problem with converting a char array of integers to an int

[kenter13]

Hi

My problem:

I am reading some binary image and keep their pixel values in a char array. Then convert them to integer to use for my purposes. This is working for low pixel values. But I am having some problems with high pixel values!.
For example, if the original pixel value is 3375, my result is 61.
another example: org. pix. value: 3452, result:-127.

I have a int array and char array that contains original image values. For casting, I am doing this way:

within a loop

intArr[i][j] = (int)orgImgCharArray[i][j]

It is working values like 100, 50, 150 but not for higher values.

Thanks
Kenter

[mtweeman]

Char type supports numbers from 0 to 255 and I think that's the reason. Can't you just put this values directly to int array?

[kenter13]

Yeah your right. I am using ifstream::read member function to read the binary data. And it takes char array as parameter. Do you have a suggestion to do this in a different way?

Thanks
Kenter

[spaggy]

what's the format of your image file. If it's a .bmp then the pixel should be stored in triplets (RGB). And each of those is between 0 and 255.

[kenter13]

It is not a known format. It is a format which used in my university. And it is for monochromic images. So no rgb vales. And the values for some images ranges from 0-255 and for some the range is higher.

[mtweeman]

And what about wide characters? Do you try to do it using them? In wide characters each character stores symbol on 2 bytes, so you can write a numbers from 0 to 65535.

[Disch]

1 'char' is 1 byte. If you want to read a variable that's larger than 1 byte, you have a few options.

The endian-safe way is to read the variable one byte at a time and use bitshifting to construct the larger value.

Here's an imperfect, but functional function you can use:

template <typename T>
T ReadInt(istream& s)
{
  T ret = 0;
  for(int i = 0; i < sizeof(T); ++i)
  {
    ret <<= 8;
    ret |= (unsigned char)s.get();
  }
  return ret;
}

// ... then to use, say you want to read an 'int' from the file:

int i = ReadInt<int>(myfile);

[kenter13]

Thanks a lot for the answers. I will try these suggestions.

Cheers
Kenter

[ne555]

@Disch:
What is the difference whit this:

T i;
file.read( (char *)&i, sizeof(i) );

[mtweeman]

It allows you to read binary data from a file. read() (write() function too) expects pointer to a char, so adress of the object 'i' has to be thrown on a pointer to a char. Second argument of read() function is size of object you want to store. It's basically a quantity of chars to save.

[Disch]

@ne555:

My approach is endian-safe. Reading directly like your code is subject to system's endianness.

For example, say you have the following 4 bytes in a file: 01 23 45 67

With my approach, you will read 0x67452301 on any system.

WIth your approach you will get 0x67452301 on little endian systems only, and will get 0x1234567 on big endian systems.

[ne555]

Thanks