C++ How to create a bitmap file - c++

Question

I am trying to figure out how to create a bitmap file in C++ VS. Currently I have taken in the file name and adding the ".bmp" extension to create the file. I want to know how I could change the pixels of the file by making it into different colors or patterns (ie. like a checkerboard) This is my function that I have and I believe that I have to send 3 different Bytes at a time in order to establish the color of the pixel.
void makeCheckerboardBMP(string fileName, int squaresize, int n) {
ofstream ofs;
ofs.open(fileName + ".bmp");
writeHeader(ofs, n, n);
for(int row = 0; row < n; row++) {
for(int col = 0; col < n; col++) {
if(col % 2 == 0) {
ofs << 0;
ofs << 0;
ofs << 0;
} else {
ofs << 255;
ofs << 255;
ofs << 255;
}
}
}
}
void writeHeader(ostream& out, int width, int height){
if (width % 4 != 0) {
cerr << "ERROR: There is a windows-imposed requirement on BMP that the width be a
multiple of 4.\n";
cerr << "Your width does not meet this requirement, hence this will fail. You can fix
this\n";
cerr << "by increasing the width to a multiple of 4." << endl;
exit(1);
}
BITMAPFILEHEADER tWBFH;
tWBFH.bfType = 0x4d42;
tWBFH.bfSize = 14 + 40 + (width*height*3);
tWBFH.bfReserved1 = 0;
tWBFH.bfReserved2 = 0;
tWBFH.bfOffBits = 14 + 40;
BITMAPINFOHEADER tW2BH;
memset(&tW2BH,0,40);
tW2BH.biSize = 40;
tW2BH.biWidth = width;
tW2BH.biHeight = height;
tW2BH.biPlanes = 1;
tW2BH.biBitCount = 24;
tW2BH.biCompression = 0;
out.write((char*)(&tWBFH),14);
out.write((char*)(&tW2BH),40);
}

Answer 1

These are the two functions I am using for my code (one greyscale, one RGB saving).
Might give you a hint whats going wrong.
Note: they are done to work, not to be efficient.
void SaveBitmapToFile( BYTE* pBitmapBits, LONG lWidth, LONG lHeight,WORD wBitsPerPixel, LPCTSTR lpszFileName )
{
RGBQUAD palette[256];
for(int i = 0; i < 256; ++i)
{
palette[i].rgbBlue = (byte)i;
palette[i].rgbGreen = (byte)i;
palette[i].rgbRed = (byte)i;
}
BITMAPINFOHEADER bmpInfoHeader = {0};
// Set the size
bmpInfoHeader.biSize = sizeof(BITMAPINFOHEADER);
// Bit count
bmpInfoHeader.biBitCount = wBitsPerPixel;
// Use all colors
bmpInfoHeader.biClrImportant = 0;
// Use as many colors according to bits per pixel
bmpInfoHeader.biClrUsed = 0;
// Store as un Compressed
bmpInfoHeader.biCompression = BI_RGB;
// Set the height in pixels
bmpInfoHeader.biHeight = lHeight;
// Width of the Image in pixels
bmpInfoHeader.biWidth = lWidth;
// Default number of planes
bmpInfoHeader.biPlanes = 1;
// Calculate the image size in bytes
bmpInfoHeader.biSizeImage = lWidth* lHeight * (wBitsPerPixel/8);
BITMAPFILEHEADER bfh = {0};
// This value should be values of BM letters i.e 0x4D42
// 0x4D = M 0×42 = B storing in reverse order to match with endian
bfh.bfType = 'B'+('M' << 8);
// <<8 used to shift ‘M’ to end
// Offset to the RGBQUAD
bfh.bfOffBits = sizeof(BITMAPINFOHEADER) + sizeof(BITMAPFILEHEADER) + sizeof(RGBQUAD) * 256;
// Total size of image including size of headers
bfh.bfSize = bfh.bfOffBits + bmpInfoHeader.biSizeImage;
// Create the file in disk to write
HANDLE hFile = CreateFile( lpszFileName,GENERIC_WRITE, 0,NULL,
CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL,NULL);
if( !hFile ) // return if error opening file
{
return;
}
DWORD dwWritten = 0;
// Write the File header
WriteFile( hFile, &bfh, sizeof(bfh), &dwWritten , NULL );
// Write the bitmap info header
WriteFile( hFile, &bmpInfoHeader, sizeof(bmpInfoHeader), &dwWritten, NULL );
// Write the palette
WriteFile( hFile, &palette[0], sizeof(RGBQUAD) * 256, &dwWritten, NULL );
// Write the RGB Data
if(lWidth%4 == 0)
{
WriteFile( hFile, pBitmapBits, bmpInfoHeader.biSizeImage, &dwWritten, NULL );
}
else
{
char* empty = new char[ 4 - lWidth % 4];
for(int i = 0; i < lHeight; ++i)
{
WriteFile( hFile, &pBitmapBits[i * lWidth], lWidth, &dwWritten, NULL );
WriteFile( hFile, empty, 4 - lWidth % 4, &dwWritten, NULL );
}
}
// Close the file handle
CloseHandle( hFile );
}
void SaveBitmapToFileColor( BYTE* pBitmapBits, LONG lWidth, LONG lHeight,WORD wBitsPerPixel, LPCTSTR lpszFileName )
{
BITMAPINFOHEADER bmpInfoHeader = {0};
// Set the size
bmpInfoHeader.biSize = sizeof(BITMAPINFOHEADER);
// Bit count
bmpInfoHeader.biBitCount = wBitsPerPixel;
// Use all colors
bmpInfoHeader.biClrImportant = 0;
// Use as many colors according to bits per pixel
bmpInfoHeader.biClrUsed = 0;
// Store as un Compressed
bmpInfoHeader.biCompression = BI_RGB;
// Set the height in pixels
bmpInfoHeader.biHeight = lHeight;
// Width of the Image in pixels
bmpInfoHeader.biWidth = lWidth;
// Default number of planes
bmpInfoHeader.biPlanes = 1;
// Calculate the image size in bytes
bmpInfoHeader.biSizeImage = lWidth* lHeight * (wBitsPerPixel/8);
BITMAPFILEHEADER bfh = {0};
// This value should be values of BM letters i.e 0x4D42
// 0x4D = M 0×42 = B storing in reverse order to match with endian
bfh.bfType = 'B'+('M' << 8);
// <<8 used to shift ‘M’ to end
// Offset to the RGBQUAD
bfh.bfOffBits = sizeof(BITMAPINFOHEADER) + sizeof(BITMAPFILEHEADER);
// Total size of image including size of headers
bfh.bfSize = bfh.bfOffBits + bmpInfoHeader.biSizeImage;
// Create the file in disk to write
HANDLE hFile = CreateFile( lpszFileName,GENERIC_WRITE, 0,NULL,
CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL,NULL);
if( !hFile ) // return if error opening file
{
return;
}
DWORD dwWritten = 0;
// Write the File header
WriteFile( hFile, &bfh, sizeof(bfh), &dwWritten , NULL );
// Write the bitmap info header
WriteFile( hFile, &bmpInfoHeader, sizeof(bmpInfoHeader), &dwWritten, NULL );
// Write the palette
//WriteFile( hFile, &palette[0], sizeof(RGBQUAD) * 256, &dwWritten, NULL );
// Write the RGB Data
if(lWidth%4 == 0)
{
WriteFile( hFile, pBitmapBits, bmpInfoHeader.biSizeImage, &dwWritten, NULL );
}
else
{
char* empty = new char[ 4 - lWidth % 4];
for(int i = 0; i < lHeight; ++i)
{
WriteFile( hFile, &pBitmapBits[i * lWidth], lWidth, &dwWritten, NULL );
WriteFile( hFile, empty, 4 - lWidth % 4, &dwWritten, NULL );
}
}
// Close the file handle
CloseHandle( hFile );
}

Answer 2

Given your writeHeader is properly implemented this is almost correct. You need to fix 2 issues though:
You are writing one int per color channel. This should be one byte instead. You need to cast the literals to unsigned char.
Scanlines in bitmaps need to be DWORD-aligned. After your inner loop over col you need to write additional bytes to account for this, unless the size in bytes of the row is a multiple of four.

Answer 3

You need to force the output to be written in binary format, not text, this is chosen when you open your file/create your stream and to output all the values as bytes, not integers, this can be done in a number of ways possibly the easiest being write chr(0) or chr(255) - you also need to start your file with a header section - there are a number of formats that make this too long to go into in an answer here - some of them are down to preference as much as anything. There is a good summary in Wikipedia.
Basically you have to inform the receiving applications which format you are using, the number of rows, columns and how the colours are stored.