = channel8to4(@(cx16.r1)) ; (red) -> $00:0R
cx16.r0H = channel8to4(@(cx16.r1+1))<<4 ; (green) -> $G0:0R
cx16.r0H |= channel8to4(@(cx16.r1+2)) ; (blue) -> $GB:0R
return cx16.r0
}
sub channel8to4(ubyte channelvalue) -> ubyte {
; accurately convert a single 8 bit color channel value to 4 bits, see https://threadlocalmutex.com/?p=48
return msb(channelvalue * $000f + 135)
}
sub set_monochrome(uword screencolorRGB, uword drawcolorRGB) {
vera_palette_ptr = $fa00
cx16.vpoke(1, vera_palette_ptr, lsb(screencolorRGB)) ; G,B

1
docs/source/todo.rst
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@ -2,7 +2,6 @@
TODO
====
- palette: mnore accurate conversion to 4 bit colorspage in set_rgb8, by doing (x * 15 + 135) >> 8, see https://threadlocalmutex.com/?p=48
- fix the compiler crash on s1[n+1] = s1[2] where s1 is a subroutine param (regular variable works)
- improve the working of %option merge: should be able to merge your own stuff into say textio. , and improve the docs about it too.
- give error when using %option merge in module scope.