Make a guess at the value before doing the binary search, cuts the number of conditionals by half. Not as much gain as I hoped but something.

stb_image_resize.h

@@ -677,20 +677,32 @@ static float stbir__linear_to_srgb(float f)
         return 1.055f * (float)pow(f, 1 / 2.4f) - 0.055f;
 }
 
+// Used as a starting point to save time in the binary search in stbir__linear_to_srgb_uchar.
+static unsigned char stbr__linear_uchar_to_srgb_uchar[] = {
+    0, 12, 21, 28, 33, 38, 42, 46, 49, 52, 55, 58, 61, 63, 66, 68, 70, 73, 75, 77, 79, 81, 82, 84, 86, 88, 89, 91, 93, 94,
+    96, 97, 99, 100, 102, 103, 104, 106, 107, 109, 110, 111, 112, 114, 115, 116, 117, 118, 120, 121, 122, 123, 124, 125, 126,
+    127, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 142, 143, 144, 145, 146, 147, 148, 149, 150,
+    151, 151, 152, 153, 154, 155, 156, 157, 157, 158, 159, 160, 161, 161, 162, 163, 164, 165, 165, 166, 167, 168, 168, 169,
+    170, 171, 171, 172, 173, 174, 174, 175, 176, 176, 177, 178, 179, 179, 180, 181, 181, 182, 183, 183, 184, 185, 185, 186,
+    187, 187, 188, 189, 189, 190, 191, 191, 192, 193, 193, 194, 194, 195, 196, 196, 197, 197, 198, 199, 199, 200, 201, 201,
+    202, 202, 203, 204, 204, 205, 205, 206, 206, 207, 208, 208, 209, 209, 210, 210, 211, 212, 212, 213, 213, 214, 214, 215,
+    215, 216, 217, 217, 218, 218, 219, 219, 220, 220, 221, 221, 222, 222, 223, 223, 224, 224, 225, 226, 226, 227, 227, 228,
+    228, 229, 229, 230, 230, 231, 231, 232, 232, 233, 233, 234, 234, 235, 235, 236, 236, 237, 237, 237, 238, 238, 239, 239,
+    240, 240, 241, 241, 242, 242, 243, 243, 244, 244, 245, 245, 245, 246, 246, 247, 247, 248, 248, 249, 249, 250, 250, 251,
+    251, 251, 252, 252, 253, 253, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255
+};
+
 static unsigned char stbir__linear_to_srgb_uchar(float f)
 {
     int x = (int) (f * (1 << 28)); // has headroom so you don't need to clamp
-    int v = 0;
+    int v = stbr__linear_uchar_to_srgb_uchar[(int)(f * 255)]; // Make a guess at the value with a table.
     int i;
 
-    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;
+    // Refine the guess with a short binary search.
+    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;
 }

tests/resample_test.cpp

@@ -799,10 +799,6 @@ void test_suite(int argc, char **argv)
 	else
 		barbara = "barbara.png";
 
-	test_format<unsigned short>(barbara, 0.5, 2.0, STBIR_TYPE_UINT16, STBIR_COLORSPACE_SRGB);
-    test_32();
-
-
 	// check what cases we need normalization for
 #if 1
 	{
@@ -877,6 +873,8 @@ void test_suite(int argc, char **argv)
 
 	test_premul();
 
+	test_32();
+
 	// Some tests to make sure errors don't pop up with strange filter/dimension combinations.
 	stbir_resize(image88, 8, 8, 0, output88, 4, 16, 0, STBIR_TYPE_UINT8, 1, STBIR_ALPHA_CHANNEL_NONE, 0, STBIR_EDGE_CLAMP, STBIR_EDGE_CLAMP, STBIR_FILTER_BOX, STBIR_FILTER_CATMULLROM, STBIR_COLORSPACE_SRGB, &g_context);
 	stbir_resize(image88, 8, 8, 0, output88, 4, 16, 0, STBIR_TYPE_UINT8, 1, STBIR_ALPHA_CHANNEL_NONE, 0, STBIR_EDGE_CLAMP, STBIR_EDGE_CLAMP, STBIR_FILTER_CATMULLROM, STBIR_FILTER_BOX, STBIR_COLORSPACE_SRGB, &g_context);