Celestia12 Celestia12 - 1 year ago 123
C++ Question

Write a PPM file with C++ and get distorted result

I have to read, modify and write a PPM image. I tried to check if I got all the data of the PPM image in my array right by trying to write it to a new file. But the image i got looks like a broken TV:a pattern of this:

I have no idea what I did wrong, I'd like to know at least a way to check what could be wrong. These are my codes:
Main:
Image instance;

string filename = "Image03.ppm";
string format = "ppm";
bool fileWritten = false;
if (instance.load(filename, format)) {
cout << "The file has been successfully loaded" << endl;

cout << "The dimensions of the image are:" << endl;
cout << instance.getWidth() << "x" << instance.getHeight() << endl;


...

bool imaging::Image::load(const string & filename, const string & format){
string extension = filename.substr(filename.find_last_of(".") + 1);
bool flag = false; // to stop the loop when a different letter is found.
bool result = false; // stores the return value
unsigned int sz = extension.size();
for (unsigned int i = 0; i < sz; ++i) {
if (tolower(extension[i]) != tolower(format[i]) && !flag) {
cout << "The extension of the file does not match the required format." << endl;

flag = true;

}
}

if (!flag) {
string filename1 = filename;
const char *cstr = filename1.c_str();
int pw = 0;
int ph = 0;
float* buff = ReadPPM(cstr, &pw, &ph);
width = pw;
height = ph;
bool h = WritePPM(buff, width, height, "kokoo.ppm");


...

bool WritePPM(const float * data, int w, int h, const char * filename) {
ofstream myfile;
myfile.open(filename, ios::out | ios::binary);

myfile << "P6\n" << w << "\n" << h << "\n255\n";
for (int i = 0; i < w*h; i++) {
myfile.write((char*)&data[i], sizeof(float));
}
myfile.close();
return 0;
}


...

float * ReadPPM(const char * filename, int* w, int* h) {
ifstream myfile;
myfile.open(filename, ios::binary);
if (myfile.is_open())
{
int i = 0;
string letter;
cout << "File successfully open" << endl;

myfile.ignore(2, ' ');
int width;
int height;
myfile >> width;
myfile >> height;
myfile.ignore(4, ' ');

*w = width;
*h = height;

int size = width*height * 3;
int k = myfile.peek();
while (k == 32) { //32 is ascii for whitespace.
myfile.ignore(256,' ');
k = myfile.peek();
}
unsigned char * temp = new unsigned char[size];
myfile.read((char*)temp, size);
float * buff = new float[size];



while (i < size) {
buff[i] = (float)temp[i]/255;
i++;
cout << "before" << buff[i] << endl;
cin.get();
}

delete[] temp;
return buff;
myfile.close();
}
else
{
cout << "Error opening file";
cin.get();
return 0;
}




}


Even if my code looks silly, it's why I'm new to this language and except if I did something terribly wrong, I like how it looks, it's an assignment anyway.

Answer Source

If you really have to do convert to a float array, here is what you are doing wrong:

Your loop in WritePPM is totally wrong. You forgot the 3, you forgot to scale back by multiplying with 255, and you can't write float values to a PPM. It should be something like:

for (int i = 0; i < 3*w*h; i++) {
    unsigned char c = (unsigned char) (255.0f * data[i] + 0.5f); 
    myfile.write((char*) &c, sizeof(c));
}

I didn't check all the other code, but you definitely still have some other bugs, such as closing the file after the return statement in ReadPPM.

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