Converting from signed char to unsigned char and back again?

Question

I\'m working with JNI and have an array of type jbyte, where jbyte is represented as an signed char i.e. ranging from -128 to 127. The jbytes represent image pixels. For ima

Answer 1

There are two ways to interpret the input data; either -128 is the lowest value, and 127 is the highest (i.e. true signed data), or 0 is the lowest value, 127 is somewhere in the middle, and the next "higher" number is -128, with -1 being the "highest" value (that is, the most significant bit already got misinterpreted as a sign bit in a two's complement notation.

Assuming you mean the latter, the formally correct way is

signed char in = ...
unsigned char out = (in < 0)?(in + 256):in;

which at least gcc properly recognizes as a no-op.