Base64 to BMP generates skewed image

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1

I have a code that retrieves an Base64 String from an API call. Since the image can be bigger than the allowed variable storage size, I store the String in a .txt file.

After cleaning this file I start the process of decoding the Base64 and transforming it into a BMP image file. Unfortunally, the result is not what is to be expected. I'm able to open the file, meaning it's not corrupted/doesn't have errors, but some lines are just plain incorrect with pixels in the wrong positions.

img
img940×240 6.58 KB

Here is the Base64 String stored on my .txt file: https://nopaste.net/zXJROd2pLy

Here is my code:

#include "base64.hpp" //densaugeo base64 repository

void makeFiguraFromTxt(String fname){
  limpaBase64(imgTemp); //Other function of mine that just cleans the .txt file
  File file = SD.open(imgTemp, FILE_READ); //File with Base64 String
  File bmpFile = SD.open(fname, FILE_WRITE); //File where BMP image will be stored

  uint32_t largura; //width
  //Header -----------------------------------------------------------------
  uint8_t buffer_header[54]; //bmp has the first 54 bytes as header
  if (file.available()) {
    size_t bytesRead = file.read(buffer_header, sizeof(buffer_header));
    String chunk = String((char*)buffer_header);

    unsigned int decoded_length = decode_base64_length((unsigned char *)chunk.c_str(), chunk.length());
    unsigned char decoded_data[decoded_length];

    decode_base64((unsigned char *)chunk.c_str(), decoded_length, decoded_data);

    bmpFile.write((uint8_t*)decoded_data, decoded_length);

    largura = decoded_data[18] | (decoded_data[19] << 8) | (decoded_data[20] << 16) | (decoded_data[21] << 24);
  }

  //Stride (amount of bytes per line)
  unsigned int stride = ceil((largura * 1)/32)*4;

  uint8_t buffer[stride];
  while (file.available()){
    int bytesRead = file.read(buffer, sizeof(buffer));
    String chunk = String((char*)buffer);

    Serial.print(chunk);

    unsigned int decodedLength = decode_base64_length((unsigned char *)chunk.c_str(), chunk.length());
    unsigned char *decodedData = (unsigned char *)malloc(decodedLength);
    
    decode_base64((unsigned char *)chunk.c_str(), decodedLength, decodedData);

    bmpFile.write((uint8_t *)decodedData, decodedLength);

    free(decodedData);
  }

  bmpFile.flush();
  bmpFile.close();

  file.close();
}

I'm not sure why this is happening, so I would like some insight if possible. My suspicion initially was that the lines need to be multiple of 4, but the stride was supposed to solve it.

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