How to load an JPEG image into a char array C++?

Question

i want to store an JPEG image into a normal unsigned char array, i'd used ifstream to store it; however, when i checked if the array i'd stored is correct or not ( by rewrite it again to an JPEG image), the image that i rewrote by using the stored array couldn't show correctly, so i think the problem must come from the technique that i use to store the image into an array is not correct. I want an array which can be stored perfectly so that i can use it to rewrite back into a JPEG image again.I'd really appreciate if anyone can help me solve this problem!

int size = 921600;
    unsigned char output[size];
    int i = 0;

    ifstream DataFile;
    DataFile.open("abc.jpeg");
    while(!DataFile.eof()){
        DataFile >> output[i];
        i++;
    }
    /* i try to rewrite the above array into a new image here */
    FILE * image2;
    image2 = fopen("def.jpeg", "w");
    fwrite(output,1,921600, image2);
    fclose(image2);

JPEG has its specific structure. It's not just raw pixel values stored in some file. Consider using an existing library that does it for you.. https://en.wikipedia.org/wiki/JPEG#Syntax_and_structure — AlexG, Nov 09 '18 at 11:20
Why don't you use fopen() then fread() instead of your while loop, read the entire file into the buffer? — SPlatten, Nov 09 '18 at 11:45
The very first mistake in your code is you didn't open the file in binary mode. — PaulMcKenzie, Nov 09 '18 at 12:36

Sam Varshavchik · Accepted Answer · 2018-11-09T13:20:46.663

2

There are multiple problems in the shown code.

while(!DataFile.eof()){

This is always a bug. See the linked question for a detailed explanation.

    DataFile >> output[i];

The formatted extraction operator, >>, by definition, skips over all white space characters and ignores them. Your jpg file surely has bytes 0x09, 0x20, and a few others, somewhere in it, and this automatically skips over and does not read them.

In order to do this correctly, you need to use read() and gcount() to read your binary file. Using gcount() correctly should also result in your code detecting the end-of-file condition properly.

edited Nov 09 '18 at 13:20

answered Nov 09 '18 at 11:37

Sam Varshavchik

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"This is always a bug". Not always, but in the form shown above yes. – Lightness Races in Orbit Nov 09 '18 at 11:58
2

File needs to be opened in binary mode too – Lightness Races in Orbit Nov 09 '18 at 11:59

score 1 · Answer 2 · answered Nov 10 '18 at 08:52

Make sure to add error check when opening files. Find the file size and read in to the buffer according to the filesize.

You might also look in to using std::vector<unsigned char> for character storage.

int main()
{
    std::ifstream DataFile("abc.jpeg", std::ios::binary);
    if(!DataFile.good())
        return 0;

    DataFile.seekg(0, std::ios::end);
    size_t filesize = (int)DataFile.tellg();
    DataFile.seekg(0);

    unsigned char output[filesize];
    //or std::vector
    //or unsigned char *output = new unsigned char[filesize];
    if(DataFile.read((char*)output, filesize))
    {
        std::ofstream fout("def.jpeg", std::ios::binary);
        if(!fout.good())
            return 0;
        fout.write((char*)output, filesize);
    }

    return 0;
}

How to load an JPEG image into a char array C++?

2 Answers2