+static uint8_t gamma_corr(uint8_t in) {
+ unsigned lo, hi = 0xff40, target;
+ int i = 15;
+ in = (in << 2) | (in >> 6);
+ /* equivalent float code:
+ if (in >= 252)
+ return 253;
+ return round(pow(in / 256.0, 0.8) * 256);
+ */
+ lo = target = in << 8;
+ do {
+ unsigned mid = (lo + hi) >> 1;
+ unsigned pow = pow5(mid);
+ if (pow > target) hi = mid;
+ else lo = mid;
+ } while (--i);
+ return (pow4((lo + hi) >> 1) + 0x80) >> 8;
+}
+#else
+/**
+ * This is a gamma correction that xan3 applies to all palette entries.
+ *
+ * There is a peculiarity, namely that the values are clamped to 253 -
+ * it seems likely that this table was calculated by a buggy fixed-point
+ * implementation, the one above under RUNTIME_GAMMA behaves like this for
+ * example.
+ * The exponent value of 0.8 can be explained by this as well, since 0.8 = 4/5
+ * and thus pow(x, 0.8) is still easy to calculate.
+ * Also, the input values are first rotated to the left by 2.
+ */
+static const uint8_t gamma_lookup[256] = {
+ 0x00, 0x09, 0x10, 0x16, 0x1C, 0x21, 0x27, 0x2C,
+ 0x31, 0x35, 0x3A, 0x3F, 0x43, 0x48, 0x4C, 0x50,
+ 0x54, 0x59, 0x5D, 0x61, 0x65, 0x69, 0x6D, 0x71,
+ 0x75, 0x79, 0x7D, 0x80, 0x84, 0x88, 0x8C, 0x8F,
+ 0x93, 0x97, 0x9A, 0x9E, 0xA2, 0xA5, 0xA9, 0xAC,
+ 0xB0, 0xB3, 0xB7, 0xBA, 0xBE, 0xC1, 0xC5, 0xC8,
+ 0xCB, 0xCF, 0xD2, 0xD5, 0xD9, 0xDC, 0xDF, 0xE3,
+ 0xE6, 0xE9, 0xED, 0xF0, 0xF3, 0xF6, 0xFA, 0xFD,
+ 0x03, 0x0B, 0x12, 0x18, 0x1D, 0x23, 0x28, 0x2D,
+ 0x32, 0x36, 0x3B, 0x40, 0x44, 0x49, 0x4D, 0x51,
+ 0x56, 0x5A, 0x5E, 0x62, 0x66, 0x6A, 0x6E, 0x72,
+ 0x76, 0x7A, 0x7D, 0x81, 0x85, 0x89, 0x8D, 0x90,
+ 0x94, 0x98, 0x9B, 0x9F, 0xA2, 0xA6, 0xAA, 0xAD,
+ 0xB1, 0xB4, 0xB8, 0xBB, 0xBF, 0xC2, 0xC5, 0xC9,
+ 0xCC, 0xD0, 0xD3, 0xD6, 0xDA, 0xDD, 0xE0, 0xE4,
+ 0xE7, 0xEA, 0xED, 0xF1, 0xF4, 0xF7, 0xFA, 0xFD,
+ 0x05, 0x0D, 0x13, 0x19, 0x1F, 0x24, 0x29, 0x2E,
+ 0x33, 0x38, 0x3C, 0x41, 0x45, 0x4A, 0x4E, 0x52,
+ 0x57, 0x5B, 0x5F, 0x63, 0x67, 0x6B, 0x6F, 0x73,
+ 0x77, 0x7B, 0x7E, 0x82, 0x86, 0x8A, 0x8D, 0x91,
+ 0x95, 0x99, 0x9C, 0xA0, 0xA3, 0xA7, 0xAA, 0xAE,
+ 0xB2, 0xB5, 0xB9, 0xBC, 0xBF, 0xC3, 0xC6, 0xCA,
+ 0xCD, 0xD0, 0xD4, 0xD7, 0xDA, 0xDE, 0xE1, 0xE4,
+ 0xE8, 0xEB, 0xEE, 0xF1, 0xF5, 0xF8, 0xFB, 0xFD,
+ 0x07, 0x0E, 0x15, 0x1A, 0x20, 0x25, 0x2A, 0x2F,
+ 0x34, 0x39, 0x3D, 0x42, 0x46, 0x4B, 0x4F, 0x53,
+ 0x58, 0x5C, 0x60, 0x64, 0x68, 0x6C, 0x70, 0x74,
+ 0x78, 0x7C, 0x7F, 0x83, 0x87, 0x8B, 0x8E, 0x92,
+ 0x96, 0x99, 0x9D, 0xA1, 0xA4, 0xA8, 0xAB, 0xAF,
+ 0xB2, 0xB6, 0xB9, 0xBD, 0xC0, 0xC4, 0xC7, 0xCB,
+ 0xCE, 0xD1, 0xD5, 0xD8, 0xDB, 0xDF, 0xE2, 0xE5,
+ 0xE9, 0xEC, 0xEF, 0xF2, 0xF6, 0xF9, 0xFC, 0xFD
+};
+#endif
+