renderd7/bmpconverter.hpp

#pragma once
#include "lodepng.h"

#include <iostream>

//returns 0 if all went ok, non-0 if error
//output image is always given in RGBA (with alpha channel), even if it's a BMP without alpha channel
unsigned decodeBMP(std::vector<unsigned char>& image, unsigned& w, unsigned& h, const std::vector<unsigned char>& bmp) {
  static const unsigned MINHEADER = 54; //minimum BMP header size

  if(bmp.size() < MINHEADER) return -1;
  if(bmp[0] != 'B' || bmp[1] != 'M') return 1; //It's not a BMP file if it doesn't start with marker 'BM'
  unsigned pixeloffset = bmp[10] + 256 * bmp[11]; //where the pixel data starts
  //read width and height from BMP header
  w = bmp[18] + bmp[19] * 256;
  h = bmp[22] + bmp[23] * 256;
  //read number of channels from BMP header
  if(bmp[28] != 24 && bmp[28] != 32) return 2; //only 24-bit and 32-bit BMPs are supported.
  unsigned numChannels = bmp[28] / 8;

  //The amount of scanline bytes is width of image times channels, with extra bytes added if needed
  //to make it a multiple of 4 bytes.
  unsigned scanlineBytes = w * numChannels;
  if(scanlineBytes % 4 != 0) scanlineBytes = (scanlineBytes / 4) * 4 + 4;

  unsigned dataSize = scanlineBytes * h;
  if(bmp.size() < dataSize + pixeloffset) return 3; //BMP file too small to contain all pixels

  image.resize(w * h * 4);

  /*
  There are 3 differences between BMP and the raw image buffer for LodePNG:
  -it's upside down
  -it's in BGR instead of RGB format (or BRGA instead of RGBA)
  -each scanline has padding bytes to make it a multiple of 4 if needed
  The 2D for loop below does all these 3 conversions at once.
  */
  for(unsigned y = 0; y < h; y++)
  for(unsigned x = 0; x < w; x++) {
    //pixel start byte position in the BMP
    unsigned bmpos = pixeloffset + (h - y - 1) * scanlineBytes + numChannels * x;
    //pixel start byte position in the new raw image
    unsigned newpos = 4 * y * w + 4 * x;
    if(numChannels == 3) {
      image[newpos + 0] = bmp[bmpos + 2]; //R
      image[newpos + 1] = bmp[bmpos + 1]; //G
      image[newpos + 2] = bmp[bmpos + 0]; //B
      image[newpos + 3] = 255;            //A
    } else {
      image[newpos + 0] = bmp[bmpos + 2]; //R
      image[newpos + 1] = bmp[bmpos + 1]; //G
      image[newpos + 2] = bmp[bmpos + 0]; //B
      image[newpos + 3] = bmp[bmpos + 3]; //A
    }
  }
  return 0;
}

std::vector<unsigned char> ConvertFile(std::string filename) {

  std::vector<unsigned char> bmp;
  lodepng::load_file(bmp, filename);
  
  std::vector<unsigned char> image;
  unsigned w, h;
  unsigned error = decodeBMP(image, w, h, bmp);

  if(error) {
    std::cout << "BMP decoding error " << error << std::endl;
    
  }

  std::vector<unsigned char> png;
  error = lodepng::encode(png, image, w, h);

  if(error) {
    std::cout << "PNG encoding error " << error << ": " << lodepng_error_text(error) << std::endl;
    
  }

  return png;

}

std::vector<unsigned char> ConvertData(std::vector<unsigned char> data) {
  
  std::vector<unsigned char> image;
  unsigned w, h;
  unsigned error = decodeBMP(image, w, h, data);

  if(error) {
    std::cout << "BMP decoding error " << error << std::endl;
    
  }

  std::vector<unsigned char> png;
  error = lodepng::encode(png, image, w, h);

  if(error) {
    std::cout << "PNG encoding error " << error << ": " << lodepng_error_text(error) << std::endl;
    
  }

  return png;

}