Alexander Shuev Alexander Shuev - 1 month ago 16
C++ Question

ifstream::read not reading unsigned char, even with reinterpret_cast

I am trying to get my code to read a PPM image (P3) and it isn't working as it should. The idea is to get a 3 unsigned chars and store them in RGB. But at the moment it is only resulting in taking the first character and ignoring the rest.

Image Image::readImg(std::string const &filename) {
std::ifstream ifs;
ifs.open(filename.c_str(), std::ios::binary);
Image _in;
try {
if (ifs.fail()) {
throw("Can't open input file");
}
std::string header;
int w, h, max;
ifs >> header;
if (strcmp(header.c_str(), "P3") != 0) throw("Can't read input file");
ifs >> w >> h >> max;
_in.init(w, h);
ifs.ignore(256, '\n');
unsigned char pix[3];
for (int i = 0; i < h; ++i){
for (int j = 0; j < w; ++j){
ifs.read(reinterpret_cast<char *>(pix), 3);
_in.pixels[i][j].R = pix[0];
_in.pixels[i][j].G = pix[1];
_in.pixels[i][j].B = pix[2];
}
}
std::cout << "|" << _in.pixels[0][0].R << " " << _in.pixels[0][0].G << " " << _in.pixels[0][0].B << "|";
ifs.close();
}
catch (const char *err) {
fprintf(stderr, "%s\n", err);
ifs.close();
}
return _in;
}


Note the std::cout is supposed to output in my scenario |186 0 255|, but instead I get |1 8 6|.




EDIT: The file (original.ppm), when opened in Notepad++, looks like this (UNIX / UTF-8):

P3
1024 768
255
186 0 255 186 0 255 186 0 255 186 0 255 186 0 255 186 0 255 186 1 255
186 1 254 186 1 254 185 2 254 185 2 254 185 1 254 185 2 253 185 3 253
185 2 252 185 3 252 185 3 252 185 3 252 185 3 251 184 4 251 184 4 251
184 4 251 184 4 251 184 5 250 184 5 250 183 5 250 183 6 249 183 6 249
183 6 249 183 6 248 183 7 249 183 7 249 183 7 248 183 7 247 183 8 247
182 7 247 182 8 246 183 9 247 183 9 246 183 9 246 182 9 246 181 9 246
182 9 246 181 10 245 182 10 245 181 10 244 181 10 245 181 11 244 181 11 244
...


The result should be:
_in.pixels[0][0].R = 186
_in.pixels[0][0].G = 0
_in.pixels[0][0].B = 255
and to continue collecting the RGB of all the pixels in the file.

Answer

[Updated] This should work:

int pix[3]; // not an unsigned char
...
ifs >> pix[0] >> pix[1] >> pix[2];

instead of:

ifs.read(reinterpret_cast<char *>(pix), 3);

Like you do for width and height?

ifs >> w >> h >> max;
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