Why is the Right Side of my Image Being Wrapped Around? - c++

Question

essentially, I'm making a class that takes a BMP file for the constructor. From THIS POST I get all of the header data out of the way and then read RGB data into a vector. I then calculate the intensities as 0.25R + 0.5G + 0.25B. I put these numbers into a space-separated file, line by line. With the original above my result below and Using GNUPlot to open and plot the image gives me this result.
original
distortion
As you can see, the right side of the image is consistently being wrapped around further as the image is written to file (or somewhere before this process). I've pasted the code below, any help?
std::vector<char> MImage::readBMP(std::string const file){
static constexpr size_t HEADER_SIZE = 54;
std::ifstream bmp(file, std::ios::binary);
std::array<char, HEADER_SIZE> header;
bmp.read(header.data(), header.size());
auto fileSize = *reinterpret_cast<uint32_t*>(&header[2]);
auto dataOffset = *reinterpret_cast<uint32_t*>(&header[10]);
auto width = *reinterpret_cast<uint32_t*>(&header[18]);
auto height = *reinterpret_cast<uint32_t*>(&header[22]);
auto depth = *reinterpret_cast<uint16_t*>(&header[28]);
/*
std::cout << "fileSize: " << fileSize << std::endl;
std::cout << "dataOffset: " << dataOffset << std::endl;
std::cout << "width: " << width << std::endl;
std::cout << "height: " << height << std::endl;
std::cout << "depth: " << depth << "-bit" << std::endl;
*/
std::vector<char> img(dataOffset - HEADER_SIZE);
//bmp.read(img.data(), img.size());
auto dataSize = ((width * 3 + 3) & (~3)) * height;
img.resize(dataSize);
bmp.read(img.data(), img.size());
char temp = 0;
for (int i = dataSize - 4; i >= 0; i -= 3)
{
temp = img[i];
img[i] = img[i + 2];
img[i + 2] = temp;
}
// Convert to intensity
int k = 0;
int size = (int)img.size();
for (int j = 0; k+2 < size; j++)
{
//0.25B + 0.5G + 0.25R
img[j] = ((abs(img[k]) >> 2) + (abs(img[k + 1]) >> 1) + (abs(img[k + 2]) >> 2));
//OutputDebugStringA((to_string(img[j]) + "\n").c_str());
k += 3;
}
img.resize(dataSize / 3);
//OutputDebugStringA((to_string(img.size()) + "\n").c_str());
int a, b, c = 0;
//Testing #img data
ofstream TestPic;
TestPic.open("testpic.txt");
for (a = 0; a < HEIGHT; a++) {
for (b = 0; b < WIDTH; b++) {
TestPic << (int)img[c];
if (b < WIDTH-1) {
TestPic << " ";
}
c++;
}
TestPic << "\n";
}
TestPic.close();
return img; }
GNUPlot command: plot [0:630] [0:354] 'testpic.txt' matrix with image pixels

Answer 1

The problem you are seeing is caused by improper data alignment. Each scanline of .bmp file must have a byte-size divisible by 4. You are calculating the input data size correctly with this line:
auto dataSize = ((width * 3 + 3) & (~3)) * height;
However, while converting the img array you do not compensate/throw away the padding at the end of each scanline.
The best advice is either to use a standard bmp loader/converter like the one in STB libraries (the stb_image.h file) or, if you want to do it yourself, allocate the other array (img2 or something like that) and iterate img array scanline by scanline, writing greyscale values to img2.
By the way, this answer, which you have mentioned, contains a solution for your exact problem (it is called the "padding fix").