fix stbir__linear_to_srgb_uchar:

    1. table stored threshhold of transition from i to i+1, but wants to be i-1 to i
    2. table was computed by dividing uchar by 256.0 instead of 255.0, causing it to be 100% wrong

stb_image_resize.h

@@ -450,38 +450,38 @@ static float stbir__srgb_uchar_to_linear_float[256] = {
 // sRGB transition values, scaled by 1<<28
 static int stbir__srgb_offset_to_linear_scaled[256] =
 {
-	    40579,    121738,    202897,    284056,    365216,    446375,    527534,    608693,
-	   689852,    771011,    852421,    938035,   1028466,   1123787,   1224073,   1329393,
-	  1439819,   1555418,   1676257,   1802402,   1933917,   2070867,   2213313,   2361317,
-	  2514938,   2674237,   2839271,   3010099,   3186776,   3369359,   3557903,   3752463,
-	  3953090,   4159840,   4372764,   4591913,   4817339,   5049091,   5287220,   5531775,
-	  5782804,   6040356,   6304477,   6575216,   6852618,   7136729,   7427596,   7725263,
-	  8029775,   8341176,   8659511,   8984821,   9317151,   9656544,  10003040,  10356683,
-	 10717513,  11085572,  11460901,  11843540,  12233529,  12630908,  13035717,  13447994,
-	 13867779,  14295110,  14730025,  15172563,  15622760,  16080655,  16546285,  17019686,
-	 17500894,  17989948,  18486882,  18991734,  19504536,  20025326,  20554138,  21091010,
-	 21635972,  22189062,  22750312,  23319758,  23897432,  24483368,  25077600,  25680162,
-	 26291086,  26910406,  27538152,  28174360,  28819058,  29472282,  30134062,  30804430,
-	 31483418,  32171058,  32867378,  33572412,  34286192,  35008744,  35740104,  36480296,
-	 37229356,  37987316,  38754196,  39530036,  40314860,  41108700,  41911584,  42723540,
-	 43544600,  44374792,  45214140,  46062680,  46920440,  47787444,  48663720,  49549300,
-	 50444212,  51348480,  52262136,  53185204,  54117712,  55059688,  56011160,  56972156,
-	 57942704,  58922824,  59912552,  60911908,  61920920,  62939616,  63968024,  65006168,
-	 66054072,  67111760,  68179272,  69256616,  70343832,  71440936,  72547952,  73664920,
-	 74791848,  75928776,  77075720,  78232704,  79399760,  80576904,  81764168,  82961576,
-	 84169152,  85386920,  86614904,  87853120,  89101608,  90360384,  91629480,  92908904,
-	 94198688,  95498864,  96809440,  98130456,  99461928, 100803872, 102156320, 103519296,
-	104892824, 106276920, 107671616, 109076928, 110492880, 111919504, 113356808, 114804824,
-	116263576, 117733080, 119213360, 120704448, 122206352, 123719104, 125242720, 126777232,
-	128322648, 129879000, 131446312, 133024600, 134613888, 136214192, 137825552, 139447968,
-	141081456, 142726080, 144381808, 146048704, 147726768, 149416016, 151116496, 152828192,
-	154551168, 156285408, 158030944, 159787808, 161556000, 163335568, 165126512, 166928864,
-	168742640, 170567856, 172404544, 174252704, 176112384, 177983568, 179866320, 181760640,
-	183666528, 185584032, 187513168, 189453952, 191406400, 193370544, 195346384, 197333952,
-	199333264, 201344352, 203367216, 205401904, 207448400, 209506752, 211576960, 213659056,
-	215753056, 217858976, 219976832, 222106656, 224248464, 226402272, 228568096, 230745952,
-	232935872, 235137872, 237351968, 239578176, 241816512, 244066992, 246329648, 248604512,
-	250891568, 253190848, 255502368, 257826160, 260162240, 262510608, 264871312, 267244336,
+	        0,     40738,    122216,    203693,    285170,    366648,    448125,    529603,
+	   611080,    692557,    774035,    855852,    942009,   1033024,   1128971,   1229926,
+	  1335959,   1447142,   1563542,   1685229,   1812268,   1944725,   2082664,   2226148,
+	  2375238,   2529996,   2690481,   2856753,   3028870,   3206888,   3390865,   3580856,
+	  3776916,   3979100,   4187460,   4402049,   4622919,   4850123,   5083710,   5323731,
+	  5570236,   5823273,   6082892,   6349140,   6622065,   6901714,   7188133,   7481369,
+	  7781466,   8088471,   8402427,   8723380,   9051372,   9386448,   9728650,  10078021,
+	 10434603,  10798439,  11169569,  11548036,  11933879,  12327139,  12727857,  13136073,
+	 13551826,  13975156,  14406100,  14844697,  15290987,  15745007,  16206795,  16676389,
+	 17153826,  17639142,  18132374,  18633560,  19142734,  19659934,  20185196,  20718552,
+	 21260042,  21809696,  22367554,  22933648,  23508010,  24090680,  24681686,  25281066,
+	 25888850,  26505076,  27129772,  27762974,  28404716,  29055026,  29713942,  30381490,
+	 31057708,  31742624,  32436272,  33138682,  33849884,  34569912,  35298800,  36036568,
+	 36783260,  37538896,  38303512,  39077136,  39859796,  40651528,  41452360,  42262316,
+	 43081432,  43909732,  44747252,  45594016,  46450052,  47315392,  48190064,  49074096,
+	 49967516,  50870356,  51782636,  52704392,  53635648,  54576432,  55526772,  56486700,
+	 57456236,  58435408,  59424248,  60422780,  61431036,  62449032,  63476804,  64514376,
+	 65561776,  66619028,  67686160,  68763192,  69850160,  70947088,  72053992,  73170912,
+	 74297864,  75434880,  76581976,  77739184,  78906536,  80084040,  81271736,  82469648,
+	 83677792,  84896192,  86124888,  87363888,  88613232,  89872928,  91143016,  92423512,
+	 93714432,  95015816,  96327688,  97650056,  98982952, 100326408, 101680440, 103045072,
+	104420320, 105806224, 107202800, 108610064, 110028048, 111456776, 112896264, 114346544,
+	115807632, 117279552, 118762328, 120255976, 121760536, 123276016, 124802440, 126339832,
+	127888216, 129447616, 131018048, 132599544, 134192112, 135795792, 137410592, 139036528,
+	140673648, 142321952, 143981456, 145652208, 147334208, 149027488, 150732064, 152447968,
+	154175200, 155913792, 157663776, 159425168, 161197984, 162982240, 164777968, 166585184,
+	168403904, 170234160, 172075968, 173929344, 175794320, 177670896, 179559120, 181458992,
+	183370528, 185293776, 187228736, 189175424, 191133888, 193104112, 195086128, 197079968,
+	199085648, 201103184, 203132592, 205173888, 207227120, 209292272, 211369392, 213458480,
+	215559568, 217672656, 219797792, 221934976, 224084240, 226245600, 228419056, 230604656,
+	232802400, 235012320, 237234432, 239468736, 241715280, 243974080, 246245120, 248528464,
+	250824112, 253132064, 255452368, 257785040, 260130080, 262487520, 264857376, 267239664,
 };
 
 static float stbir__srgb_to_linear(float f)
@@ -506,23 +506,16 @@ static unsigned char stbir__linear_to_srgb_uchar(float f)
 	int v = 0;
 	int i;
 
-	// Everything below 128 is off by 1. This fixes that.
-	int fix = 0;
+	i = v + 128; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
+	i = v +  64; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
+	i = v +  32; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
+	i = v +  16; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
+	i = v +   8; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
+	i = v +   4; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
+	i = v +   2; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
+	i = v +   1; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
 
-	// Adding 1 to 0 with the fix gives incorrect results for input 0. This fixes that.
-	if (x < 81000)
-		return 0;
-
-	i =    128; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i; else fix = 1;
-	i = v + 64; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
-	i = v + 32; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
-	i = v + 16; if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
-	i = v + 8;  if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
-	i = v + 4;  if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
-	i = v + 2;  if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
-	i = v + 1;  if (x >= stbir__srgb_offset_to_linear_scaled[i]) v = i;
-
-	return (unsigned char)v + fix;
+	return (unsigned char) v;
 }
 
 static float stbir__filter_trapezoid(float x, float scale)

tests/resample_test.cpp

@@ -810,21 +810,22 @@ void test_suite(int argc, char **argv)
 	}
 #endif
 
-	for (i = 0; i < 256; i++) {
-		float f = stbir__srgb_to_linear(float(i) / 255);
-		int n = stbir__linear_to_srgb_uchar(f);
-		STBIR_ASSERT(n == i);
-	}
-
 #if 0 // linear_to_srgb_uchar table
 	for (i=0; i < 256; ++i) {
-		float f = stbir__srgb_to_linear((i+0.5f)/256.0f);
+		float f = stbir__srgb_to_linear((i-0.5f)/255.0f);
 		printf("%9d, ", (int) ((f) * (1<<28)));
 		if ((i & 7) == 7)
 			printf("\n");
 	}
 #endif
 
+	for (i = 0; i < 256; i++) {
+		float f = stbir__srgb_to_linear(float(i) / 255);
+		int n = stbir__linear_to_srgb_uchar(f);
+		STBIR_ASSERT(n == i);
+	}
+
+
 	test_filters();
 
 	test_subpixel_1();