Different base64 encoding for same Image - c++

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.