diff --git a/CMakeLists.txt b/CMakeLists.txt
index e3dbb7c..acbfb72 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -61,7 +61,8 @@ endif ()
 
 # -lrt is only for glibc < 2.17
 # -liconv may or may not be a part of libc
-foreach (extra iconv rt)
+# -lm may or may not be a part of libc
+foreach (extra iconv rt m)
 	find_library (extra_lib_${extra} ${extra})
 	if (extra_lib_${extra})
 		list (APPEND project_libraries ${extra_lib_${extra}})
diff --git a/degesch.c b/degesch.c
index 3262691..0e70813 100644
--- a/degesch.c
+++ b/degesch.c
@@ -51,6 +51,7 @@ enum
 #include "common.c"
 #include "kike-replies.c"
 
+#include <math.h>
 #include <langinfo.h>
 #include <locale.h>
 #include <pwd.h>
@@ -2108,17 +2109,23 @@ filter_color_cube_for_acceptable_nick_colors (size_t *len)
 	// This is a pure function and we don't use threads, static storage is fine
 	static int table[6 * 6 * 6];
 	size_t len_counter = 0;
-	for (int x = 0; x < 6 * 6 * 6; x++)
+	for (int x = 0; x < N_ELEMENTS (table); x++)
 	{
-		// FIXME this isn't exactly right, the values aren't linear
 		int r =  x / 36;
 		int g = (x / 6) % 6;
 		int b = (x % 6);
 
-		// Use the luma value of colours within the cube to filter colours that
-		// look okay-ish on terminals with both black and white backgrounds
-		double luma = 0.2126 * r / 6. + 0.7152 * g / 6. + 0.0722 * b / 6.;
-		if (luma >= .3 && luma <= .5)
+		// The first step is 95/255, the rest are 40/255,
+		// as an approximation we can double the first step
+		double linear_R = pow ((r + !!r) / 6., 2.2);
+		double linear_G = pow ((g + !!g) / 6., 2.2);
+		double linear_B = pow ((b + !!b) / 6., 2.2);
+
+		// Use the relative luminance of colours within the cube to filter
+		// colours that look okay-ish on terminals with both black and white
+		// backgrounds (use the test-nick-colors script to calibrate)
+		double Y = 0.2126 * linear_R + 0.7152 * linear_G + 0.0722 * linear_B;
+		if (Y >= .25 && Y <= .4)
 			table[len_counter++] = 16 + x;
 	}
 	*len = len_counter;
diff --git a/test-nick-colors b/test-nick-colors
new file mode 100755
index 0000000..a09f74a
--- /dev/null
+++ b/test-nick-colors
@@ -0,0 +1,23 @@
+#!/bin/sh
+# Check whether the terminal colours filtered by our algorithm are legible
+export example=$(
+	tcc "-run -lm" - <<-EOF
+	#include <stddef.h>
+	#include <stdio.h>
+	#include <math.h>
+
+	$(perl -0777 -ne 'print $& if /^.*?\nfilter_color(?s:.*?)^}$/m' degesch.c)
+
+	void main () {
+		size_t len = 0;
+		int *table = filter_color_cube_for_acceptable_nick_colors (&len);
+		for (size_t i = 0; i < len; i++)
+			printf ("<@\\x1b[38;5;%dmIRCuser\\x1b[m> I'm typing!\n", table[i]);
+	}
+	EOF
+)
+
+# Both should give acceptable results,
+# which results in a bad compromise that the main author himself needs
+xterm -bg black -fg white -e 'echo $example; cat' &
+xterm -bg white -fg black -e 'echo $example; cat' &