I have a RGBA image file which I want to convert to BMP in C++

Question

I am new to interpreting image formats. I have read about imagemagick but are there any other libraries available which I can use ?
Or is it much like reading rgba file and rearranging the data. any code snippet? Any help here would be of great help.

EDIT: I have written below code to convert RGBA image to BMP but output image is invalid when viewed. Can i get help here? Referred post by @bRad Gibson IntegerfileToBMP

#include <iostream>
#include <fstream>
#include <cstdint>
#include <iterator>
#include <vector>
using namespace std;

typedef unsigned char  BYTE; // 1byte
typedef unsigned short  WORD; // 2bytes uint16_t
typedef unsigned long  DWORD; //4bytes uint32_t
typedef unsigned long int LONG; 

struct BMP
{
    BMP();
    struct
    {
        WORD ID;
        DWORD fileSizeInBytes;
        WORD reserved1;
        WORD reserved2;
        DWORD pixelArrayOffsetInBytes;
    } FileHeader;

    enum class CompressionMethod : DWORD {   BI_RGB              = 0x00, 
                                                BI_RLE8             = 0x01,
                                                BI_RLE4             = 0x02,
                                                BI_BITFIELDS        = 0x03,
                                                BI_JPEG             = 0x04,
                                                BI_PNG              = 0x05,
                                                BI_ALPHABITFIELDS   = 0x06 };

    struct
    {
        DWORD headerSizeInBytes;
        DWORD bitmapWidthInPixels;
        DWORD bitmapHeightInPixels;
        WORD colorPlaneCount;
        WORD bitsPerPixel;
        CompressionMethod compressionMethod;
        DWORD bitmapSizeInBytes;
        int32_t horizontalResolutionInPixelsPerMeter;
        int32_t verticalResolutionInPixelsPerMeter;
        DWORD paletteColorCount;
        DWORD importantColorCount;
    } DIBHeader;
};

BMP::BMP()
{
    //Initialized fields
    FileHeader.ID                                   = 0x4d42; // == 'BM' (little-endian)
    FileHeader.reserved1                            = 0;
    FileHeader.reserved2                            = 0;
    FileHeader.pixelArrayOffsetInBytes              = sizeof( FileHeader ) + sizeof( DIBHeader );
    DIBHeader.headerSizeInBytes                     = 40;
    DIBHeader.colorPlaneCount                       = 1;
    DIBHeader.bitsPerPixel                          = 32;
    DIBHeader.compressionMethod                     = CompressionMethod::BI_RGB;
    DIBHeader.horizontalResolutionInPixelsPerMeter  = 2835; // == 72 ppi
    DIBHeader.verticalResolutionInPixelsPerMeter    = 2835; // == 72 ppi
    DIBHeader.paletteColorCount                     = 0;
    DIBHeader.importantColorCount                   = 0;
}

int main()
{
    const std::string rgbaFilename = "rgbaFile.rgba";
    const std::string bitmapFilename = "bitmapFile.bmp";

    //Read RGBA file
    ifstream readRGBA(rgbaFilename, ios::binary);

    if(!readRGBA)
        return 0;

    std::vector<char> buffer((
            std::istreambuf_iterator<char>(readRGBA)), 
            (std::istreambuf_iterator<char>()));

    //Construct .bmp
    BMP bmp;
    size_t charCount = buffer.size();

    bmp.DIBHeader.bitmapSizeInBytes = charCount * sizeof( buffer[ 0 ] );
    bmp.FileHeader.fileSizeInBytes = bmp.FileHeader.pixelArrayOffsetInBytes + bmp.DIBHeader.bitmapSizeInBytes;
    bmp.DIBHeader.bitmapWidthInPixels = static_cast< uint32_t >( ceil( (sqrt( (float)charCount ) ) ));
    bmp.DIBHeader.bitmapHeightInPixels = static_cast< uint32_t >( ceil( charCount / static_cast< float >( bmp.DIBHeader.bitmapWidthInPixels ) ) );

    std::ofstream writeBMP( bitmapFilename, std::ofstream::binary );

    if( !writeBMP )
        return 0;

    writeBMP.write( reinterpret_cast< char * >( &bmp ), sizeof( bmp ) );
    writeBMP.write( reinterpret_cast< char * >( &buffer[ 0 ] ), bmp.DIBHeader.bitmapSizeInBytes );
    return 0;
}

Answer 1

There are many image libraries to convert image data. As you noted, ImageMagick is one such library which is capable of converting RAW (RGB[A]) data into a BMP.

However, the BMP format is relatively simple, compared to many other image format. RAW data can be converted into a BMP with a small amount of code, so if you would like to reduce your external dependencies (eg. not use external code), writing your own BMP serialization code does not necessarily warrant integrating an external library.

See here for a simple example that reads and writes the format to/from RAW data.