同一张图片使用不同的base64编码

Question

I need to convert base64 to opencv::mat file. So I took a set of 10 frames from a video and appended base64(A) (using base64.b64encode in python).

After that, I used the code from here to convert that base64 to Mat. Images looked fine.

But they have larger file size than the original images, plus when I encoded these final images to base64(B) (using base64.b64encode in python), the encoded base64 is different from original base64(A) . I can't understand why? This is also affecting the output of my application that is using the cv::mat output.

For base64 to Mat I am using code from here (asposted above).

Edited : Following is my python script to convert set of jpeg to base64 (.txt)

 def img2txt(file_directory):    
        imageFiles = glob.glob(file_directory+"/*.jpg")
        imageFiles.sort()
        fileWrite='base64encoding.txt'
        #print fileWrite
        for i in range(0,len(imageFiles)):
            image = open(imageFiles[i],'rb')
            image_read = image.read()
            image_64_encode = base64.b64encode(image_read)
            with open (fileWrite, 'a') as f: 
                f.writelines(image_64_encode+'\n')

base64decode function : from here

static const std::string base64_chars =
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"abcdefghijklmnopqrstuvwxyz"
"0123456789+/";


static inline bool is_base64(unsigned char c) {
    return (isalnum(c) || (c == '+') || (c == '/'));
}

std::string base64_decode(std::string const& encoded_string) {
    int in_len = encoded_string.size();
    int i = 0;
    int j = 0;
    int in_ = 0;
    unsigned char char_array_4[4], char_array_3[3];
    std::string ret;

    while (in_len-- && (encoded_string[in_] != '=') && is_base64(encoded_string[in_])) {
        char_array_4[i++] = encoded_string[in_]; in_++;
        if (i == 4) {
            for (i = 0; i < 4; i++)
                char_array_4[i] = base64_chars.find(char_array_4[i]);

            char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
            char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
            char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];

            for (i = 0; (i < 3); i++)
                ret += char_array_3[i];
            i = 0;
        }
    }

    if (i) {
        /*for (j = i; j < 4; j++)
            char_array_4[j] = 0;*/

        for (j = 0; j < i; j++)
            char_array_4[j] = base64_chars.find(char_array_4[j]);

        char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
        char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
        //char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];

        for (j = 0; (j < i - 1); j++) 
            ret += char_array_3[j];
    }

    return ret;
}

C++ Main function :

int main()
{  
  ifstream in("TestBase64.txt");
  if(!in) {
    cout << "Cannot open input file.\n";
    return 1;
    }
    int i=0;
    string encoded_string;
    while (getline(in, encoded_string)) 
    {
        string decoded_string = base64_decode(encoded_string);
        vector<uchar> data(decoded_string.begin(), decoded_string.end());
        cv::imwrite("/Frames_from_B_to_Mat/Frames_from_B_to_Mat"+b+".jpg");
        i++; 
    }

    return 0;
}

Answer 1

On the line:

vector<uchar> data(decoded_string.begin(), decoded_string.end());

you are presumably holding the JPEG-encoded representation of one image in data . So you may as well just write this to a binary file, rather than use cv::imwrite() which is for writing a Mat to a file.

If, for some inexplicable reason, you want to use cv::imwrite() , you need to pass it a Mat . So you would end up decoding the JPEG representation to a Mat and then encoding to JPEG and writing - which seems silly:

cv::Mat img = cv::imdecode(data, cv::IMREAD_COLOR);
cv::imwrite('result.jpg',img);

TLDR;

What I'm saying is that your data is already JPEG-encoded, you read it from a JPEG file.