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improve 256-color grey quantization

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nick black 2021-07-09 00:46:48 -04:00
parent b96dc6d1bc
commit 55f8614391
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GPG Key ID: 5F43400C21CBFACC
2 changed files with 21 additions and 33 deletions
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22
src/lib/internal.h
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@ -497,16 +497,12 @@ nfbcellidx(const ncplane* n, int row, int col){
return fbcellidx(logical_to_virtual(n, row), n->lenx, col);
}
// is the rgb value greyish (excluding white and black)?
// is the rgb value greyish? note that pure white and pure black are both
// considered greyish according to the definition of this function =].
static inline bool
rgb_greyish_p(unsigned r, unsigned g, unsigned b){
const unsigned GREYMASK = 0xf8;
if((r & GREYMASK) == (g & GREYMASK) && (g & GREYMASK) == (b & GREYMASK)){
if(r < 16){
return false; // black
}else if(r > 240){
return false; // white
}
return true;
}
return false;
@ -522,15 +518,14 @@ rgb_quantize_256(unsigned r, unsigned g, unsigned b){
// if all 5 MSBs match, return grey from 24-member grey ramp or pure
// black/white from original 16 (0 and 15, respectively)
if(rgb_greyish_p(r, g, b)){
// 256 / 26 == 9.846
int gidx = r * 5 / 49 - 1;
if(gidx < 0){
// 8 and 238 come from https://terminalguide.namepad.de/attr/fgcol256/,
// which suggests 10-nit intervals otherwise.
if(r < 8){
return 0;
}
if(gidx >= 24){
}else if(r > 238){
return 15;
}
return 232 + gidx;
return 232 + (r - 8) / 10;
}
r /= 43;
g /= 43;
@ -551,6 +546,9 @@ rgb_quantize_8(unsigned r, unsigned g, unsigned b){
static const int CYAN = 6;
static const int WHITE = 7;
if(rgb_greyish_p(r, g, b)){
if(r < 64){
return BLACK;
}
return WHITE;
}
if(r < 128){ // we have no red

32
src/tests/internal.cpp
View File

@ -5,27 +5,17 @@
TEST_CASE("Internals") {
SUBCASE("RGBtoANSIWhite") {
unsigned r, g, b;
for(r = 250 ; r < 256 ; ++r){
g = b = r;
CHECK(15 == rgb_quantize_256(r, g, b));
}
}
SUBCASE("RGBtoANSIBlack") {
unsigned r, g, b;
for(r = 0 ; r < 10 ; ++r){
g = b = r;
CHECK(0 == rgb_quantize_256(r, g, b));
}
}
SUBCASE("RGBtoANSIGrey") {
unsigned r, g, b;
for(r = 10 ; r < 244 ; ++r){
g = b = r;
CHECK(231 + (r * 5) / 49 == rgb_quantize_256(r, g, b));
SUBCASE("RGBtoANSIWhiteGreyBlack") {
unsigned r;
for(r = 0 ; r < 256 ; ++r){
int g = rgb_quantize_256(r, r, r);
if(r < 8){
CHECK(0 == g);
}else if(r > 238){
CHECK(15 == g);
}else{
CHECK(g == (r - 8) / 10 + 232);
}
}
}