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 image processing  we usually want pixel components to range from 0 to 255  I therefore want to convert the jbyte value to the range 0 to 255  i e  the same range as unsigned char   do some calculations on the value and then store the result as a jbyte again   How can I do these conversion safely    I managed to get this code to work  where a pixel value is incremented by 30 but clamped to the value 255  but I don t understand if it s safe or portable     define CLAMP255 v   v  gt  255   255    v  lt  0   0   v     jbyte pixel        pixel   CLAMP 255  unsigned char pixel   30     I m interested to know how to do this in both C and C

User · Answer

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.

User · Answer

I m not 100  sure that I understand your question  so tell me if I m wrong   If I got it right  you are reading jbytes that are technically signed chars  but really pixel values ranging from 0 to 255  and you re wondering how you should handle them without corrupting the values in the process   Then  you should do the following    convert jbytes to unsigned char before doing anything else  this will definetly restore the pixel values you are trying to manipulate use a larger signed integer type  such as int while doing intermediate calculations  this to make sure that over- and underflows can be detected and dealt with  in particular  not casting to a signed type could force to compiler to promote every type to an unsigned type in which case you wouldn t be able to detect underflows later on  when assigning back to a jbyte  you ll want to clamp your value to the 0-255 range  convert to unsigned char and then convert again to signed char  I m not certain the first conversion is strictly necessary  but you just can t be wrong if you do both   For example   inline int fromJByte jbyte pixel           cast to unsigned char re-interprets values as 0-255        cast to int will make intermediate calculations safer     return static cast lt int gt  static cast lt unsigned char gt  pixel       inline jbyte fromInt int pixel        if pixel  lt  0          pixel   0       if pixel  gt  255          pixel   255       return static cast lt jbyte gt  static cast lt unsigned char gt  pixel       jbyte in       int intermediate   fromJByte in    30  jbyte out   fromInt intermediate

User · Answer

Do you realize  that CLAMP255 returns 0 for v  lt  0 and 255 for v    0  IMHO  CLAMP255 should be defined as      define CLAMP255 v   v  gt  255   255    v  lt  0   0   v     Difference  If v is not greater than 255 and not less than 0  return v instead of 255

User · Answer

Yes this is safe   The c language uses a feature called integer promotion to increase the number of bits in a value before performing calculations  Therefore your CLAMP255 macro will operate at integer  probably 32 bit  precision  The result is assigned to a jbyte  which reduces the integer precision back to 8 bits fit in to the jbyte

User · Answer

This is one of the reasons why C   introduced the new cast style  which includes static cast and reinterpret cast  There s two things you can mean by saying conversion from signed to unsigned  you might mean that you wish the unsigned variable to contain the value of the signed variable modulo the maximum value of your unsigned type   1  That is if your signed char has a value of -128 then CHAR MAX 1 is added for a value of 128 and if it has a value of -1  then CHAR MAX 1 is added for a value of 255  this is what is done by static cast  On the other hand you might mean to interpret the bit value of the memory referenced by some variable to be interpreted as an unsigned byte  regardless of the signed integer representation used on the system  i e  if it has bit value 0b10000000 it should evaluate to value 128  and 255 for bit value 0b11111111  this is accomplished with reinterpret cast   Now  for the two s complement representation this happens to be exactly the same thing  since -128 is represented as 0b10000000 and -1 is represented as 0b11111111 and likewise for all in between  However other computers  usually older architectures  may use different signed representation such as sign-and-magnitude or ones  complement  In ones  complement the 0b10000000 bitvalue would not be -128  but -127  so a static cast to unsigned char would make this 129  while a reinterpret cast would make this 128  Additionally in ones  complement the 0b11111111 bitvalue would not be -1  but -0   yes this value exists in ones  complement   and would be converted to a value of 0 with a static cast  but a value of 255 with a reinterpret cast  Note that in the case of ones  complement the unsigned value of 128 can actually not be represented in a signed char  since it ranges from -127 to 127  due to the -0 value   I have to say that the vast majority of computers will be using two s complement making the whole issue moot for just about anywhere your code will ever run  You will likely only ever see systems with anything other than two s complement in very old architectures  think  60s timeframe   The syntax boils down to the following   signed char x   -100  unsigned char y   y    unsigned char x                        C static y     unsigned char    amp x                    C reinterpret y   static cast lt unsigned char gt  x            C   static y   reinterpret cast lt unsigned char amp  gt  x      C   reinterpret   To do this in a nice C   way with arrays   jbyte memory buffer nr pixels   unsigned char  pixels   reinterpret cast lt unsigned char  gt  memory buffer     or the C way   unsigned char  pixels    unsigned char  memory buffer

[c++] Converting from signed char to unsigned char and back again?

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