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More accurate 8-bit <=> 5-bit RGB color conversion (#1827)

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Rangi
2025-09-08 15:13:25 -04:00
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parent 65d408eb5d
commit 223b3d1921
20 changed files with 31 additions and 15 deletions
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@@ -461,6 +461,24 @@ Previously we had
.Pp
Instead, now we have
.Ql p ** q ** r == p ** (q ** r) .
.Ss 8-bit and 5-bit color conversion
Changed in 1.0.0.
.Pp
RGBGFX takes 8-bit RGB colors as its PNG input, and outputs 5-bit GBC colors.
Its
.Ql -r/--reverse
mode does the opposite 5-bit to 8-bit conversion.
Instead of the previous inaccurate conversions, we now do accurate rounding to the nearest equivalent.
.Pp
Previously to convert an 8-bit color channel to 5-bit, we truncated it as
.Ql c >> 3 ;
and to reverse a 5-bit color channel to 8-bit, we extended it as
.Ql (c << 3) | (c >> 2) .
.Pp
Instead, now we round 8-bit to 5-bit as
.Ql (c * 31 + 127) / 255 ,
and round 5-bit to 8-bit as
.Ql (c * 255 + 15) / 31 .
.Sh BUGS
These are misfeatures that may have been possible by mistake.
They do not get deprecated, just fixed.