trying to write a ppm image in c++ - c++

Question

I want to read and write a ppm image and calculate the average number of color.
My read method works fine and also the average number of each color is correct, but my write method gives me a wrong result.
The buffer which holds the data has type Vec3<float>*. Every 3D vector holds the r, g and b values.
Here is the read method:
Image * ReadPPM(const char * filename) {
Image* image;
ifstream file;
unsigned int width, height;
string version;
float maxvalue;
file.open(filename, ios::in | ios::binary);
if (!file) {
cerr << "file could not be open" << endl;
exit(EXIT_FAILURE);
}
// read the header
file >> version;
// check the header
// PPM "header" is valid
if (version.compare("P6") != 0)
{
cout << "Invalid image format (must be 'P6')";
exit(EXIT_FAILURE);
}
file >> width >> height >> maxvalue;
size_t size = height * width*3;
unsigned char * buffer = new unsigned char[size];
Vec3<float>* finalBuffer = new Vec3<float>[size / 3];
file.get();
file.read( (char *)buffer, size);
int j = 0;
for (int i = 0; i < size; i+=3) {
Vec3<float> vec(
(int)buffer[i]/maxvalue,
(int)buffer[i+1] / maxvalue,
(int)buffer[i+2] / maxvalue
);
finalBuffer[j] = vec;
j++;
}
file.clear();
file.close();
image = new Image(width, height, finalBuffer);
return image;
}
Here is the write method:
bool Image::operator >> (string filename) {
ofstream file;
file.open(filename,ios::out | ios::binary);
if (!file) {
cerr << "Cannot open file" << endl;
return false;
}
file << "P6" <<"\n";
file << getWidth() << "\n";
file << getHeight() << "\n";
file << "255" << "\n";
size_t size = height * width;
void*img = getRawDataPtr(); //returns void* buffer
unsigned char * temp = new unsigned char[size*3];
Vec3<float>* buff = (Vec3<float>*)img;
for (int i = 0; i<size; i++) {
Vec3<float> vector;
vector[0] =buff[i].x;
vector[1] = buff[i].y;
vector[2] = buff[i].z; // /255.f;
buff[i] = vector;
}
for (int i = 0; i < size; i++) {
temp[i * 3] = static_cast<unsigned char>(buff[i].x);
temp[i * 3 + 1] = static_cast<unsigned char>(buff[i].y);
temp[i * 3 + 2] = static_cast<unsigned char>(buff[i].z);
}
file.write((char *)temp, size * 3);
if (file.fail()) {
cerr << "Could not write data" << endl;
return false;
}
file.clear();
file.close();
return true;
delete[] buff;
delete[] temp;
}

Answer 1

Your values are normalized between 0 and 1 when reading the file. When writing, you just cast the float values into unsigned char, so you can only obtain 0 and 1 values: I guess you final image is filled with black.
void*img = getRawDataPtr(); //returns void* buffer
unsigned char * temp = new unsigned char[size*3];
Vec3<float>* buff = (Vec3<float>*)img;
Why creating a dynamic array you will have to manage? You are coding in C++, so use a container like std::vector.
for (int i = 0; i<size; i++) {
Vec3<float> vector;
vector[0] =buff[i].x;
vector[1] = buff[i].y;
vector[2] = buff[i].z; // /255.f;
buff[i] = vector;
}
What is the purpose of this loop? You are creating a Vec3<float> filled without transformation from the Vec3<float> in the current pixel in which you assign without transformation your temporary value: this just does nothing.
return true;
delete[] buff;
delete[] temp;
As I said in the comments, the code after return is not executed, so you will never release the memory you allocated for temp. Use a std::vector and you will not have to do it. buff is your original image: you should not delete it here.
Once cleaned, your code may become:
std::vector<unsigned char> temp(size*3);
Vec3<float>* buff = static_cast<Vec3<float>*>(getRawDataPtr());
for (int i = 0; i < size; i++) {
temp[i * 3] = static_cast<unsigned char>(buff[i].x * 255);
temp[i * 3 + 1] = static_cast<unsigned char>(buff[i].y * 255);
temp[i * 3 + 2] = static_cast<unsigned char>(buff[i].z * 255);
}
file.write(reinterpret_cast<char*>(&temp[0]), size * 3);
if (file.fail()) {
cerr << "Could not write data" << endl;
return false;
}
file.close();
return true;